Problems with CIImageAccumulator from MTKView texture - swift3

Question

I want capture the output of an MTKView via the view's texture into a CIImageAccumulator to achieve a gradual painting build up effect. The problem is that the accumulator seems to be messing with the color/alpha/colorspace of the original, as shown below:
From the image above, the way I capture the darker-looking brushstroke is via the view's currentDrawable.texture property:
lastSubStrokeCIImage = CIImage(mtlTexture: self.currentDrawable!.texture, options: nil)!.oriented(CGImagePropertyOrientation.downMirrored)
subStrokeUIView.image = UIImage(ciImage: lastSubStrokeCIImage)
Now, once I take the same image and pipe it into a CIIAcumulator for later processing (I only do this once per drawing segment), the result is the brighter-looking result shown in the upper portion of the attachment:
lazy var ciSubCurveAccumulator: CIImageAccumulator =
{
[unowned self] in
return CIImageAccumulator(extent: CGRect(x: 0, y: 0, width: self.frame.width * self.contentScaleFactor, height: self.frame.height * self.window!.screen.scale ) , format: kCIFormatBGRA8)
}()!
ciSubCurveAccumulator.setImage(lastSubStrokeCIImage)
strokeUIView.image = UIImage(ciImage: ciSubCurveAccumulator.image())
I have tried using a variety of kCIFormats in the CIImageAccumulator definition, all to no avail. What is the CIImageAccumulator doing to mess with the original, and how can I fix it? Note that I intend the use ciSubCurveAccumulator to gradually build up a continuous brushstroke of consistent color. For simplicity of the question, I'm not showing the accumulating part. This problem is stopping me dead on my tracks.
Any suggestions would kindly be appreciated

Answer 1

The problem came down to two things: one, I needed to set up MTLRenderPipelineDescriptor() for compositeOver compositing and two, I needed to set introduce a CIFilter to hold intermediate compositing over the accumulating CIImageAccumulator. This CIFilter also needed to be set up to CIOverCompositing. Below is a snippet of code that captures all of the above:
lazy var ciSubCurveAccumulator: CIImageAccumulator =
{
[unowned self] in
return CIImageAccumulator(extent: CGRect(x: 0, y: 0, width: self.frame.width * self.contentScaleFactor, height: self.frame.height * self.window!.screen.scale ) , format: kCIFormatBGRA8)
}()! // set up CIImageAccumulator
let lastSubStrokeCIImage = CIImage(mtlTexture: self.currentDrawable!.texture, options: nil)!.oriented(CGImagePropertyOrientation.downMirrored)
let compositeFilter : CIFilter = CIFilter(name: "CISourceOverCompositing")!
compositeFilter.setValue(lastSubStrokeCIImage, forKey: kCIInputImageKey) // foreground image
compositeFilter.setValue(ciSubCurveAccumulator.image(), forKey: kCIInputBackgroundImageKey) // background image
let bboxChunkSubCurvesScaledAndYFlipped = CGRect(...) // capture the part of the texture that was drawn to
ciSubCurveAccumulator.setImage(compositeFilter.value(forKey: kCIOutputImageKey) as! CIImage, dirtyRect: bboxChunkSubCurvesScaledAndYFlipped) // comp bbox with latest updates
Wrapping the above bit of code inside draw() allows gradual painting to accumulate quite nicely. Hopefully this helps someone at some point.