Using metal to snapshot SCNRenderer produces darker output image - rendering

Question

I'm using a Metal render pass to snapshot my SceneKit scene attached to a SCNRenderer. The method is faster than using the UIImage-producing SCNRenderer.snapshot(), but the output of the two methods is different; my method produces a darker image. I thought this could be to do with either a color-space difference, or alpha issue.
The image on the right shows my custom method, in which the color doesn't look right.
The color space seems to be the same in the UIImage produced by both the standard method's result, and my own (kCGColorSpaceModelRGB; sRGB IEC61966-2.1), so I don't think this is the issue.
I'll share elements of the custom render code that I believe are relevant.
I configure the MTLRenderPassDescriptor as follows:
renderPassDescriptor.colorAttachments[0].loadAction = MTLLoadAction.clear
renderPassDescriptor.colorAttachments[0].clearColor = MTLClearColorMake(0, 0, 0, 0)
renderPassDescriptor.colorAttachments[0].storeAction = MTLStoreAction.store
I then create a texture to render into. I create a CGContext with:
bitsPerComponent: 8
bitsPerPixel: 32
colorSpace: CGColorSpaceCreateDeviceRGB()
bitmapInfo: CGBitmapInfo.byteOrder32Little.rawValue | CGImageAlphaInfo.noneSkipFirst.rawValue
fillColor: UIColor.clear.cgColor
This is an area I'm concerned about. I've tried other color spaces, CGBitmapInfo and CGImageAlphaInfo flags, and other fill colors. The fill color does have an effect on the output, but I do need transparency, so clear does feel correct.
I create a MTLTextureDescriptor.texture2DDescriptor with .rgba8Unorm as the pixel format, with usage MTLTextureUsage(rawValue: MTLTextureUsage.renderTarget.rawValue | MTLTextureUsage.shaderRead.rawValue).
I then go on to hand my texture to the render pass descriptor and run a render command.
renderPassDescriptor.colorAttachments[0].texture = texture
let commandBuffer = commandQueue.makeCommandBuffer()!
renderer.render(atTime: time, viewport: viewport, commandBuffer: commandBuffer,
passDescriptor: renderPassDescriptor)
commandBuffer.commit()
In my normal pipeline, I go on here to create a CVPixelBuffer, but I introduced the creation of a CGImage to be able to more easily preview the image in the Xcode debugger. I do this using the following:
var data = Array<UInt8>(repeatElement(0, count: 4*mtlTexture.width*mtlTexture.height))
mtlTexture.getBytes(&data, bytesPerRow: 4*mtlTexture.width, from: MTLRegionMake2D(0, 0, mtlTexture.width, mtlTexture.height), mipmapLevel: 0)
let bitmapInfo = CGBitmapInfo(rawValue: (CGBitmapInfo.byteOrder32Big.rawValue | CGImageAlphaInfo.premultipliedLast.rawValue))
let colorSpace = CGColorSpaceCreateDeviceRGB()
let context = CGContext(data: &data,
width: mtlTexture.width,
height: mtlTexture.height,
bitsPerComponent: 8,
bytesPerRow: 4*mtlTexture.width,
space: colorSpace,
bitmapInfo: bitmapInfo.rawValue)
return context?.makeImage()
And this CGImage (or the CVPixelBuffer) is where I first observe the darkened image. So I believe that either the initial Metal render pass is creating the color disparity, or, I'm always performing wrong conversions to each other format I use.
An issue that is perhaps related can be found here:
https://github.com/MetalPetal/MetalPetal/issues/76
That issue seems to be taking place in a render view, and I don't use a SceneView or anything called a renderView. I have a SCNRenderer and I turn snapshots into images to write to video buffers, but the color issue presents itself earlier than those steps. The post does mention that the render view should use the format bgra8Unorm_srgb, so I wonder if that should be introduced in my pipeline, but I just can't work out where it belongs. Changing the pixelFormat from rgba8Unorm to bgra8Unorm_srgb in my MTLTextureDescriptor doesn't seem to make any difference.
Does this effect look familiar to anyone, or can anyone shed light on this?

Answer 1

It should work if you chose CGColorSpaceCreateWithName(kCGColorSpaceSRGB) for the bitmap context and MTLPixelFormatRGBA8Unorm_sRGB for the texture format.