/**
* Compute the shift and mask values for the alpha channel.
*/
void alpha_shiftmask_get(int* shift, unsigned* mask, adv_color_def def_ordinal)
{
union adv_color_def_union def;
unsigned pos;
unsigned len;
def.ordinal = def_ordinal;
assert(def.nibble.type == adv_color_type_rgb);
len = def.nibble.alpha_size * 8;
if (def.nibble.red_pos == 0 || def.nibble.green_pos == 0 || def.nibble.blue_pos == 0) {
pos = color_def_bytes_per_pixel_get(def_ordinal) * 8 - len;
} else {
pos = 0;
}
rgb_shiftmask_get(shift, mask, len, pos);
}
static void ui_color_alpha_set(adv_pixel* map, const adv_color_rgb* fore, const adv_color_rgb* back, adv_color_def buffer_def, unsigned translucency)
{
unsigned i;
int t;
int red_shift, green_shift, blue_shift, alpha_shift;
adv_pixel red_mask, green_mask, blue_mask, alpha_mask;
union adv_color_def_union udef;
udef.ordinal = buffer_def;
rgb_shiftmask_get(&red_shift, &red_mask, udef.nibble.red_len, udef.nibble.red_pos);
rgb_shiftmask_get(&green_shift, &green_mask, udef.nibble.green_len, udef.nibble.green_pos);
rgb_shiftmask_get(&blue_shift, &blue_mask, udef.nibble.blue_len, udef.nibble.blue_pos);
alpha_shiftmask_get(&alpha_shift, &alpha_mask, buffer_def);
t = translucency;
for (i = 0; i < 256; ++i) {
int cr, cg, cb, ca;
double F, B, A;
double I, T;
double a, b, c;
/*
I = ui foreground/background alpha channel
T = ui/game alpha channel
a = ui foreground component
b = ui background component
c = game component
the color is computed with
a * UI_FOREGROUND + b * UI_BACKGROUND + c * GAME
with the corner conditions :
T=1 -> a=I,b=1-I,c=0
T=0 -> a=I,b=0,c=1-I
I=0 -> a=0,b=T,c=1-T
I=1 -> a=1,b=0,c=0
where always a+b+c = 1
the following relations are arbitrarily chosen :
a = I
b = T * (1-I)
c = (1-T) * (1-I)
*/
I = i / 255.0;
T = t / 255.0;
a = I;
b = T * (1 - I);
c = (1 - T) * (1 - I);
A = 1 - c; /* alpha channel */
if (A == 0) {
F = 0;
B = 0;
} else {
/* the alpha component is implicitely applied */
F = a / A;
B = b / A;
}
cr = fore->red * F + back->red * B;
cg = fore->green * F + back->green * B;
cb = fore->blue * F + back->blue * B;
ca = 255 * A;
if (cr > 255)
cr = 255;
if (cg > 255)
cg = 255;
if (cb > 255)
cb = 255;
if (ca > 255)
ca = 255;
map[i] = rgb_nibble_insert(cr, red_shift, red_mask)
| rgb_nibble_insert(cg, green_shift, green_mask)
| rgb_nibble_insert(cb, blue_shift, blue_mask)
| rgb_nibble_insert(ca, alpha_shift, alpha_mask);
}
}
