How to change Android bitmap in NDK so that I can use it on Java side?

Question

I call the C ++ function on the JNI and pass the RGBA_8888 bitmap, lock it, change the values, unlock, return, and then show in Java with this C ++ code:

AndroidBitmap_getInfo(env, map, &info) < 0); AndroidBitmap_lockPixels(env, map, (void**)&pixel); for(i=info.width*info.height-1;i>=0;i--) { pixel[i] = 0xf1f1f1f1; } AndroidBitmap_unlockPixels(env, map); 

The problem is that the bitmap images do not look the way I expect and the pixel values ​​(verified with getPixel) do not match when I test them in Java from what I installed in C ++. When I set the bitmap values ​​to 0xffffffff, I get the correct value in Java, but for many others I do not. For example, 0xf1f1f1f1 turns into 0xF1FFFFFF.

What do I need to do to make it work?

PS: I am using Android 2.3.4

Answer 1

The problem seems to be related to pre-multiplex alpha.

Further testing showed that the values ​​of r, g, b scale alpha, so if I transfer the bitmap to jni, I need to multiply the values ​​of r, g, b by 255 and divide them by alpha in order to have a real value with which I can perform alpha operations. When I pass them back, I have to do the opposite.

Only for copying and operations without alpha, which is not necessary since alpha will be 255 for maximum opacity and * 255/255 denies itself.