Issues with sampling Lut in Skia Effect

[Jurek Bielicki]

Hi Skia Team,

I'm working on a custom Skia effect for Android OS.

To implement the effect I need to pass Lut (Look Up Table) in the form of  4 float bitmap (SkColorType::kRGBA_F32_SkColorType), as below (only x-horizontal coord is in focus for simplicity):

fn(std::vector<float> lut, int w, int h) {

SkImageInfo info = SkImageInfo::Make(w, h, SkColorType::kRGBA_F32_SkColorType,
        SkAlphaType::kOpaque_SkAlphaType);

SkPixmap map(info, lut.data(), w * 4 * sizeof(float));

bmp.allocPixels(info);
bmp.writePixels(map);
return SkImage::MakeFromBitmap(bmp);

},

where only green channel - ctr % 4 == 1, for every 4th pixel starting from third - x % 4 == 2 is populated in lut:

std::vector<float> lut(w * h * 4, 0);

int ctr = 0;

for (auto &e : lut) {

    int x = (ctr / 4) % w;

    if (x % 4 == 2 && ctr % 4 == 1)

         e = (float) x / (float) w;

    ctr++;

}

In the effect I can properly sample and display either:

• uniform shader uContentTexture;  // containing processed content
• uniform shader fLutTexture;  // containing passed lok up table,
  

I can see that each pixel is displayed as expected.

But when I sample Lut and use obtained value to sample content, the position is correct, but color is wrong (seem to be incorectly interpolated), code below splits display to three parts:

• top - correctly displayed content (black, red, green, blue repeatedly)
• middle - correctly displayed lut (black, black, green, black repeatedly)
• bottom - incorrectly coloured every 4th pixel (so location is good)

    uniform shader uContentTexture;
    uniform shader fLut;
    uniform fWidth;
    vec4 main(float2 p) {
        if (p.y < 250) {
            return sample(uContentTexture, p);
        }
        if (p.y < 500) {
            return sample(fLut, p);
        }
        float4 lut = sample(fLut, p);
        return sample(uContentTexture, float2(fWidth * lut.y + 0.5, p.y));
    }

I suspect some inconsistency in interpolation of content color based on lut.x value, but have no clue how to verify and correct such misalignment. 

I am aware of Skia/Sksl specific sampling rules described here: https://groups.google.com/g/skia-discuss/c/8l_y6zEteX8

Any help would be really appreciated.

Best Regards,
Jurek

[Brian Osman]

Hmm, are you able to recreate the problem on fiddle.skia.org ? That would make it easier for us to see how everything fits together.

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[Brian Salomon]

Also, the GPU backend does not support F32 textures so I believe your texture will get downgraded to F16.

To view this discussion on the web visit https://groups.google.com/d/msgid/skia-discuss/CAMcBjo5xQ9MKJRUn%3DYqC%3DOKeKkNnk8%2BivysFmRTwzh9RbWVHFw%40mail.gmail.com.

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[Jurek Bielicki]

Using F32 was the reason (no issue with OpenGL GL_RGBA32F though), replaced with F16 and stored each float with two fields on F16 texture. Thank you Brian for helpful hint!