Objective C to swift conversion for unions - ios

Question

Hi I am trying to convert below objective c code into swift but struggling to convert unions which are supported into C but not directly in swift.
I am not sure how I can convert below union type and pass it to MTLTexture getbytes?
union {
float f[2];
unsigned char bytes[8];
} u;
Also last part where I want to print these float values with log statement.
It would be great if I get working swift conversion for below code snippet.
id<MTLDevice> device = MTLCreateSystemDefaultDevice();
id<MTLCommandQueue> queue = [device newCommandQueue];
id<MTLCommandBuffer> commandBuffer = [queue commandBuffer];
MTKTextureLoader *textureLoader = [[MTKTextureLoader alloc] initWithDevice:device];
id<MTLTexture> sourceTexture = [textureLoader newTextureWithCGImage:image.CGImage options:nil error:nil];
CGColorSpaceRef srcColorSpace = CGColorSpaceCreateDeviceRGB();
CGColorSpaceRef dstColorSpace = CGColorSpaceCreateDeviceGray();
CGColorConversionInfoRef conversionInfo = CGColorConversionInfoCreate(srcColorSpace, dstColorSpace);
MPSImageConversion *conversion = [[MPSImageConversion alloc] initWithDevice:device
srcAlpha:MPSAlphaTypeAlphaIsOne
destAlpha:MPSAlphaTypeAlphaIsOne
backgroundColor:nil
conversionInfo:conversionInfo];
MTLTextureDescriptor *grayTextureDescriptor = [MTLTextureDescriptor texture2DDescriptorWithPixelFormat:MTLPixelFormatR16Unorm
width:sourceTexture.width
height:sourceTexture.height
mipmapped:false];
grayTextureDescriptor.usage = MTLTextureUsageShaderWrite | MTLTextureUsageShaderRead;
id<MTLTexture> grayTexture = [device newTextureWithDescriptor:grayTextureDescriptor];
[conversion encodeToCommandBuffer:commandBuffer sourceTexture:sourceTexture destinationTexture:grayTexture];
MTLTextureDescriptor *textureDescriptor = [MTLTextureDescriptor texture2DDescriptorWithPixelFormat:grayTexture.pixelFormat
width:sourceTexture.width
height:sourceTexture.height
mipmapped:false];
textureDescriptor.usage = MTLTextureUsageShaderWrite | MTLTextureUsageShaderRead;
id<MTLTexture> texture = [device newTextureWithDescriptor:textureDescriptor];
MPSImageLaplacian *imageKernel = [[MPSImageLaplacian alloc] initWithDevice:device];
[imageKernel encodeToCommandBuffer:commandBuffer sourceTexture:grayTexture destinationTexture:texture];
MPSImageStatisticsMeanAndVariance *meanAndVariance = [[MPSImageStatisticsMeanAndVariance alloc] initWithDevice:device];
MTLTextureDescriptor *varianceTextureDescriptor = [MTLTextureDescriptor
texture2DDescriptorWithPixelFormat:MTLPixelFormatR32Float
width:2
height:1
mipmapped:NO];
varianceTextureDescriptor.usage = MTLTextureUsageShaderWrite;
id<MTLTexture> varianceTexture = [device newTextureWithDescriptor:varianceTextureDescriptor];
[meanAndVariance encodeToCommandBuffer:commandBuffer sourceTexture:texture destinationTexture:varianceTexture];
[commandBuffer commit];
[commandBuffer waitUntilCompleted];
union {
float f[2];
unsigned char bytes[8];
} u;
MTLRegion region = MTLRegionMake2D(0, 0, 2, 1);
[varianceTexture getBytes:u.bytes bytesPerRow:2 * 4 fromRegion:region mipmapLevel: 0];
NSLog(#"mean: %f", u.f[0] * 255);
NSLog(#"variance: %f", u.f[1] * 255 * 255);
It will be great if I get swift representation for this?

Answer 1

You can use a Struct instead, like this. And add an extension to get description for logging.
struct u {
var bytes: [UInt8] = [0,0,0,0, 0,0,0,0]
var f: [Float32] {
set {
var f = newValue
memcpy(&bytes, &f, 8)
}
get {
var f: [Float32] = [0,0]
var b = bytes
memcpy(&f, &b, 8)
return Array(f)
}
}
}
extension u: CustomStringConvertible {
var description: String {
let bytesString = (bytes.map{ "\($0)"}).joined(separator: " ")
return "floats : \(f[0]) \(f[1]) - bytes : \(bytesString)"
}
}
var test = u()
print(test)
test.f = [3.14, 1.618]
print(test)
test.bytes = [195, 245, 72, 64, 160, 26, 207, 63]
print(test)
Log:
floats : 0.0 0.0 - bytes : 0 0 0 0 0 0 0 0
floats : 3.14 1.618 - bytes : 195 245 72 64 160 26 207 63
floats : 3.14 1.618 - bytes : 195 245 72 64 160 26 207 63

Answer 2

You don't need the whole union for getBytes to work, only u.bytes is used there, which can be converted as
var bytes = [UInt8](repeating: 0, count: 8)
that's the array of length 8 (with arbitrary initial value 0 in each element), and you pass it to getBytes as an UnsafeMutableRawPointer:
varianceTexture.getBytes(&bytes, ...)
As for union, there are many ways to represent it. For example:
var u = ([Float](repeating: 0.0, count: 2), [UInt8](repeating: 0, count: 8))
And in that case you pass it as
varianceTexture.getBytes(&u.1, ...)
Or you could make it a class or struct in a similar way.