swscale crops image on yuv2rgb

[xlp]

Summary of the bug:
swscale leaves a small right portion of the image out on yuv2rgb conversion. aka "crops it", so a bar of the destination-image on right side stays unwritten. this seems to effect only certain resolutions/linesizes - below is one given that seems to cause issues

How to reproduce:

// HAS NO ISSUES
// int32_t width = 1600;
// int32_t height = 900;
// int srcLinesize[8] = {1664, 832, 832, 0, 0, 0, 0, 0};

// HAS ISSUES
int32_t width = 1220;
int32_t height = 1200;
int srcLinesize[8] = {1280, 640, 640, 0, 0, 0, 0, 0};

struct SwsContext* sws_scaler_ctx = sws_getContext(width, height, AV_PIX_FMT_YUV420P,
                                width, height, AV_PIX_FMT_RGB0,
                                SWS_FAST_BILINEAR, 0, 0, 0);

size_t destBufferSize = width * 4 * height;
uint8_t* destBuffer = (uint8_t*) malloc(destBufferSize);
memset(destBuffer, 0x00, destBufferSize);
uint8_t* destLines[4] = {destBuffer, 0, 0, 0};
int destData[4] = {width * 4, 0, 0, 0};

// Make a dummy source with green footage
uint8_t* srcData[8];
for (int i = 0; i < 8; i++) {
    if (srcLinesize[i] > 0) {
        size_t srcBufferSize = srcLinesize[i]*height;
        srcData[i] = (uint8_t*) malloc(srcBufferSize);
        memset(srcData[i], 0x00, srcBufferSize);
    } else {
        srcData[i] = 0x00;
    }
}

sws_scale(sws_scaler_ctx, (const uint8_t* const*)srcData, srcLinesize, 0, height, destLines, destData);

Produced Result:

Green area with unwritten gap at the right side (here in the size of 4x1200)

Expected result:

Completely green area, as the provided YUV source contains

[Carl Eugen Hoyos]

Is the issue not reproducible with ffmpeg, the application?

[xlp]

Yes i've tried running the mp4 (which initially caused the issue on my side) thru ffmpeg -i source.mp4 frame%05d.png, to find that the issue is not present there.

[Niklas Haas]

The problem here, and the reason it is not reproducible with just ffmpeg, is that the dst buffer stride is not aligned to a multiple of 8, which breaks the unscaled special converters.

This seems to fall into a general class of bugs revolving around calling libswscale directly with unaligned buffers,

cf #6763, #10852, #6763

It seems that the only general fix to such issues is to either avoid the use of asm entirely for unaligned buffers, or just print a big scary warning and issue a memcpy from a suitably aligned internal buffer.

I am leaning towards the latter, as a definitive fix for these issues, all of which have in common that they can only happen when one deliberately ignores best practices around image buffer alignment.