# Image Processing

This page documents the functionality present in this library that deals with the management and manipulation of images. One thing to note is that there is no explicit image object. Instead, everything deals with array2d objects that contain various kinds of pixels.

## Pixel Types

Most image handling routines in dlib will accept images containing any pixel type. This is made possible by defining a traits class, pixel_traits, for each possible pixel type. This traits class enables image processing routines to determine how to handle each kind of pixel and therefore only pixels which have a pixel_traits definition may be used. The following list defines all the pixel types which come with pixel_traits definitions.

- RGB
  There are two RGB pixel types in dlib, rgb_pixel and bgr_pixel. Each defines a 24bit RGB pixel type. The bgr_pixel is identical to rgb_pixel except that it lays the color channels down in memory in BGR order rather than RGB order and is therefore useful for interfacing with other image processing tools which expect this format (e.g. OpenCV).
- RGB Alpha
  The rgb_alpha_pixel is an 8bit per channel RGB pixel with an 8bit alpha channel.
- HSI
  The hsi_pixel is a 24bit pixel which represents a point in the Hue Saturation Intensity (HSI) color space.
- Grayscale
  Any built in scalar type may be used as a grayscale pixel type. For example, unsigned char, int, double, etc.

Pixels
rgb_pixel
bgr_pixel
rgb_alpha_pixel
hsi_pixel
pixel_traits
get_pixel_intensity
assign_pixel
assign_pixel_intensity
Image I/O
jpeg_loader
load_bmp
load_dng
load_image
load_jpeg
load_png
png_loader
save_bmp
save_dng
save_png
Object Detection
get_frontal_face_detector
object_detector
full_object_detection
scan_image
scan_image_movable_parts
find_points_above_thresh
scan_image_pyramid
scan_image_boxes
scan_fhog_pyramid
scan_image_custom
find_candidate_object_locations
test_box_overlap
remove_unobtainable_rectangles
setup_hashed_features
Scan Image Pyramid Tools
compute_box_dimensions create_single_box_detection_template create_overlapped_2x2_detection_template create_grid_detection_template determine_object_boxes setup_grid_detection_templates setup_grid_detection_templates_verbose
Feature Extraction
get_surf_points
SURF Tools
hessian_pyramid compute_surf_descriptor haar_x haar_y get_interest_points interest_point surf_point compute_dominant_angle draw_surf_points
hog_image
extract_fhog_features
fine_hog_image
poly_image
hashed_feature_image
binned_vector_feature_image
nearest_neighbor_feature_image
randomly_sample_image_features
Edges and Thresholds
edge_orientation
hysteresis_threshold
sobel_edge_detector
suppress_non_maximum_edges
threshold_image
auto_threshold_image
Morphology
label_connected_blobs
segment_image
binary_dilation
binary_erosion
binary_open
binary_close
binary_intersection
binary_union
binary_difference
binary_complement
Filtering
gaussian_blur
spatially_filter_image
spatially_filter_image_separable
float_spatially_filter_image_separable
separable_3x3_filter_block_grayscale
separable_3x3_filter_block_rgb
sum_filter
sum_filter_assign
max_filter
spatially_filter_image_separable_down
Scaling and Rotating
pyramid_up
pyramid_down
pyramid_disable
interpolate_nearest_neighbor
interpolate_bilinear
interpolate_quadratic
transform_image
rotate_image
resize_image
flip_image_left_right
flip_image_up_down
add_image_left_right_flips
add_image_rotations
upsample_image_dataset
flip_image_dataset_left_right
rotate_image_dataset
extract_image_chips
Colormaps
randomly_color_image
heatmap
jet
Miscellaneous
cv_image
toMat
draw_line
draw_rectangle
tile_images
draw_fhog
fill_rect
assign_image
assign_image_scaled
assign_all_pixels
assign_border_pixels
equalize_histogram
get_histogram
zero_border_pixels
integral_image
integral_image_generic
integral_image_generic
dlib/image_transforms.h
dlib/image_transforms/integral_image_abstract.h
This object is an alternate way of representing image data that allows for very fast computations of sums of pixels in rectangular regions. To use this object you load it with a normal image and then you can use the get_sum_of_area() member function to compute sums of pixels in a given area in constant time.
integral_image
dlib/image_transforms.h
dlib/image_transforms/integral_image_abstract.h
This is a specialization of the integral_image_generic template for the case where sums of pixel values should be represented with longs. E.g. if you use 8bit pixels in your original images then this is the appropriate kind of integral image to use with them.
haar_x
dlib/image_transforms.h
dlib/image_transforms/integral_image_abstract.h
This is a function that operates on an integral_image and allows you to compute the response of a Haar wavelet oriented along the X axis.
haar_y
dlib/image_transforms.h
dlib/image_transforms/integral_image_abstract.h
This is a function that operates on an integral_image and allows you to compute the response of a Haar wavelet oriented along the Y axis.
draw_surf_points
dlib/image_keypoint/draw_surf_points.h
dlib/image_keypoint/draw_surf_points_abstract.h
This routine adds a bunch of surf_point objects onto an image_window object so they can be visualized.
surf_ex.cpp.html
compute_dominant_angle
dlib/image_keypoint.h
dlib/image_keypoint/surf_abstract.h
Computes and returns the dominant angle (i.e. the angle of the dominant gradient) at a given point and scale in an image. This function is part of the main processing of the SURF algorithm.
get_surf_points
dlib/image_keypoint.h
dlib/image_keypoint/surf_abstract.h
This function runs the complete SURF algorithm on an input image and returns the points it found. For a description of what exactly the SURF algorithm does you should read the following paper:

