Flattening image layers

Question

I am working on a basic image editing tool with support for layers, and I need to know how to merge the layers into a single image. Obviously, if the pixels have no alpha value, then whichever is on top wins, but I dont know what to do when the pixels have alpha values. So here's the question: Given two (or more) pixels represented using RGBA values, how do I merge them into a single pixel in this context (layers)?

TIA

Answer 1

For each component in an layer, if the alpha value is a (range 0..1) then you'll see a proportion a of that component + a proportion 1-a of what's underneath.

Try working from the bottom layer to the top layer.

EDIT:

#define MAX_PIXEL 255

int numLayers; // Number of layers.
Color* layers; // Pointer to the layers.

Color flattened;

flattened.R = 0;
flattened.G = 0;
flattened.B = 0;
flattened.A = MAX_PIXEL;

// Layer 0 is the bottom layer.
for (int i = 0; i < numLayers; i++) {
    int alpha;

    alpha = layers[i].A;

    flattened[i].R = (layers[i].R * alpha + flattened[i].R * (MAX_PIXEL - alpha)) / MAX_PIXEL;
    flattened[i].G = (layers[i].G * alpha + flattened[i].G * (MAX_PIXEL - alpha)) / MAX_PIXEL;
    flattened[i].B = (layers[i].B * alpha + flattened[i].B * (MAX_PIXEL - alpha)) / MAX_PIXEL;
}