当alpha值不是1或0时,Metal MTLTexture用黑色替换半透明区域

Question

在使用Apple的纹理导入器或我自己的时候,用软件(带有透明的bg)或Photoshop(保存为PNG)绘制的白色软边圆圈在渲染时会将其半透明颜色替换为黑色.

下面是Xcode的Metal调试器的屏幕抓取,你可以在发送到着色器之前看到纹理.

图像位于此处(我的排名不够高,无法嵌入)

在Xcode,finder中,当放入UIImageView时,源纹理没有环.但是在UIImage - > CGContex - > MTLTexture过程(我特别考虑MTLTexture部分)的某处,透明部分变暗了.

在过去的几天里,我一直在撞墙,改变我所能做的一切,但我无法理解.

为了透明(ha),这是我的个人导入代码

import UIKit
import CoreGraphics

class MetalTexture {

    class func imageToTexture(imageNamed: String, device: MTLDevice) -> MTLTexture {
        let bytesPerPixel = 4
        let bitsPerComponent = 8

        var image = UIImage(named: imageNamed)!

        let width = Int(image.size.width)
        let height = Int(image.size.height)
        let bounds = CGRectMake(0, 0, CGFloat(width), CGFloat(height))

        var rowBytes = width * bytesPerPixel
        var colorSpace = CGColorSpaceCreateDeviceRGB()

        let context = CGBitmapContextCreate(nil, width, height, bitsPerComponent, rowBytes, colorSpace, CGBitmapInfo(CGImageAlphaInfo.PremultipliedLast.rawValue))

        CGContextClearRect(context, bounds)
        CGContextTranslateCTM(context, CGFloat(width), CGFloat(height))
        CGContextScaleCTM(context, -1.0, -1.0)
        CGContextDrawImage(context, bounds, image.CGImage)

        var texDescriptor = MTLTextureDescriptor.texture2DDescriptorWithPixelFormat(.RGBA8Unorm, width: width, height: height, mipmapped: false)

        var texture = device.newTextureWithDescriptor(texDescriptor)
        texture.label = imageNamed

        var pixelsData = CGBitmapContextGetData(context)

        var region = MTLRegionMake2D(0, 0, width, height)
        texture.replaceRegion(region, mipmapLevel: 0, withBytes: pixelsData, bytesPerRow: rowBytes)

        return texture
    }
}

但我不认为这是问题所在(因为它是Swift的Apple副本,而且我使用了它们而没有差别).

任何潜在客户都会非常有帮助.

Answer 1

感谢Jessy，我决定研究一下如何混合Alpha，并弄清楚了。我的纹理在GPU调试器中看起来仍然变暗，但是在实际应用中，一切看起来都正确。我对管道状态描述符进行了更改，您可以在下面看到。

pipelineStateDescriptor.colorAttachments[0].blendingEnabled = true
pipelineStateDescriptor.colorAttachments[0].rgbBlendOperation = .Add
pipelineStateDescriptor.colorAttachments[0].alphaBlendOperation = .Add
pipelineStateDescriptor.colorAttachments[0].sourceRGBBlendFactor = .DestinationAlpha
pipelineStateDescriptor.colorAttachments[0].sourceAlphaBlendFactor = .DestinationAlpha
pipelineStateDescriptor.colorAttachments[0].destinationRGBBlendFactor = .OneMinusSourceAlpha
pipelineStateDescriptor.colorAttachments[0].destinationAlphaBlendFactor = .OneMinusBlendAlpha

Answer 2

由于您使用的是CGContext被配置为使用与alpha通道预乘的颜色，采用标准RGB = src.rgb * src.a + dst.rgb * (1-src.a)会造成变暗的区域，因为该src.rgb值预乘用src.a了。这意味着您想要的是src.rgb + dst.rgb * (1-src.a)这样配置的：

pipeline.colorAttachments[0].isBlendingEnabled = true
pipeline.colorAttachments[0].rgbBlendOperation = .add
pipeline.colorAttachments[0].alphaBlendOperation = .add
pipeline.colorAttachments[0].sourceRGBBlendFactor = .one
pipeline.colorAttachments[0].sourceAlphaBlendFactor = .sourceAlpha
pipeline.colorAttachments[0].destinationRGBBlendFactor = .oneMinusSourceAlpha
pipeline.colorAttachments[0].destinationAlphaBlendFactor = .oneMinusSourceAlpha

该.one办法离开RGB原样。预乘颜色的原因是，您只需在创建时就将颜色乘一次，而不是每次混合时。您的配置也可以间接实现此目的。