Video frame dump and alloc issue related to alignment

[zhen9910]

• Issue

examples/dump-frames works fine if frame width and height are multiple of 32 bytes. But I find it generated corrupted image if input or output size is 300*300 pixels.

I used 300*300 in the test is because many pre-trained object detection models are typically designed to accept inputs of a specific size that is not multiple of 32. For example, ssd_mobilenet_v1_1's the expected input size is 300x300 pixels.

• Reproduce
  run dump-frames example with a video with format 300*300 pixels, then you find the output ppm images are corrupted.

cargo run --example dump-frames ../sample-300x300.mp4 

• Analysis
  After some check, it looks video function alloc() calls "av_frame_get_buffer" with fixed alignment 32 bytes, so it created frame with linesize[0] = 960, but it should be 900.

I then tried a test to set this alignment to 1, and it worked fine to dump 300*300 pixel images.

rust-ffmpeg/src/util/frame/video.rs

Line 29 in 0e4c39a

av_frame_get_buffer(self.as_mut_ptr(), 32);

• Suggestion
  So I am asking could you please modify this alloc() function to add an additional argument "align" ? or add a new version of alloc2() function, such as

pub unsafe fn alloc2(&mut self, format: format::Pixel, width: u32, height: u32, align: u32) {
        self.set_format(format);
        self.set_width(width);
        self.set_height(height);

        av_frame_get_buffer(self.as_mut_ptr(), align as c_int);
    }

thanks