如何将16位图像加载到金属纹理？

Question

推荐的使用方法MTKTextureLoader.newTexture不适用于16位图像.

1. 该named:版本以静默方式将图像转换为8位像素格式
2. 该cgImage:版本与终止Image decoding failed

UIImage和NSImage都支持加载16位图像,并且有一个方便的.cgimage方法,可以在一个内核中转换为CGImage,从而解决了在两个平台上获得CGImage的问题.

如何编写一个转换CGImage并返回16位Metal纹理的函数？

Answer 1

loadEXRTexture下面的函数加载扩展范围图像并将其像素转换为半精度浮点,将生成的像素数据存储MTLTexture为.rgba16Float格式.它试图通过使用CGImageSource从Quartz默认值改变其行为的选项来保留浮点图像的原始范围(它将"解码"函数应用于将源数据压缩到适合绘制到位图上下文的范围内).它假定图像源创建的图像具有以RGB顺序打包的三个浮点组件.

func convertRGBF32ToRGBAF16(_ src: UnsafePointer<Float>, _ dst: UnsafeMutablePointer<UInt16>, pixelCount: Int) {
    for i in 0..<pixelCount {
        storeAsF16(src[i * 3 + 0], dst + (i * 4) + 0)
        storeAsF16(src[i * 3 + 1], dst + (i * 4) + 1)
        storeAsF16(src[i * 3 + 2], dst + (i * 4) + 2)
        storeAsF16(1.0, dst + (i * 4) + 3)
    }
}

func loadEXRTexture(_ url: URL, device: MTLDevice) -> MTLTexture? {
    guard let imageSource = CGImageSourceCreateWithURL(url as CFURL, nil) else { return nil }

    let options = [ kCGImageSourceShouldCache : true, kCGImageSourceShouldAllowFloat : true ] as CFDictionary
    guard let image = CGImageSourceCreateImageAtIndex(imageSource, 0, options) else { return nil }

    let descriptor = MTLTextureDescriptor.texture2DDescriptor(pixelFormat: .rgba16Float,
                                                              width: image.width,
                                                              height: image.height,
                                                              mipmapped: false)
    descriptor.usage = .shaderRead
    guard let texture = device.makeTexture(descriptor: descriptor) else { return nil }

    if image.bitsPerComponent == 32 && image.bitsPerPixel == 96 {
        let srcData: CFData! = image.dataProvider?.data
        CFDataGetBytePtr(srcData).withMemoryRebound(to: Float.self, capacity: image.width * image.height * 3) { srcPixels in
            let dstPixels = UnsafeMutablePointer<UInt16>.allocate(capacity: 4 * image.width * image.height)
            convertRGBF32ToRGBAF16(srcPixels, dstPixels, pixelCount: image.width * image.height)
            texture.replace(region: MTLRegionMake2D(0, 0, image.width, image.height),
                            mipmapLevel: 0,
                            withBytes: dstPixels,
                            bytesPerRow: MemoryLayout<UInt16>.size * 4 * image.width)
            dstPixels.deallocate()
        }
    }

    return texture
}

你需要在桥接头中包含这个实用程序,因为据我所知,Swift没有__fp16类型或任何等价物:

#include <stdint.h>
static inline void storeAsF16(float value, uint16_t *pointer) { *(__fp16 *)pointer = value; }