how to display MTKView with rgba16Float MTLPixelFormat

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攒了一身酷
攒了一身酷 2021-01-24 07:42

I have an MTKView set to use MTLPixelFormat.rgba16Float. I\'m having display issues which can be best described with the following graphic:

So the intended UIC

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  • 2021-01-24 08:41

    So, it looks like you are very close to the complete solution. But what you have is not quite correct. Here is a Metal function that will convert from sRGB to a linear value which you can then write in your Metal shader (I still suggest that you write to a sRGB texture but you can also write to a 16 bit texture). Note that sRGB is not a simple 2.2 gamma curve.

    // Convert a non-linear log value to a linear value.
    // Note that normV must be normalized in the range [0.0 1.0].
    
    static inline
    float sRGB_nonLinearNormToLinear(float normV)
    {
      if (normV <= 0.04045f) {
        normV *= (1.0f / 12.92f);
      } else {
        const float a = 0.055f;
        const float gamma = 2.4f;
        //const float gamma = 1.0f / (1.0f / 2.4f);
        normV = (normV + a) * (1.0f / (1.0f + a));
        normV = pow(normV, gamma);
      }
    
      return normV;
    }
    
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  • 2021-01-24 08:47

    The key turned out to be in undoing the gamma correction that is inherently embedded in a UIColor

    let colorSRGB = UIColor(red: 0.92, green: 0.79, blue: 0.18, alpha: 1.0)
    let rgbaSRGB = colorSRGB.getRGBAComponents()!
    let gammapower : CGFloat = 2.2
    
    let r = pow(rgbaSRGB.red, gammapower)
    let g = pow(rgbaSRGB.green, gammapower)
    let b = pow(rgbaSRGB.blue, gammapower)
    let a = pow(rgbaSRGB.alpha, gammapower)
    
    let colorNoGamma: UIColor = UIColor(red: CGFloat(r), green: CGFloat(g), blue: CGFloat(b), alpha: CGFloat(a))
    

    Once I pass colorNoGamma to be applied in an MKTView with MTLPixelFormat.rgba16Float, the results will match a UIView displaying colorSRGB. It makes sense once you think through it...Thanks to @MoDJ for leading me in the right path.

    Note that, on the MTKView side, I can keep the texture settings as originally defined, namely:

    let texDescriptor = MTLTextureDescriptor.texture2DDescriptor(pixelFormat: MTLPixelFormat.rgba8Unorm, width: Int(width), height: Int(height), mipmapped: isMipmaped)
    target = texDescriptor.textureType
    

    And, if I want to convert the currentDrawable into a texture to be displayed in a UIImageView, then I want to make sure I don't apply ANY colorspace settings. I.e:

    let strokeCIImage = CIImage(mtlTexture: metalTextureComposite, options: [:])!.oriented(CGImagePropertyOrientation.downMirrored)
    let imageCropCG = cicontext.createCGImage(strokeCIImage, from: box, format: kCIFormatABGR8, colorSpace: colorSpace)  // kCIFormatRGBA8 gives same result. Not sure what's proper
    let layerStroke = CALayer()
    layerStroke.frame = bbox
    layerStroke.contents = imageCropCG
    

    Note the argument options: [:] means I pass no colorspace/premultiplication/renderer settings as I did in the original post where I defined kciOptions

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