> SURF: Speeded Up Robust Features By Herbert Bay, Tinne Tuytelaars, and Luc Van Gool

Also note that there are numerous flavors of the SURF algorithm you can put together using the functions in dlib. The get_surf_points() function is just an example of one way you might do so.

surf_ex.cpp.html
compute_surf_descriptor
dlib/image_keypoint.h
dlib/image_keypoint/surf_abstract.h
Computes the 64 dimensional SURF descriptor vector of a box centered at a given center point, tilted at a given angle, and sized according to a given scale.
hog_image
dlib/image_keypoint.h
dlib/image_keypoint/hog_abstract.h
This object is a tool for performing the image feature extraction algorithm described in the following paper:

> Histograms of Oriented Gradients for Human Detection by Navneet Dalal and Bill Triggs

object_detector_ex.cpp.html
train_object_detector.cpp.html
extract_fhog_features
dlib/image_transforms.h
dlib/image_transforms/fhog_abstract.h
This function implements the HOG feature extraction method described in the paper:

> Object Detection with Discriminatively Trained Part Based Models by P. Felzenszwalb, R. Girshick, D. McAllester, D. Ramanan in IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol. 32, No. 9, Sep. 2010

This means that it takes an input image and outputs Felzenszwalb's 31 dimensional version of HOG features.
fhog_ex.cpp.html
fine_hog_image
dlib/image_keypoint.h
dlib/image_keypoint/fine_hog_image_abstract.h
This object is a version of the hog_image that allows you to extract HOG features at a finer resolution.
poly_image
dlib/image_keypoint.h
dlib/image_keypoint/poly_image_abstract.h
This object is a tool for extracting local feature descriptors from an image. In particular, it fits polynomials to local pixel patches and allows you to query the coefficients of these polynomials.
hashed_feature_image
dlib/image_keypoint.h
dlib/image_keypoint/hashed_feature_image_abstract.h
This object is a tool for performing image feature extraction. In particular, it wraps another image feature extractor and converts the wrapped image feature vectors into sparse indicator vectors. It does this by hashing each feature vector and then returns a new vector which is zero everywhere except for the position determined by the hash.

The following feature extractors can be wrapped by the hashed_feature_image:

- hog_image
- fine_hog_image
- poly_image

object_detector_ex.cpp.html
train_object_detector.cpp.html
binned_vector_feature_image
dlib/image_keypoint.h
dlib/image_keypoint/binned_vector_feature_image_abstract.h
This object is a tool for performing image feature extraction. In particular, it wraps another image feature extractor and converts the wrapped image feature vectors into a high dimensional sparse vector. For example, if the lower level feature extractor outputs the vector [3,4,5] and this vector is hashed into the second bin of four bins then the output sparse vector is:

> [0,0,0,0, 3,4,5,1, 0,0,0,0, 0,0,0,0].

That is, the output vector has a dimensionality that is equal to the number of hash bins times the dimensionality of the lower level vector plus one. The value in the extra dimension concatenated onto the end of the vector is always a constant value of of 1 and serves as a bias value. This means that, if there are N hash bins, these vectors are capable of representing N different linear functions, each operating on the vectors that fall into their corresponding hash bin.

The following feature extractors can be wrapped by the binned_vector_feature_image:

- hog_image
- fine_hog_image
- poly_image

nearest_neighbor_feature_image
dlib/image_keypoint.h
dlib/image_keypoint/nearest_neighbor_feature_image_abstract.h
This object is a tool for performing image feature extraction. In particular, it wraps another image feature extractor and converts the wrapped image feature vectors into sparse indicator vectors. It does this by finding the nearest neighbor for each feature vector and returning an indicator vector that is zero everywhere except for the position indicated by the nearest neighbor.

