MTKTextureLoader messes PNG RGB vs BGR formats - objective-c

Question

I'm trying to port my OpenGL code to Metal one. As part of it I need to draw PNG textures, and I'm using MTKTextureLoader to load them. Here is my pipeline code:
texturePipelineDescriptor.vertexFunction = textureVertexFunc;
texturePipelineDescriptor.fragmentFunction = textureFragmentFunc;
texturePipelineDescriptor.colorAttachments[0].pixelFormat = MTLPixelFormatRGBA8Unorm;
texturePipelineDescriptor.colorAttachments[0].blendingEnabled = YES;
texturePipelineDescriptor.colorAttachments[0].rgbBlendOperation = MTLBlendOperationAdd;
texturePipelineDescriptor.colorAttachments[0].sourceRGBBlendFactor = MTLBlendFactorSourceAlpha;
texturePipelineDescriptor.colorAttachments[0].destinationRGBBlendFactor = MTLBlendFactorOneMinusSourceAlpha;
And here is my loader code:
NSData* imageData = [NSData dataWithBytes:imageBuffer length:imageBufferSize];
id<MTLTexture> newMTLTexture = [m_metal_renderer.metalTextureLoader newTextureWithData:imageData options:nil error:&error];
And here is the result. The entire image should be orange, like the small parts.
If I change this line:
texturePipelineDescriptor.colorAttachments[0].pixelFormat = MTLPixelFormatRGBA8Unorm;
To this line:
texturePipelineDescriptor.colorAttachments[0].pixelFormat = MTLPixelFormatBGRA8Unorm;
I will get the following results:
As you can see,images changed R and B channels, When I loading the textures, in ALL OF THEM
newMTLTexture.pixelFormat = MTLPixelFormatBGRA8Unorm;
regardless whether they are drawn correctly or not.
I would suspect there is something wrong with PNG files, but they seems absolutely normal in any PNG viewer.
Any clues what I can do to resolve it? Is it a bug In MTLTextureLoader? Is there another way of loading PNG picture?
P.S. The software is written for MacOS and not iOS.

Answer 1

I found the problem. It seems that some of the PNG files were stored in RGB (24 bit) format, while another were stored in RGBA (32 bit) format. All color became right after converting images to 32bit format.
I would expect MTKTextureLoader to be able to handle that, but apparently it is not. If someone knows the option I missed or another way to correctly load those PNG's to Metal (Other than adding 4th channel manually) it will be great!