Core Image workingColorSpace & outputColorSpace

Question

I am rendering video frames using Metal Core Image shaders. One of the requirements I have is to be able to pick a particular color (and user selected nearby range) from the CIImage, keep that color in the output and turn every other else black and white (color splash). But I am confused about the right approach that would work for videos shot in all kinds of color spaces (including 10 bit HDR):

1. First job is to extract the color value from the CIImage at any given pixel location. From my understanding, this can be extracted using the following API:

   func render(_ image: CIImage, 
     toBitmap data: UnsafeMutableRawPointer, 
     rowBytes: Int, 
     bounds: CGRect, 
     format: CIFormat, 
     colorSpace: CGColorSpace?)
   

The API says passing NULL to colorSpace will cause the output to be in ciContext outputColorSpace. It's not clear how to correctly use this API to extract the exact color at given pixel locations, given the possibility of both 8 bit and 10 bit input images?

2. Having extracted the value, the next issue is how to pass the values to Metal Core Image shader? Shaders use normalized color ranges dependent on the workingcolorSpace of ciContext. Do I need to create a 1D texture with the color that should be passed to shader or there is a better way?

Answer 1

Based on your comment, here is another alternative:

You can read the pixel value as floats using the context's working color space. By using float values, you ensure that the bit depth of the input doesn't matter and that extended color values are correctly represented. So for instance, a 100% red in BT.2020 would result in an extended sRGB value of (1.2483, -0.3880, -0.1434).

To read the value, you could use our small helper library CoreImageExtensions (or check out the implementation to see how to use render to get float values):

let pixelColor = context.readFloat32PixelValue(from: image, at: coordinate, colorSpace: context.workingColorSpace)
// you can convert that to CIVector, which can be passed to a kernel
let vectorValue = CIVector(x: pixelColor.r, y: pixelColor.g, ...)

In your Metal kernel, you can use a float4 input parameter for that color.

You can store and use the color value on later rendering calls as long as you are using the same workingColorSpace for the context.