How to mix two int colors correctly

Question

I'm trying to blend two colors that are coded as integers. Here is my little function:

int blend (int a, int b, float ratio) {
    if (ratio > 1f) {
        ratio = 1f;
    } else if (ratio < 0f) {
        ratio = 0f;
    }
    float iRatio = 1.0f - ratio;

    int aA = (a >> 24 & 0xff);
    int aR = ((a & 0xff0000) >> 16);
    int aG = ((a & 0xff00) >> 8);
    int aB = (a & 0xff);

    int bA = (b >> 24 & 0xff);
    int bR = ((b & 0xff0000) >> 16);
    int bG = ((b & 0xff00) >> 8);
    int bB = (b & 0xff);

    int A = ((int)(aA * iRatio) + (int)(bA * ratio));
    int R = ((int)(aR * iRatio) + (int)(bR * ratio));
    int G = ((int)(aG * iRatio) + (int)(bG * ratio));
    int B = ((int)(aB * iRatio) + (int)(bB * ratio));

    return A << 24 | R << 16 | G << 8 | B;
}

Everything seems to work fine, but certain arguments produce wrong colors. For example:

    int a = 0xbbccdd;
    int b = 0xbbccdd;
    int c = blend(a, b, 0.5f); // gives 0xbaccdc, although it should be 0xbbccdd

My guess is that either multiplication by float ratios or casting are to blame here, but I can't figure out what's wrong with them...

So what's the correct way to blend two colors in java?

Answer 1

My guess is that the casting to int should be done after the addition. Like this

int a = (int)((aA * iRatio) + (bA * ratio));

I would also suggest using Java naming conventions when using variables. Only constants should be caps.