如何將灰度位圖轉換爲alpha蒙版？

Question

我有一張灰度JPG圖片，我想將其加載到格式爲Bitmap.Config.ALPHA_8的位圖中。這是可能的，我該怎麼做？

從PNG（可以有空的R，G，B通道）加載alpha通道很簡單，但我想用JPG進行壓縮。

這是一個後續問題How to combine two opaque bitmaps into one with alpha channel?

Answer 1

ColorMatrix營救！

引述的Android文檔，嘉洛斯：

用於轉化的位圖的 顏色+ alpha分量5x4的矩陣。矩陣被存儲在單個陣列中，並且其處理如下：[ a，b，c，d，e，f，g，h，i，j，k，l，m， n，o，當應用於 顏色[r，g，b，a]時，得到的顏色被計算爲（在鉗位後） R'= a R + b G + c B + d A + e; G'= f R + g G + h B + i A + j; B'= k R + l G + m B + n A + o; A'= p R + q G + r B + s A + t;

設置從紅色通道（或綠色，或藍色，對灰度無關緊要）採用alpha值的顏色矩陣，然後在Paint.setColorFilter()中使用它。下面是一個或多或少的完整示例：

final BitmapFactory.Options options = new BitmapFactory.Options(); 
options.inPreferredConfig = Bitmap.Config.ARGB_8888; 
options.inScaled = false; 

// Load source grayscale bitmap 
Bitmap grayscale = BitmapFactory.decodeResource(getResources(), R.drawable.my_grayscale_mask, options); 
// Place for alpha mask. It's specifically ARGB_8888 not ALPHA_8, 
// ALPHA_8 for some reason didn't work out for me. 
Bitmap alpha = Bitmap.createBitmap(grayscale.getWidth(), grayscale.getHeight(), 
     Bitmap.Config.ARGB_8888); 
float[] matrix = new float[] { 
     0, 0, 0, 0, 0, 
     0, 0, 0, 0, 0, 
     0, 0, 0, 0, 0, 
     1, 0, 0, 0, 0}; 
Paint grayToAlpha = new Paint(); 
grayToAlpha.setColorFilter(new ColorMatrixColorFilter(new ColorMatrix(matrix))); 
Canvas alphaCanvas = new Canvas(alpha); 
// Make sure nothing gets scaled during drawing 
alphaCanvas.setDensity(Bitmap.DENSITY_NONE); 
// Draw grayscale bitmap on to alpha canvas, using color filter that 
// takes alpha from red channel 
alphaCanvas.drawBitmap(grayscale, 0, 0, grayToAlpha); 
// Bitmap alpha now has usable alpha channel!