The following feature extractors can be wrapped by the nearest_neighbor_feature_image:

- hog_image
- fine_hog_image
- poly_image

hessian_pyramid
dlib/image_keypoint.h
dlib/image_keypoint/hessian_pyramid_abstract.h
This object represents an image pyramid where each level in the pyramid holds determinants of Hessian matrices for the original input image. This object can be used to find stable interest points in an image.

This object is an implementation of the fast Hessian pyramid as described in the paper:

> SURF: Speeded Up Robust Features By Herbert Bay, Tinne Tuytelaars, and Luc Van Gool

This implementation was also influenced by the very well documented OpenSURF library and its corresponding description of how the fast Hessian algorithm functions:

> Notes on the OpenSURF Library by Christopher Evans

get_interest_points
dlib/image_keypoint.h
dlib/image_keypoint/hessian_pyramid_abstract.h
This function extracts interest points from a hessian_pyramid.
interest_point
dlib/image_keypoint.h
dlib/image_keypoint/hessian_pyramid_abstract.h
This is a simple struct used to represent the interest points returned by the get_interest_points function.
surf_point
dlib/image_keypoint.h
dlib/image_keypoint/surf_abstract.h
This is a simple struct used to represent the SURF points returned by the get_surf_points function.
surf_ex.cpp.html
pixel_traits
dlib/pixel.h
dlib/pixel.h
As the name implies, this is a traits class for pixel types. It allows you to determine what sort of pixel type you are dealing with.
hsi_pixel
dlib/pixel.h
dlib/pixel.h
This is a simple struct that represents an HSI colored graphical pixel.
rgb_alpha_pixel
dlib/pixel.h
dlib/pixel.h
This is a simple struct that represents an RGB colored graphical pixel with an alpha channel.
rgb_pixel
dlib/pixel.h
dlib/pixel.h
This is a simple struct that represents an RGB colored graphical pixel.
bgr_pixel
dlib/pixel.h
dlib/pixel.h
This is a simple struct that represents a BGR colored graphical pixel.

The difference between this object and the rgb_pixel is just that this struct lays its pixels down in memory in BGR order rather than RGB order. You only care about this if you are doing something like using the cv_image object to map an OpenCV image into a more object oriented form.

cv_image
dlib/opencv.h
dlib/opencv/cv_image_abstract.h
This object is meant to be used as a simple wrapper around the OpenCV IplImage struct or Mat object. Using this class template you can turn an OpenCV image into something that looks like a normal dlib style image object.

So you should be able to use cv_image objects with many of the image processing functions in dlib as well as the GUI tools for displaying images on the screen.

Note that you can do the reverse conversion, from dlib to OpenCV, using the toMat routine. Also note that you have to #include OpenCV's header before you #include dlib/opencv.h.

toMat
dlib/opencv.h
dlib/opencv/to_open_cv_abstract.h
This routine converts a dlib style image into an instance of OpenCV's cv::Mat object. This is done by setting up the Mat object to point to the same memory as the dlib image.

Note that you can do the reverse conversion, from OpenCV to dlib, using the cv_image object. Also note that you have to #include OpenCV's header before you #include dlib/opencv.h.

heatmap
dlib/image_transforms.h
dlib/image_transforms/colormaps_abstract.h
Converts a grayscale image into a heatmap. This is useful if you want to display a grayscale image with more than 256 values. In particular, this function uses the following color mapping:
image_ex.cpp.html
jet
dlib/image_transforms.h
dlib/image_transforms/colormaps_abstract.h
Converts a grayscale image into an image using the jet color scheme. This is useful if you want to display a grayscale image with more than 256 values. In particular, this function uses the following color mapping:
randomly_color_image
dlib/image_transforms.h
dlib/image_transforms/colormaps_abstract.h
Randomly generates a mapping from gray level pixel values to the RGB pixel space and then uses this mapping to create a colored version an image.

This function is useful for displaying the results of some image segmentation. For example, the output of label_connected_blobs or segment_image.

assign_pixel
dlib/pixel.h
dlib/pixel.h
assign_pixel() is a templated function that can assign any pixel type to another pixel type. It will perform whatever conversion is necessary to make the assignment work. (E.g. color to grayscale conversion)
assign_pixel_intensity
dlib/pixel.h
dlib/pixel.h
assign_pixel_intensity() is a templated function that can change the intensity of a pixel. So if the pixel in question is a grayscale pixel then it simply assigns that pixel the given value. However, if the pixel is not a grayscale pixel then it converts the pixel to the HSI color space and sets the I channel to the given intensity and then converts this HSI value back to the original pixel's color space.
get_pixel_intensity
dlib/pixel.h
dlib/pixel.h
get_pixel_intensity() is a templated function that returns the grayscale intensity of a pixel. If the pixel isn't a grayscale pixel then it converts the pixel to grayscale and returns that value.
png_loader
dlib/image_io.h
dlib/image_loader/png_loader_abstract.h
This object loads a Portable Network Graphics (PNG) image file into an array2d of pixels.

