Android SetPixels

android/platform_frameworks_base/core/jni/android/graphics/Bitmap.cpp
SetPixels()

bool GraphicsJNI::SetPixels(JNIEnv* env, jintArray srcColors,
                            int srcOffset, int srcStride,
                            int x, int y, int width, int height,
                            const SkBitmap& dstBitmap) {
    SkAutoLockPixels alp(dstBitmap);
    void* dst = dstBitmap.getPixels();
    FromColorProc proc = ChooseFromColorProc(dstBitmap.config());

    if (NULL == dst || NULL == proc) {
        return false;
    }

    const jint* array = env->GetIntArrayElements(srcColors, NULL);
    const SkColor* src = (const SkColor*)array + srcOffset;

    // reset to to actual choice from caller
    dst = dstBitmap.getAddr(x, y);
    // now copy/convert each scanline
    for (int y = 0; y < height; y++) {
        proc(dst, src, width, x, y);
        src += srcStride;
        dst = (char*)dst + dstBitmap.rowBytes();
    }

    env->ReleaseIntArrayElements(srcColors, const_cast<jint*>(array),
                                 JNI_ABORT);
    return true;
}

↓
FromColor_D32()

static void FromColor_D32(void* dst, const SkColor src[], int width,
                          int, int) {
    SkPMColor* d = (SkPMColor*)dst;

    for (int i = 0; i < width; i++) {
        *d++ = SkPreMultiplyColor(*src++);
    }
}

↓
android/platform_external_skia/src/core/SkColor.cpp
SkPreMultiplyARGB()
aが0x00の場合, r=0x00,g=0x00,b=0x00
aが0x01〜0xFEの場合, r=r*a, g=g*a, b=b*a
aが0xFFの場合, r=r, g=g, b=b
αブレンディングをしている。

SkPMColor SkPreMultiplyARGB(U8CPU a, U8CPU r, U8CPU g, U8CPU b) {
    if (a != 255) {
        r = SkMulDiv255Round(r, a);
        g = SkMulDiv255Round(g, a);
        b = SkMulDiv255Round(b, a);
    }
    return SkPackARGB32(a, r, g, b);
}

↓
android/platform_external_skia/include/core/SkMath.h

/** Return a*b/255, rounding any fractional bits.
 Only valid if both a and b are 0..255
*/

static inline U8CPU SkMulDiv255Round(U8CPU a, U8CPU b) {
    SkASSERT((uint8_t)a == a);
    SkASSERT((uint8_t)b == b);
    unsigned prod = SkMulS16(a, b) + 128;
    return (prod + (prod >> 8)) >> 8;
}