问题
I cannot explain this (screenshot from Eclipse debug):
Pixel at (0,0) does not have the values it was set for!
All other pixels are fine, they do have the same value they were assigned to.
EDIT I did some more digging, and the code for setPixel is calling a native function:
1391 public void setPixel(int x, int y, int color) {
1392 checkRecycled("Can't call setPixel() on a recycled bitmap");
1393 if (!isMutable()) {
1394 throw new IllegalStateException();
1395 }
1396 checkPixelAccess(x, y);
1397 nativeSetPixel(mNativeBitmap, x, y, color, mIsPremultiplied);
1398 }
so I run setPixel(0,0) with different values, starting with the one that was OK for setPixel(0,1).
First I changed only one of the arguments. The conclusion is that the alpha value is the culprit, but not sure exactly how. I tried a lot of other alpha values and it seems that around 0xB0 and up the results are coming back OK.
I tried the same values on other pixels and the problem is not dependent on the pixel coordinates, it fails as well for others. The problem seems to be data dependent.
source.setPixel(0, 0, Color.argb(0x40, 0x1A, 0x11, 0x12));
int sp00 = source.getPixel(0, 0); // sp00 1075580948 [0x401c1014] BAD
source.setPixel(0, 0, Color.argb(0xFE, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -16781602 [0xfeffeede] OK!
source.setPixel(0, 0, Color.argb(0x40, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 1090514911 [0x40ffefdf] BAD
source.setPixel(0, 0, Color.argb(0xFE, 0x1A, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -31789346 [0xfe1aeede] OK!
source.setPixel(0, 0, Color.argb(0xFE, 0xFF, 0x11, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -16838178 [0xfeff11de] OK!
source.setPixel(0, 0, Color.argb(0xFE, 0xFF, 0xEE, 0x12));
sp00 = source.getPixel(0, 0); // sp00 -16781806 [0xfeffee12] OK!
source.setPixel(0, 0, Color.argb(0x00, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 0 [0x0] Strange, why Color has to police the alpha value?
source.setPixel(0, 0, Color.argb(0x10, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 285208543 [0x10ffefdf] BAD
source.setPixel(0, 0, Color.argb(0x20, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 553643999 [0x20ffefdf] BAD
source.setPixel(0, 0, Color.argb(0x30, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 822079455 [0x30ffefdf] BAD
source.setPixel(0, 0, Color.argb(0x50, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 1358950367 [0x50ffefdf] BAD
source.setPixel(0, 0, Color.argb(0x60, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 1627385823 [0x60ffefdf] BAD
source.setPixel(0, 0, Color.argb(0x70, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 1895821279 [0x70ffefdf] BAD
source.setPixel(0, 0, Color.argb(0x80, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -2130711075 [0x80ffeddd] BAD but change of pattern
source.setPixel(0, 0, Color.argb(0x90, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -1862275619 [0x90ffeddd] BAD
source.setPixel(0, 0, Color.argb(0xA0, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -1593840162 [0xa0ffedde] BAD but change of pattern again
source.setPixel(0, 0, Color.argb(0xB0, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -1325404450 [0xb0ffeede] OK!
source.setPixel(0, 0, Color.argb(0xC0, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -1056968994 [0xc0ffeede] OK!
source.setPixel(0, 0, Color.argb(0xD0, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -788533538 [0xd0ffeede] OK!
source.setPixel(0, 0, Color.argb(0xE0, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -520098082 [0xe0ffeede] OK!
source.setPixel(0, 0, Color.argb(0xF0, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -251662626 [0xf0ffeede] OK!
source.setPixel(0, 0, Color.argb(0xA7, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -1476399395 [0xa7ffeedd] BAD
source.setPixel(0, 0, Color.argb(0xA3, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -1543508258 [0xa3ffeede] OK!
source.setPixel(0, 0, Color.argb(0xA1, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -1577062690 [0xa1ffeede] OK!
source.setPixel(0, 0, Color.argb(0xAB, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -1409290274 [0xabffefde] BAD
source.setPixel(0, 0, Color.argb(0xA9, 0xFF, 0xEE, 0xDE));
sp00 = source.getPixel(0, 0); // sp00 -1442844962 [0xa9ffeede] OK!
回答1:
Well, more or less I found what is happening.
It is called "pre-multiplied Alpha" and Bitmaps before api19 do not offer any way to control this feature, they are premultiplied by default and this cannot be changed.
On api19 there are 2 new methods to Bitmap: isPremultiplied() and setPremultiplied(boolean)
According to the new doc:
When a pixel is pre-multiplied, the RGB components have been multiplied by the alpha component. For instance, if the original color is a 50% translucent red (128, 255, 0, 0), the pre-multiplied form is (128, 128, 0, 0).
Also this post and this other post give some more explanation. According to this, some more tests show:
Bitmap source = Bitmap.createBitmap(2, 2, Config.ARGB_8888);
source.setPixel(0, 0, Color.argb(0x02, 0x10, 0x20, 0x30));
source.setPixel(0, 1, Color.argb(0x03, 0x10, 0x20, 0x30));
source.setPixel(1, 0, Color.argb(0x05, 0x78, 0x96, 0x64));
source.setPixel(1, 1, Color.argb(128, 255, 200, 150));
int sp00 = source.getPixel(0, 0); // sp00 33554432 [0x2000000]
int sp01 = source.getPixel(0, 1); // sp01 50331733 [0x3000055]
int sp10 = source.getPixel(1, 0); // sp10 90610022 [0x5669966]
int sp11 = source.getPixel(1, 1); // sp11 -2130720875 [0x80ffc795]
For lower color values the rounding causes the info to be lost (see sp00 above).
Also, for lower values, the value of alpha itself is not retrieved to the original.
On top of this, the given formulas do not explain the values I see.
Finally to use unmodified pixels I now use this code to set the Bitmap pixles:
Bitmap source = Bitmap.createBitmap(2, 2, Config.ARGB_8888);
IntBuffer data = IntBuffer.wrap(new int[] {
Color.argb(0x06, 0xff, 0xa0, 0x8d),
Color.argb(0x2a, 0xab, 0xce, 0x9f),
Color.argb(0x8f, 0xfe, 0x05, 0x18),
Color.argb(0xff, 0xc8, 0xcf, 0xd4)
});
source.copyPixelsFromBuffer(data);
and to retrieve the pixels I use:
IntBuffer sourceData = IntBuffer.allocate(4);
source.copyPixelsToBuffer(sourceData);
Using these methods do not premultiply the color.
来源：https://stackoverflow.com/questions/22283390/puzzled-by-pixel-value-in-bitmap-pre-multiplied-color-using-setpixel