Note that you must define DLIB_PNG_SUPPORT if you want to use this object. You must also set your build environment to link to the libpng library. However, if you use CMake and dlib's default CMakeLists.txt file then it will get setup automatically.

jpeg_loader
dlib/image_io.h
dlib/image_loader/jpeg_loader_abstract.h
This object loads a JPEG image file into an array2d of pixels.

Note that you must define DLIB_JPEG_SUPPORT if you want to use this object. You must also set your build environment to link to the libjpeg library. However, if you use CMake and dlib's default CMakeLists.txt file then it will get setup automatically.

load_jpeg
dlib/image_io.h
dlib/image_loader/jpeg_loader_abstract.h
This function loads a JPEG image file into an array2d of pixels.

Note that you must define DLIB_JPEG_SUPPORT if you want to use this object. You must also set your build environment to link to the libjpeg library. However, if you use CMake and dlib's default CMakeLists.txt file then it will get setup automatically.

load_dng
dlib/image_io.h
dlib/image_loader/image_loader_abstract.h
This global function loads a dlib DNG file (a lossless compressed image format) into an array2d of pixels.
save_dng
dlib/image_io.h
dlib/image_saver/image_saver_abstract.h
This global function saves an image as a dlib DNG file (a lossless compressed image format).

This routine can save images containing any type of pixel. However, the DNG format can natively store only the following pixel types: rgb_pixel, hsi_pixel, rgb_alpha_pixel, uint8, uint16, float, and double. All other pixel types will be converted into one of these types as appropriate before being saved to disk.

save_png
dlib/image_io.h
dlib/image_saver/save_png_abstract.h
This global function writes an image to disk as a PNG (Portable Network Graphics) file.

Note that you must define DLIB_PNG_SUPPORT if you want to use this function. You must also set your build environment to link to the libpng library. However, if you use CMake and dlib's default CMakeLists.txt file then it will get setup automatically.

This routine can save images containing any type of pixel. However, save_png() can only natively store the following pixel types: rgb_pixel, rgb_alpha_pixel, uint8, and uint16. All other pixel types will be converted into one of these types as appropriate before being saved to disk.

load_image
dlib/image_io.h
dlib/image_loader/load_image_abstract.h
This global function takes a file name, looks at its extension, and then loads it into an array2d of pixels using the appropriate image loading routine. The supported types are BMP, PNG, JPEG, and the dlib DNG file format.

Note that you can only load PNG and JPEG files if you link against libpng and libjpeg respectively.

image_ex.cpp.html
load_bmp
dlib/image_io.h
dlib/image_loader/image_loader_abstract.h
This global function loads a MS Windows BMP file into an array2d of pixels.
load_png
dlib/image_io.h
dlib/image_loader/png_loader_abstract.h
This function loads a Portable Network Graphics (PNG) image file into an array2d of pixels.

Note that you must define DLIB_PNG_SUPPORT if you want to use this object. You must also set your build environment to link to the libpng library. However, if you use CMake and dlib's default CMakeLists.txt file then it will get setup automatically.

save_bmp
dlib/image_io.h
dlib/image_saver/image_saver_abstract.h
This global function saves an image as a MS Windows BMP file.

This routine can save images containing any type of pixel. However, it will convert all color pixels into rgb_pixel and grayscale pixels into uint8 type before saving to disk.

