Reading Pixels as YUV420

[Jason Williams]

Hey All,

I am trying to read pixels from an image in YUV420 format and I am not getting the data I expect. I have the following code:

void readPixelsCallback(SkImage::ReadPixelsContext context, std::unique_ptr<const SkImage::AsyncReadResult> result) {

    auto* pixels = static_cast<std::vector<uint8_t>*>(context);

    if (!result) {

        assert(false);

        return;

    }

    // the data in result is ordered Y, U, then V.

    pixels->resize(result->rowBytes(0) + result->rowBytes(1) + result->rowBytes(2));

    memcpy(pixels->data(), result->data(0), result->rowBytes(0));

    auto* data = pixels->data() + result->rowBytes(0);

    memcpy(data, result->data(1), result->rowBytes(1));

    data += result->rowBytes(1);

    memcpy(data, result->data(2), result->rowBytes(2));

}

std::vector<uint8_t> ContextSkia::readPixelsYUV(sk_sp<SkImage> image, int srcX, int srcY, int width, int height) {

    assert(0 <= srcX && srcX < image->width());

    assert(0 <= srcY && srcY < image->height());

    assert(width <= image->width() - srcX);

    assert(height <= image->height() - srcY);

    int clampedWidth = (width > 0) ? width : image->width();

    int clampedHeight = (height > 0) ? height : image->height();

    int readWidth = srcX + clampedWidth;

    int readHeight = srcY + clampedHeight;

    std::vector<uint8_t> pixels(0);

    image->asyncRescaleAndReadPixelsYUV420(SkYUVColorSpace::kRec709_Limited_SkYUVColorSpace,

                                           SkColorSpace::MakeSRGB(),

                                           SkIRect::MakeXYWH(srcX, srcY, readWidth, readHeight),

                                           SkISize::Make(clampedWidth, clampedHeight),

                                           SkImage::RescaleGamma::kSrc,

                                           SkImage::RescaleMode::kNearest,

                                           readPixelsCallback,

                                           &pixels);

    return pixels;

}

The result's rowBytes for the Y plane is always equal to the image width, but I am expecting it to be the image's width * height. In my tests srcX and srcY are 0 and my width and heights are even values. What am I missing here?
 (BTW: I tried to do this with a  Skia Fiddle and the callback always gets an error not sure what I am doing wrong there either)

[Brian Osman]

Throughout Skia, "rowBytes" refers to the number of bytes for one (horizontal) row of pixels, not the entire image. So if you're trying to size a buffer for an entire block of pixels, you need to multiply rowBytes by the height. rowBytes will often be equal to the width multiplied by the bytes-per-pixel, but may be larger if there is padding at the end of each row.

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[Jason Williams]

Thanks Brian.

Does this mean that result->data(0) has result->rowBytes(0) * height number of bytes in it then? My concern was that if I didn't use rowBytes(0) with that I would read beyond what is in result->data(0).

Jason

[Jim Van Verth]

Yes, result->data(0) should have result->rowBytes(0) * height bytes. Whereas result->data(1) should have result->rowBytes(1) * (height/2), and the equivalent for result->data(2).

To view this discussion on the web visit https://groups.google.com/d/msgid/skia-discuss/a9679fba-9a5e-4d98-a9b3-b9f5bc5f83a4n%40googlegroups.com.

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Jim Van Verth | Software Engineer | jvan...@google.com | 919-210-7664