iOS: CIColorKernel applyWithExtent warning message - ios

Question

I'm trying to make original CIFilter with CIKernel.
code is here.
var kernel: CIKernel?
var inputImage: CIImage?
override init() {
super.init()
self.kernel = createKernel()
}
required init(coder aDecoder: NSCoder) {
super.init(coder: aDecoder)!
self.kernel = createKernel()
}
func outputCustomImage() -> CIImage? {
if let inputImage = self.inputImage {
let dod = inputImage.extent.insetBy(dx: -1, dy: -1)
let args = [inputImage as AnyObject]
let callback: CIKernelROICallback = {
(index, rect) in
return rect.insetBy(dx: -1, dy: -1)
}
return kernel!.applyWithExtent(dod, roiCallback: callback, arguments: args)
}
return nil
}
private func createKernel() -> CIKernel {
let kernelString =
"kernel vec4 RGB_to_GBR(sampler source_image)\n" +
"{\n" +
"vec4 originalColor, twistedColor;\n" +
"originalColor = sample(source_image, samplerCoord(source_image));\n" +
"twistedColor.r = originalColor.g;\n" +
"twistedColor.g = originalColor.b;\n" +
"twistedColor.b = originalColor.r ;\n" +
"twistedColor.a = originalColor.a;\n" +
"return twistedColor;\n" +
"}\n"
return CIKernel(string: kernelString)!
}
and warning message is
Note: CIColorKernel applyWithExtent:roiCallback:arguments: ignores callback and is not recomended. Use applyWithExtent:arguments: instead.

Answer 1

I think maybe the source code you supplied is different to the code generating the warning. For the filter you're writing, you only need a CIColorKernel which would generate that warning if you're supplying an ROI callback.
A general kernel (CIKernel) is useful if you need to access other pixels - for example if you were writing a blur filter. Since you are only interested in the current pixel, stick with a color kernel and use apply(withExtent:arguments:).
Generally, if you are subclassing CIFilter, you override outputImage. Take a look at this version (I've also simplified your kernel code):
class RGB_to_GBR: CIFilter {
let kernel: CIColorKernel = {
let kernelString =
"kernel vec4 RGB_to_GBR(__sample pixel)\n" +
"{\n" +
"vec4 twistedColor = pixel.gbra;\n" +
"return twistedColor;\n" +
"}\n"
return CIColorKernel(string: kernelString)!
}()
var inputImage: CIImage?
override var outputImage: CIImage? {
guard let inputImage = inputImage else {
return nil
}
return kernel.apply(withExtent: inputImage.extent,
arguments: [inputImage])
}
}