draw_line
dlib/image_transforms.h
dlib/image_transforms/draw_abstract.h
This global function draws a line on an image.
draw_rectangle
dlib/image_transforms.h
dlib/image_transforms/draw_abstract.h
This global function draws a rectangle on an image.
tile_images
dlib/image_transforms.h
dlib/image_transforms/draw_abstract.h
This function takes an array of images and tiles them into a single large square image and returns this new big tiled image. Therefore, it is a useful method to visualize many small images at once.
draw_fhog
dlib/image_transforms.h
dlib/image_transforms/fhog_abstract.h
This function takes a FHOG feature map which was created by extract_fhog_features and converts it into an image suitable for display on the screen. In particular, we draw all the hog cells into a grayscale image in a way that shows the magnitude and orientation of the gradient energy in each cell.
fhog_ex.cpp.html
fhog_object_detector_ex.cpp.html
fill_rect
dlib/image_transforms.h
dlib/image_transforms/draw_abstract.h
This global function draws a solid rectangle on an image.
assign_border_pixels
dlib/image_transforms.h
dlib/image_transforms/assign_image_abstract.h
This global function assigns all the pixels in the border of an image to a specific value.
assign_all_pixels
dlib/image_transforms.h
dlib/image_transforms/assign_image_abstract.h
This global function assigns all the pixels in an image a specific value.
assign_image
dlib/image_transforms.h
dlib/image_transforms/assign_image_abstract.h
This global function copies one image into another and performs any necessary color space conversions to make it work right.
assign_image_scaled
dlib/image_transforms.h
dlib/image_transforms/assign_image_abstract.h
This global function copies one image into another and performs any necessary color space conversions to make it work right. Additionally, if the dynamic range of the source image is too big to fit into the destination image then it will attempt to perform the appropriate scaling.
get_histogram
dlib/image_transforms.h
dlib/image_transforms/equalize_histogram_abstract.h
This global function computes an image's histogram and returns it in the form of a column or row matrix object.
edge_orientation
dlib/image_transforms.h
dlib/image_transforms/edge_detector_abstract.h
This global function takes horizontal and vertical gradient magnitude values and returns the orientation of the gradient.
equalize_histogram
dlib/image_transforms.h
dlib/image_transforms/equalize_histogram_abstract.h
This global function performs histogram equalization on an image.
hysteresis_threshold
dlib/image_transforms.h
dlib/image_transforms/thresholding_abstract.h
This global function performs hysteresis thresholding on an image.
sobel_edge_detector
dlib/image_transforms.h
dlib/image_transforms/edge_detector_abstract.h
This global function performs spatial filtering on an image using the sobel edge detection filters.
image_ex.cpp.html
suppress_non_maximum_edges
dlib/image_transforms.h
dlib/image_transforms/edge_detector_abstract.h
This global function performs non-maximum suppression on a gradient image.
image_ex.cpp.html
zero_border_pixels
dlib/image_transforms.h
dlib/image_transforms/assign_image_abstract.h
This global function zeros the pixels on the border of an image.
spatially_filter_image
dlib/image_transforms.h
dlib/image_transforms/spatial_filtering_abstract.h
This global function performs spatial filtering on an image with a user supplied filter.
gaussian_blur
dlib/image_transforms.h
dlib/image_transforms/spatial_filtering_abstract.h
This global function blurs an image by convolving it with a Gaussian filter.
spatially_filter_image_separable
dlib/image_transforms.h
dlib/image_transforms/spatial_filtering_abstract.h
This global function performs spatial filtering on an image with a user supplied separable filter.
float_spatially_filter_image_separable
dlib/image_transforms.h
dlib/image_transforms/spatial_filtering_abstract.h
This global function performs spatial filtering on an image with a user supplied separable filter. It is optimized to work only on float valued images with float valued filters.
spatially_filter_image_separable_down
dlib/image_transforms.h
dlib/image_transforms/spatial_filtering_abstract.h
This global function performs spatial filtering on an image with a user supplied separable filter. Additionally, it produces a downsampled output.
max_filter
dlib/image_transforms.h
dlib/image_transforms/spatial_filtering_abstract.h
This function slides a rectangle over an input image and outputs a new image which contains the maximum valued pixel found inside the rectangle at each position in the input image.
sum_filter
dlib/image_transforms.h
dlib/image_transforms/spatial_filtering_abstract.h
This function slides a rectangle over an input image and adds the sum of pixel values in each rectangle location to another image.
sum_filter_assign
dlib/image_transforms.h
dlib/image_transforms/spatial_filtering_abstract.h
This function slides a rectangle over an input image and outputs a new image which contains the sum of pixels inside the rectangle at each position in the input image.
separable_3x3_filter_block_grayscale
dlib/image_transforms.h
dlib/image_transforms/spatial_filtering_abstract.h
This routine filters part of an image with a user supplied 3x3 separable filter. The output is a grayscale sub-image.
separable_3x3_filter_block_rgb
dlib/image_transforms.h
dlib/image_transforms/spatial_filtering_abstract.h
