KDColor.cpp

#include "KDColor.h"

KDColor KDColor::blend(KDColor first, KDColor second, uint8_t alpha) {
    /* This function is a hot path since it's being called for every single pixel
     * whenever we want to display a string. In this context, we're quite often
     * calling it with a value of either 0 or 0xFF, which can be very trivially
     * dealt with. Similarly, blending the same two colors yields a trivial
     * result and can be bypassed. Let's make a special case for them. */
    if (alpha == 0) {
        return second;
    }
    if (alpha == 0xFF) {
        return first;
    }
    if (first == second) {
        return first;
    }

    // We want to do first*alpha + second*(1-alpha)
    // First is RRRRR GGGGGG BBBBB
    // Second is same

    uint16_t oneMinusAlpha = 0x100 - alpha;
    uint16_t red = first.red() * alpha + second.red() * oneMinusAlpha;
    uint16_t green = first.green() * alpha + second.green() * oneMinusAlpha;
    uint16_t blue = first.blue() * alpha + second.blue() * oneMinusAlpha;
    return RGB888(red >> 8, green >> 8, blue >> 8);
}