alpha_over_rgba_rgba
Tcl_Obj* imageForeObj
Tcl_Obj* imageBackObj

/*
 * Alpha-based blending of two images, foreground, and background.  The
 * foreground's alpha channel is used to determine how much of the background
 * is seen. The result's alpha is the input's alpha attenuated by the
 * background's alpha.
 */

crimp_image*     result;
crimp_image*     imageF;
crimp_image*     imageB;
int x, y;

crimp_input (imageForeObj, imageF, rgba);
crimp_input (imageBackObj, imageB, rgba);

if (!crimp_eq_dim (imageF, imageB)) {
    Tcl_SetResult(interp, "image dimensions do not match", TCL_STATIC);
    return TCL_ERROR;
}

result = crimp_new_like (imageF);

for (y = 0; y < result->h; y++) {
    for (x = 0; x < result->w; x++) {

	int alpha = A (imageF, x, y);

	/*
	 * alpha is Opacity
	 * 255 <=> Fully opaque      <=> imageF
	 * 0   <=> Fully transparent <=> imageB
	 *
	 * => OUT = F*alpha + B*(1-alpha)
	 */

	R (result, x, y) = (R (imageF, x, y) * alpha + (255 - alpha) * R (imageB, x, y)) / 255;
	G (result, x, y) = (G (imageF, x, y) * alpha + (255 - alpha) * G (imageB, x, y)) / 255;
	B (result, x, y) = (B (imageF, x, y) * alpha + (255 - alpha) * B (imageB, x, y)) / 255;
	A (result, x, y) = alpha - (alpha * A (imageB, x, y)) / 255;
    }
}

Tcl_SetObjResult(interp, crimp_new_image_obj (result));
return TCL_OK;

/* vim: set sts=4 sw=4 tw=80 et ft=c: */
/*
 * Local Variables:
 * mode: c
 * c-basic-offset: 4
 * fill-column: 78
 * End:
 */