alpha_blend_rgb_rgb
Tcl_Obj* imageForeObj
Tcl_Obj* imageBackObj
int alpha

/*
 * Alpha-based blending of two images, foreground, and background, controlled
 * by a scalar (and extern) alpha factor.
 *
 * alpha is Opacity
 * 255 <=> Fully opaque      <=> imageF
 * 0   <=> Fully transparent <=> imageB
 *
 * => OUT = F*alpha + B*(1-alpha)
 */

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

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

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

result = crimp_new_like (imageF);
ralpha = 255 - alpha;

#define MIX(fore,back) ((((fore)*alpha) + ((back)*ralpha))/255)

for (y = 0; y < result->h; y++) {
    for (x = 0; x < result->w; x++) {
	R (result, x, y) = MIX (R (imageF, x, y), R (imageB, x, y));
	G (result, x, y) = MIX (G (imageF, x, y), G (imageB, x, y));
	B (result, x, y) = MIX (B (imageF, x, y), B (imageB, x, y));
    }
}

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:
 */