roi_align.py

from torch.autograd import Function, Variable

from .. import roi_align_cuda


class RoIAlignFunction(Function):

    @staticmethod
    def forward(ctx, features, rois, out_size, spatial_scale, sample_num=0):
        if isinstance(out_size, int):
            out_h = out_size
            out_w = out_size
        elif isinstance(out_size, tuple):
            assert len(out_size) == 2
            assert isinstance(out_size[0], int)
            assert isinstance(out_size[1], int)
            out_h, out_w = out_size
        else:
            raise TypeError(
                '"out_size" must be an integer or tuple of integers')
        ctx.spatial_scale = spatial_scale
        ctx.sample_num = sample_num
        ctx.save_for_backward(rois)
        ctx.feature_size = features.size()

        batch_size, num_channels, data_height, data_width = features.size()
        num_rois = rois.size(0)

        output = features.new_zeros(num_rois, num_channels, out_h, out_w)
        if features.is_cuda:
            roi_align_cuda.forward(features, rois, out_h, out_w, spatial_scale,
                                   sample_num, output)
        else:
            raise NotImplementedError

        return output

    @staticmethod
    def backward(ctx, grad_output):
        feature_size = ctx.feature_size
        spatial_scale = ctx.spatial_scale
        sample_num = ctx.sample_num
        rois = ctx.saved_tensors[0]
        assert (feature_size is not None and grad_output.is_cuda)

        batch_size, num_channels, data_height, data_width = feature_size
        out_w = grad_output.size(3)
        out_h = grad_output.size(2)

        grad_input = grad_rois = None
        if ctx.needs_input_grad[0]:
            grad_input = Variable(
                rois.new(batch_size, num_channels, data_height, data_width)
                .zero_())
            roi_align_cuda.backward(grad_output, rois, out_h, out_w,
                                    spatial_scale, sample_num, grad_input)

        return grad_input, grad_rois, None, None, None


roi_align = RoIAlignFunction.apply