Question

我有一张灰度JPG图片，我想将其加载到格式Bitmap.Config.ALPHA_8的位图中。这可能吗，我怎么能这样做？

从PNG（可以有空R，G，B通道）加载alpha通道很简单，但我想用JPG进行压缩。

这是How to combine two opaque bitmaps into one with alpha channel?

的后续问题

Answer 1

引用Android文档，ColorMatrix：

5x4矩阵用于转换颜色+位图的alpha分量。矩阵存储在单个中数组，其处理如下：[ a，b，c，d，e，f，g，h，i，j，k，l，m， n，o，p，q，r，s，t]当应用于颜色[r，g，b，a]，得到的颜色颜色计算为（夹紧后） 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;

设置从红色通道获取alpha值的颜色矩阵（或绿色或蓝色，对于灰度等级无关紧要......），然后在Paint.setColorFilter()中使用它。这是一个或多或少完整的例子：

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!

如何将灰度位图转换为alpha蒙版？

1 个答案: