如何在C＃UWP中使用原始像素數據合併圖層？

Question

由於通用Windows平臺默認不支持平面化位圖「圖層」，因此我一直在創建算法（基於發現的一個算法here）以獲取每個圖層的每個單獨像素，並將它們疊加在另一個上。問題是，因爲我使用Alpha來允許圖像具有透明像素，所以我不確定如何正確地合併像素，使其看起來好像一個位於另一個之上。

這是到目前爲止我的代碼：

private unsafe SoftwareBitmap CompileImage() 
    { 
     SoftwareBitmap softwareBitmap = new SoftwareBitmap(BitmapPixelFormat.Bgra8, Width, Height); 

     using (BitmapBuffer buffer = softwareBitmap.LockBuffer(BitmapBufferAccessMode.Write)) 
     { 
      using (var reference = buffer.CreateReference()) 
      { 
       byte* dataInBytes; 
       uint capacity; 
       ((IMemoryBufferByteAccess)reference).GetBuffer(out dataInBytes, out capacity); 

       BitmapPlaneDescription bufferLayout = buffer.GetPlaneDescription(0); 

       for (int index = 0; index < layers.Length; index++) 
       { 
        for (int i = 0; i < bufferLayout.Height; i++) 
        { 
         for (int j = 0; j < bufferLayout.Width; j++) 
         { 
          if(index == 0) 
          { 
           dataInBytes[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 0] = 
            layers[index].pixelData[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 0]; //B 
           dataInBytes[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 1] = 
            layers[index].pixelData[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 1]; //G 
           dataInBytes[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 2] = 
            layers[index].pixelData[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 2]; //R 
           dataInBytes[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 3] = 
            layers[index].pixelData[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 3]; //A 
          } 
          else if(index > 0) 
          { 

           //Attempts to "Average" pixel data 
           dataInBytes[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 0] = 
            (byte)(dataInBytes[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 0] * 
            layers[index].pixelData[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 0]/2); 
           dataInBytes[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 1] = 
            (byte)(dataInBytes[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 1] * 
            layers[index].pixelData[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 1]/2); 
           dataInBytes[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 2] = 
            (byte)(dataInBytes[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 2] * 
            layers[index].pixelData[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 2]/2); 
           dataInBytes[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 3] = 
            (byte)(dataInBytes[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 3] * 
            layers[index].pixelData[bufferLayout.StartIndex + bufferLayout.Stride * i + 4 * j + 3]/2); 
          } 
         } 
        } 
       } 
      } 
     } 
     return softwareBitmap; 
    } 

目前的算法將每個像素的平均BGRA值在每一層，並返回結果。這不是我打算做的。我如何去糾正它？在此先感謝

Answer 1

要應用一個alpha混合你乘以逆alpha和當前顏色的alpha，當然你應該做一個0-1的規模（這就是爲什麼圖形卡與浮動顏色）。

每例如，假設有三個層，A，B和C中，使用A作爲鹼，然後將其與乙混合：（僞碼）

//bAlpha is thge alpha value of the current pixel from B in byte value 
var alpha = bAlpha/255.0f; 
var invAlpha = 1.0f - alpha; 

//aRGB is the RGB value of the pixel on the layer A 
//bRGB is the RGB value of the pixel on the layer B 
var abRGB = aRGB * invAlpha + bRGB * alpha; 

現在應用的第三層：

//cAlpha is thge alpha value of the current pixel from C in byte value 
alpha = cAlpha/255.0f; 
invAlpha = 1.0f - alpha; 

//abRGB is the RGB value of the previously mixed layers 
//cRGB is the RGB value of the pixel on the layer C 
var abcRGB = abRGB * invAlpha + cRGB * alpha; 

當然，所有這一切都是在ARGB模式，這裏還有pARGB其中顏色已經預乘，所以你只需要乘以先前的顏色由逆α和添加當前顏色。

//bAlpha is the alpha value of the current pixel from B in byte value 
var invAlpha = 1.0f - (bAlpha/255.0f); 

//aRGB is the RGB value of the pixel on the layer A 
//bRGB is the RGB value of the pixel on the layer B 
var abRGB = aRGB * invAlpha + bRGB; 

invAlpha = 1.0f - (cAlpha/255.0f); 

//abRGB is the RGB value of the previously mixed layers 
//cRGB is the RGB value of the pixel on the layer C 
var abcRGB = abRGB * invAlpha + cRGB;