## Image

This module provides some image processing methods, which requires `opencv` to
to be installed.

### Read/Write/Show
To read or write images files, use `imread` or `imwrite`.

```python
import mmcv

img = mmcv.imread('test.jpg')
img = mmcv.imread('test.jpg', flag='grayscale')
img_ = mmcv.imread(img) # nothing will happen, img_ = img
mmcv.imwrite(img, 'out.jpg')
```

To read images from bytes

```python
with open('test.jpg', 'rb') as f:
    data = f.read()
img = mmcv.imfrombytes(data)
```

To show an image file or a loaded image

```python
mmcv.imshow('tests/data/color.jpg')
# this is equivalent to 

for i in range(10):
    img = np.random.randint(256, size=(100, 100, 3), dtype=np.uint8)
    mmcv.imshow(img, win_name='test image', wait_time=200)
```

### Color space conversion
Supported conversion methods:
- bgr2gray
- gray2bgr
- bgr2rgb
- rgb2bgr
- bgr2hsv
- hsv2bgr

```python
img = mmcv.imread('tests/data/color.jpg')
img1 = mmcv.bgr2rgb(img)
img2 = mmcv.rgb2gray(img1)
img3 = mmcv.bgr2hsv(img)
```

### Resize
There are three resize methods. All `imresize_*` methods have an argument `return_scale`,
if this argument is `False`, then the return value is merely the resized image, otherwise
is a tuple `(resized_img, scale)`.

```python
# resize to a given size
mmcv.imresize(img, (1000, 600), return_scale=True)

# resize to the same size of another image
mmcv.imresize_like(img, dst_img, return_scale=False)

# resize by a ratio
mmcv.imrescale(img, 0.5)

# resize so that the max edge no longer than 1000, short edge no longer than 800
# without changing the aspect ratio
mmcv.imrescale(img, (1000, 800))
```

### Rotate
To rotate an image by some angle, use `imrotate`. The center can be specified,
which is the center of original image by default. There are two modes of rotating,
one is to keep the image size unchanged so that some parts of the image will be
cropped after rotating, the other is to extend the image size to fit the rotated
image.

```python
img = mmcv.imread('tests/data/color.jpg')

# rotate the image clockwise by 30 degrees.
img_ = mmcv.imrotate(img, 30)

# rotate the image counterclockwise by 90 degrees.
img_ = mmcv.imrotate(img, -90)

# rotate the image clockwise by 30 degrees, and rescale it by 1.5x at the same time.
img_ = mmcv.imrotate(img, 30, scale=1.5)

# rotate the image clockwise by 30 degrees, with (100, 100) as the center.
img_ = mmcv.imrotate(img, 30, center=(100, 100))

# rotate the image clockwise by 30 degrees, and extend the image size.
img_ = mmcv.imrotate(img, 30, auto_bound=True)
```

### Flip
To flip an image, use `imflip`.

```python
img = mmcv.imread('tests/data/color.jpg')

# flip the image horizontally
mmcv.imflip(img)

# flip the image vertically
mmcv.imflip(img, direction='vertical')
```

### Crop
`imcrop` can crop the image with one or some regions, represented as (x1, y1, x2, y2).

```python
import mmcv
import numpy as np

img = mmcv.imread('tests/data/color.jpg')

# crop the region (10, 10, 100, 120)
bboxes = np.array([10, 10, 100, 120])
patch = mmcv.imcrop(img, bboxes)

# crop two regions (10, 10, 100, 120) and (0, 0, 50, 50)
bboxes = np.array([[10, 10, 100, 120], [0, 0, 50, 50]])
patches = mmcv.imcrop(img, bboxes)

# crop two regions, and rescale the patches by 1.2x
patches = mmcv.imcrop(img, bboxes, scale_ratio=1.2)
```

### Padding
There are two methods `impad` and `impad_to_multiple` to pad an image to the
specific size with given values.

```python
img = mmcv.imread('tests/data/color.jpg')

# pad the image to (1000, 1200) with all zeros
img_ = mmcv.impad(img, (1000, 1200), pad_val=0)

# pad the image to (1000, 1200) with different values for three channels.
img_ = mmcv.impad(img, (1000, 1200), pad_val=[100, 50, 200])

# pad an image so that each edge is a multiple of some value.
img_ = mmcv.impad_to_multiple(img, 32)
```