This routine filters part of an image with a user supplied 3x3 separable filter. The output is a RGB sub-image.
pyramid_down
dlib/image_transforms.h
dlib/image_transforms/image_pyramid_abstract.h
This is a simple function object to help create image pyramids. It downsamples an image by a ratio of N to N-1 where N is supplied by the user as a template argument.
pyramid_disable
dlib/image_transforms.h
dlib/image_transforms/image_pyramid_abstract.h
This object downsamples an image at a ratio of infinity to 1. That means it always outputs an image of size zero. This is useful because it can be supplied to routines which take a pyramid_down function object and it will essentially disable pyramid processing. This way, a pyramid oriented function can be turned into a regular routine which processes just the original undownsampled image.
interpolate_nearest_neighbor
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This object is a tool for performing nearest neighbor interpolation on an image.
interpolate_bilinear
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This object is a tool for performing bilinear interpolation on an image.
interpolate_quadratic
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This object is a tool for performing quadratic interpolation on an image.
transform_image
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This routine is a tool for transforming images using some kind of point mapping function (e.g. point_transform_affine) and pixel interpolation tool (e.g. interpolate_quadratic). An example application of this routine is for image rotation. Indeed, it is how rotate_image is implemented.
rotate_image
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This is a routine for rotating an image.
pyramid_up
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This routine upsamples an image. In particular, it takes a pyramid_down object (or an object with a compatible interface) as an argument and performs an upsampling which is the inverse of the supplied pyramid_down object.
resize_image
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This is a routine capable of resizing or stretching an image.
extract_image_chips
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This function extracts "chips" from an image. That is, it takes a list of rectangular sub-windows (i.e. chips) within an image and extracts those sub-windows, storing each into its own image. It also allows the user to specify the scale and rotation for the chip.
add_image_left_right_flips
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This routine takes a set of images and bounding boxes within those images and doubles the size of the dataset by adding left/right flipped copies of each image as well as the corresponding bounding boxes. Therefore, this function is useful if you are training and object detector and your objects have a left/right symmetry.
fhog_object_detector_ex.cpp.html
add_image_rotations
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This routine takes a set of images and bounding boxes within those images and grows the dataset by computing many different rotations of each image. It will also adjust the positions of the bounding boxes so that they still fall on the same objects in each rotated image.
rotate_image_dataset
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This routine takes a set of images and bounding boxes within those images and rotates the entire dataset by a user specified angle. This means that all images are rotated and the bounding boxes are adjusted so that they still sit on top of the same visual objects in the new rotated images.
flip_image_dataset_left_right
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This routine takes a set of images and bounding boxes within those images and mirrors the entire dataset left to right. This means that all images are flipped left to right and the bounding boxes are adjusted so that they still sit on top of the same visual objects in the new flipped images.
upsample_image_dataset
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This routine takes a set of images and bounding boxes within those images and upsamples the entire dataset. This means that all images are upsampled and the bounding boxes are adjusted so that they still sit on top of the same visual objects in the new images.
flip_image_left_right
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This is a routine which can flip an image from left to right. (e.g. as if viewed through a mirror).
flip_image_up_down
dlib/image_transforms.h
dlib/image_transforms/interpolation_abstract.h
This routine flips an image upside down.
auto_threshold_image
dlib/image_transforms.h
dlib/image_transforms/thresholding_abstract.h
This global function performs a simple binary thresholding on an image. Instead of taking a user supplied threshold it computes one from the image using k-means clustering.
threshold_image
dlib/image_transforms.h
dlib/image_transforms/thresholding_abstract.h
This global function performs a simple binary thresholding on an image with a user supplied threshold.
label_connected_blobs
dlib/image_transforms.h
dlib/image_transforms/label_connected_blobs_abstract.h
This function labels each of the connected blobs in an image with a unique integer label.
segment_image
dlib/image_transforms.h
dlib/image_transforms/segment_image_abstract.h
Attempts to segment an image into regions which have some visual consistency to them. In particular, this function implements the algorithm described in the paper:

