I have a grayscale JPG picture, and I'd like to load it into Bitmap of format Bitmap.Config.ALPHA_8. Is that possible, how can I do that?
It's straightforward to load alpha channel from a PNG (which can have empty R,G,B channels), but I'd like to use JPG for compression.
This is a followup question to How to combine two opaque bitmaps into one with alpha channel?
ColorMatrix to rescue!
Quoting Android docs, ColorMatrix:
5x4 matrix for transforming the color+alpha components of a Bitmap. The matrix is stored in a single array, and its treated as follows: [ a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t ] When applied to a color [r, g, b, a], the resulting color is computed as (after clamping) R' = aR + bG + cB + dA + e; G' = fR + gG + hB + iA + j; B' = kR + lG + mB + nA + o; A' = pR + qG + rB + sA + t;
Set up color matrix that takes alpha value from red channel (or green, or blue, doesn't matter for grayscale...), then use it in Paint.setColorFilter(). Here's a more or less complete example:
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!