Why ANativeWindow_Buffer.stride is different for different devices

Question

I am writing simple video player using ffmpeg for android. Following are the steps I followed

1. Read AVFrame from file
2. Convert AVFrame to RGB565 format using sws_scale
3. Get buffer from using av_image_copy_to_buffer
4. Display this buffer to SurfaceView by copying buffer to ANativeWindow_Buffer

Most of the videos are playing fine, but there is issue with videos which have lower resolution than window. For ex, when I play a 656x480 video on my OnePlus 7T (2206x1080), video looks distorted. The same video plays fine on emulator (2160x1080).

When I debugged whole pipe, I found that on OP7T, after locking ANativeWindow, ANativeWindow_Buffer.stride was set to 704 instead of 656. For all videos which plays normal, stride is same as width of buffer. The same is not the case with Android emulator.

I did some trials and tried to scale width to 600, then stride jumped to 640 and video was distorted. When I scaled width to 640, video was displayed vertically half correct.

Can anyone help me understand, How stride is calculated? And What is the reason stride is wrongly calculated?

I found one same problem here: Simple FFMpeg player for Android OP mentions that video works fine for 640, 1280, 1920.

Answer 1

It seems since my device is arm64-v8a, stride was always aligned to 64. To overcome this, I get stride after locking window and using ANative_WindowBuffer. Then I use this windowbuffer.stride to calculate dst_slice for sws_scale.

AVFrame dummy;

if ((ret = ANativeWindow_lock(window, &windowBuffer, nullptr)) < 0) {
     log_error("cannot lock window: %d", ret);
} else {
     dummy.data[0] = (uint8_t *) windowBuffer.bits;
     dummy.linesize[0] = windowBuffer.stride * 2 // For RGB565;
}

And then:

sws_scale(renderer->sws_ctx,
                        (const uint8_t* const *) frame->data,
                        frame->linesize,
                        0,
                        codecpar->height,
                        dummy.data,
                        dummy.linesize)

This will directly render scaled frame data to windowbuffer.