> Efficient Graph-Based Image Segmentation by Felzenszwalb and Huttenlocher.

find_candidate_object_locations
dlib/image_transforms.h
dlib/image_transforms/segment_image_abstract.h
This function takes an input image and generates a set of candidate rectangles which are expected to bound any objects in the image. It does this by running a version of the segment_image routine on the image and then reports rectangles containing each of the segments as well as rectangles containing unions of adjacent segments. The basic idea is described in the paper:

> Segmentation as Selective Search for Object Recognition by Koen E. A. van de Sande, et al.

Note that this function deviates from what is described in the paper slightly. See the code for details.
binary_dilation
dlib/image_transforms.h
dlib/image_transforms/morphological_operations_abstract.h
This global function performs the morphological operation of dilation on an image.
binary_erosion
dlib/image_transforms.h
dlib/image_transforms/morphological_operations_abstract.h
This global function performs the morphological operation of erosion on an image.
binary_open
dlib/image_transforms.h
dlib/image_transforms/morphological_operations_abstract.h
This global function performs a morphological opening on an image.
binary_close
dlib/image_transforms.h
dlib/image_transforms/morphological_operations_abstract.h
This global function performs a morphological closing on an image.
binary_intersection
dlib/image_transforms.h
dlib/image_transforms/morphological_operations_abstract.h
This global function computes the intersection of two binary images.
binary_union
dlib/image_transforms.h
dlib/image_transforms/morphological_operations_abstract.h
This global function computes the union of two binary images.
binary_difference
dlib/image_transforms.h
dlib/image_transforms/morphological_operations_abstract.h
This global function computes the difference of two binary images.
binary_complement
dlib/image_transforms.h
dlib/image_transforms/morphological_operations_abstract.h
This global function computes the complement of a binary image.
scan_image
dlib/image_processing.h
dlib/image_processing/scan_image_abstract.h
This global function is a tool for sliding a set of rectangles over an image space and finding the locations where the sum of pixels in the rectangles exceeds a threshold. It is useful for implementing certain kinds of sliding window classifiers.
scan_image_movable_parts
dlib/image_processing.h
dlib/image_processing/scan_image_abstract.h
This global function is a tool for sliding a set of rectangles over an image space and finding the locations where the sum of pixels in the rectangles exceeds a threshold. It is useful for implementing certain kinds of sliding window classifiers. The behavior of this routine is similar to scan_image except that it can also handle movable parts in addition to rigidly placed parts within the sliding window.
find_points_above_thresh
dlib/image_processing.h
dlib/image_processing/scan_image_abstract.h
This routine finds all points in an image with a pixel value above a threshold. It also has the ability to produce an efficient random subsample of such points if the number of them is very large.
scan_image_pyramid
dlib/image_processing.h
dlib/image_processing/scan_image_pyramid_abstract.h
This object is a tool for running a sliding window classifier over an image pyramid. This object can also be understood as a general tool for implementing the spatial pyramid models described in the paper:

> Beyond Bags of Features: Spatial Pyramid Matching for Recognizing Natural Scene Categories by Svetlana Lazebnik, Cordelia Schmid, and Jean Ponce

It also includes the ability to represent movable part models.

The following feature extractors can be used with the scan_image_pyramid object:

- hashed_feature_image
- binned_vector_feature_image
- nearest_neighbor_feature_image

object_detector_ex.cpp.html
object_detector_advanced_ex.cpp.html
scan_fhog_pyramid
dlib/image_processing.h
dlib/image_processing/scan_fhog_pyramid_abstract.h

This object is a tool for running a fixed sized sliding window classifier over an image pyramid. In particular, it slides a linear classifier over a HOG pyramid as discussed in the paper:

> Histograms of Oriented Gradients for Human Detection by Navneet Dalal and Bill Triggs, CVPR 2005

However, we augment the method slightly to use the version of HOG features from:

> Object Detection with Discriminatively Trained Part Based Models by P. Felzenszwalb, R. Girshick, D. McAllester, D. Ramanan IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol. 32, No. 9, Sep. 2010

Since these HOG features have been shown to give superior performance.
fhog_object_detector_ex.cpp.html
scan_image_boxes
dlib/image_processing.h
dlib/image_processing/scan_image_boxes_abstract.h
This object is a tool for running a classifier over an image with the goal of localizing each object present. The localization is in the form of the bounding box around each object of interest.

Unlike the scan_image_pyramid object which scans a fixed sized window over an image pyramid, the scan_image_boxes tool allows you to define your own list of "candidate object locations" which should be evaluated. This is simply a list of rectangle objects which might contain objects of interest. The scan_image_boxes object will then evaluate the classifier at each of these locations and return the subset of rectangles which appear to have objects in them.

This object can also be understood as a general tool for implementing the spatial pyramid models described in the paper:

> Beyond Bags of Features: Spatial Pyramid Matching for Recognizing Natural Scene Categories by Svetlana Lazebnik, Cordelia Schmid, and Jean Ponce

The following feature extractors can be used with the scan_image_boxes object:

- hashed_feature_image
- binned_vector_feature_image
- nearest_neighbor_feature_image

scan_image_custom
dlib/image_processing.h
dlib/image_processing/scan_image_custom_abstract.h
This object is a tool for running a classifier over an image with the goal of localizing each object present. The localization is in the form of the bounding box around each object of interest.

Unlike the scan_image_pyramid and scan_image_boxes objects, this image scanner delegates all the work of constructing the object feature vector to a user supplied feature extraction object. That is, scan_image_custom simply asks the supplied feature extractor what boxes in the image we should investigate and then asks the feature extractor for the complete feature vector for each box. That is, scan_image_custom does not apply any kind of pyramiding or other higher level processing to the features coming out of the feature extractor. That means that when you use scan_image_custom it is completely up to you to define the feature vector used with each image box.

full_object_detection
dlib/image_processing.h
dlib/image_processing/full_object_detection_abstract.h
This object represents the location of an object in an image along with the positions of each of its constituent parts.
get_frontal_face_detector
dlib/image_processing/frontal_face_detector.h
dlib/image_processing/frontal_face_detector_abstract.h
This function returns an object_detector that is configured to find human faces that are looking more or less towards the camera. It is created using the scan_fhog_pyramid object.
face_detection_ex.cpp.html
object_detector
dlib/image_processing.h
dlib/image_processing/object_detector_abstract.h
This object is a tool for detecting the positions of objects in an image. In particular, it is a simple container to aggregate an instance of an image scanner object (either scan_fhog_pyramid, scan_image_pyramid, scan_image_boxes, or scan_image_custom), the weight vector needed by one of these image scanners, and finally an instance of test_box_overlap. The test_box_overlap object is used to perform non-max suppression on the output of the image scanner object.

Note that you can use the structural_object_detection_trainer to learn the parameters of an object_detector. See the example programs for an introduction.

fhog_object_detector_ex.cpp.html
object_detector_ex.cpp.html
object_detector_advanced_ex.cpp.html
train_object_detector.cpp.html
test_box_overlap
dlib/image_processing.h
dlib/image_processing/box_overlap_testing_abstract.h
This object is a simple function object for determining if two rectangles overlap.
remove_unobtainable_rectangles
dlib/image_processing.h
dlib/image_processing/remove_unobtainable_rectangles_abstract.h
Recall that the scan_image_pyramid and scan_image_boxes objects can't produce all possible rectangles as object detections since they only consider a limited subset of all possible object positions. Therefore, when training an object detector that uses these tools you must make sure the training data does not contain any object locations that are unobtainable by the image scanning model. The remove_unobtainable_rectangles() routine is a tool to filter out these unobtainable rectangles from the training.
compute_box_dimensions
dlib/image_processing.h
dlib/image_processing/detection_template_tools_abstract.h
This function is a tool for computing a rectangle with a particular width/height ratio and area.
create_single_box_detection_template
dlib/image_processing.h
dlib/image_processing/detection_template_tools_abstract.h
This function is a tool for creating a detection template usable by the scan_image_pyramid object. This particular function creates a detection template with exactly one feature extraction region.
create_overlapped_2x2_detection_template
dlib/image_processing.h
dlib/image_processing/detection_template_tools_abstract.h
This function is a tool for creating a detection template usable by the scan_image_pyramid object. This particular function creates a detection template with four overlapping feature extraction regions.
create_grid_detection_template
dlib/image_processing.h
dlib/image_processing/detection_template_tools_abstract.h
This function is a tool for creating a detection template usable by the scan_image_pyramid object. This particular function creates a detection template with a grid of feature extraction regions.
determine_object_boxes
dlib/image_processing.h
dlib/image_processing/scan_image_pyramid_tools_abstract.h
The scan_image_pyramid object represents a sliding window classifier system. For it to work correctly it needs to be given a set of object boxes which define the size and shape of each sliding window and these windows need to be able to match the sizes and shapes of targets the user wishes to detect. Therefore, the determine_object_boxes() routine is a tool for computing a set of object boxes which can meet this requirement.

setup_grid_detection_templates
dlib/image_processing.h
dlib/image_processing/scan_image_pyramid_tools_abstract.h
This routine uses determine_object_boxes to obtain a set of object boxes and then adds them to a scan_image_pyramid object as detection templates. It also uses create_grid_detection_template to create each feature extraction region. Therefore, the detection templates will extract features from a regular grid inside each object box.
setup_grid_detection_templates_verbose
dlib/image_processing.h
dlib/image_processing/scan_image_pyramid_tools_abstract.h
This function is identical to setup_grid_detection_templates except that it also outputs information regarding the selected detection templates to standard out.
object_detector_ex.cpp.html
train_object_detector.cpp.html
setup_hashed_features
dlib/image_processing.h
dlib/image_processing/setup_hashed_features_abstract.h
This is a tool for configuring the hashed_feature_image or binned_vector_feature_image object with a random projection hash.
object_detector_ex.cpp.html
train_object_detector.cpp.html
randomly_sample_image_features
dlib/statistics.h
dlib/statistics/image_feature_sampling_abstract.h
Given a feature extractor such as the hog_image, this routine selects a random subsample of local image feature vectors from a set of images.