如何将灰度位图转换为alpha蒙版？

Question

我有一个灰度JPG图片,我想将其加载到格式的位图Bitmap.Config.ALPHA_8.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,但是我想用JPG进行压缩.

这是如何将两个不透明的位图组合成一个带alpha通道的后续问题？

Answer 1

ColorMatrix救援!

引用Android文档,ColorMatrix:

5x4矩阵,用于转换位图的颜色+ alpha分量.矩阵存储在一个数组中,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]时,得到的颜色计算为(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 + 1 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,对于灰度级无关紧要......),然后使用它Paint.setColorFilter().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!