Checks whether a bitmap is grayscale. Grayscale here means "very close to a perfect
gray".
Instead of scanning every pixel in the bitmap, we first resize the bitmap to no more than
COMPACT_BITMAP_SIZE^2 pixels using filtering. The hope is that any non-gray color elements
will survive the squeezing process, contaminating the result with color.
int height = bitmap.getHeight();
int width = bitmap.getWidth();
// shrink to a more manageable (yet hopefully no more or less colorful) size
if (height > COMPACT_BITMAP_SIZE || width > COMPACT_BITMAP_SIZE) {
if (mTempCompactBitmap == null) {
mTempCompactBitmap = Bitmap.createBitmap(
COMPACT_BITMAP_SIZE, COMPACT_BITMAP_SIZE, Bitmap.Config.ARGB_8888
);
mTempCompactBitmapCanvas = new Canvas(mTempCompactBitmap);
mTempCompactBitmapPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
mTempCompactBitmapPaint.setFilterBitmap(true);
}
mTempMatrix.reset();
mTempMatrix.setScale(
(float) COMPACT_BITMAP_SIZE / width,
(float) COMPACT_BITMAP_SIZE / height,
0, 0);
mTempCompactBitmapCanvas.drawColor(0, PorterDuff.Mode.SRC); // select all, erase
mTempCompactBitmapCanvas.drawBitmap(bitmap, mTempMatrix, mTempCompactBitmapPaint);
bitmap = mTempCompactBitmap;
width = height = COMPACT_BITMAP_SIZE;
}
final int size = height*width;
ensureBufferSize(size);
bitmap.getPixels(mTempBuffer, 0, width, 0, 0, width, height);
for (int i = 0; i < size; i++) {
if (!isGrayscale(mTempBuffer[i])) {
return false;
}
}
return true;
