Create solid color texture from a scratch

Question

I'm creating an engine and need a dummy single-pixel texture for objects which do not have one of required materials. It should be 4 channel texture and example invocation could look as follows:

let dummy = device.makeSolidTexture(device: device, color: simd_float4(0, 0, 1, 1))!
 

Code I'm using

extension MTLDevice {
    func makeSolidTexture(device: MTLDevice,
                          color: simd_float4,
                          pixelFormat: MTLPixelFormat = .bgra8Unorm) -> MTLTexture? {
        let descriptor = MTLTextureDescriptor()
        descriptor.width = 1
        descriptor.height = 1
        descriptor.mipmapLevelCount = 1
        descriptor.storageMode = .managed
        descriptor.arrayLength = 1
        descriptor.sampleCount = 1
        descriptor.cpuCacheMode = .writeCombined
        descriptor.allowGPUOptimizedContents = false
        descriptor.pixelFormat = pixelFormat
        descriptor.textureType = .type2D
        descriptor.usage = .shaderRead
        guard let texture = device.makeTexture(descriptor: descriptor) else {
            return nil
        }
        let origin = MTLOrigin(x: 0, y: 0, z: 0)
        let size = MTLSize(width: texture.width, height: texture.height, depth: texture.depth)
        let region = MTLRegion(origin: origin, size: size)
        withUnsafePointer(to: color) { ptr in
            texture.replace(region: region, mipmapLevel: 0, withBytes: ptr, bytesPerRow:  4)
        }
        return texture
    }
}

The problem is that when I inspect the texture after capturing a frame I see:

What do I miss? Why is it black?

Answer 1

It turned out that I made two mistakes there. First is that a textures MUST be aligned to 256 bytes, so I extended to size to 8 x 8 what considering the pixel format meets the requirement. My second mistake was to create a texture from float vector directly. Underlying type of the texture store the number in range 0-255, so the float vector passed to the function must be scaled and then converted to simd_uchar4 type.

Here is how it should look like

extension MTLDevice {
    func makeSolid2DTexture(device: MTLDevice,
                            color: simd_float4,
                            pixelFormat: MTLPixelFormat = .bgra8Unorm) -> MTLTexture? {
        let descriptor = MTLTextureDescriptor()
        descriptor.width = 8
        descriptor.height = 8
        descriptor.mipmapLevelCount = 1
        descriptor.storageMode = .managed
        descriptor.arrayLength = 1
        descriptor.sampleCount = 1
        descriptor.cpuCacheMode = .writeCombined
        descriptor.allowGPUOptimizedContents = false
        descriptor.pixelFormat = pixelFormat
        descriptor.textureType = .type2D
        descriptor.usage = .shaderRead
        guard let texture = device.makeTexture(descriptor: descriptor) else {
            return nil
        }
        let origin = MTLOrigin(x: 0, y: 0, z: 0)
        let size = MTLSize(width: texture.width, height: texture.height, depth: texture.depth)
        let region = MTLRegion(origin: origin, size: size)
        let mappedColor = simd_uchar4(color * 255)
        Array<simd_uchar4>(repeating: mappedColor, count: 64).withUnsafeBytes { ptr in
            texture.replace(region: region, mipmapLevel: 0, withBytes: ptr.baseAddress!, bytesPerRow: 32)
        }
        return texture
    }
}

// bgra format in range [0, 1]
let foo = device.makeSolid2DTexture(device: device, color: simd_float4(1, 0, 0, 1))!