draw_set_blend_mode_ext(src, dest);
| Argument | Description |
|---|---|
| src | Source (see constants above). |
| dest | Destination (see constants above) |
Returns: N/A
When GameMaker: Studio goes to draw a pixel there is a
source colour (the colour of the pixel we're going to draw) and a
destination colour(the colour that's already in the pixel we're
drawing to), so when determining the new colour of the pixel, the
source and destination colours are calculated according to the
chosen blend mode. Each component of the colours is stored as a
floating point value between 0 and 1, and the new colour is
calculated by multiply each component of the source colour by some
factor and by multiplying each component of destination colour by
some other factor and then adding the results together component by
component.
This function permits you to set the different component parts that
should be factored together to create a custom blend mode. The
source and destination both have a red, green, blue and alpha
component, and in the following chart the source's RGBA are
considered (Rs, Gs, Bs, As) while the destination's are (Rd, Gd,
Bd, Ad). The eleven constants that are available for use can be
either source or destination (or both) when used in this
function.
| Constant | Blend factor (Red, Green, Blue, Alpha) |
|---|---|
| bm_zero | (0, 0, 0, 0) |
| bm_one | (1, 1, 1, 1) |
| bm_src_colour | (Rs, Gs, Bs, As) |
| bm_inv_src_colour | (1-Rs, 1-Gs, 1-Bs, 1-As) |
| bm_src_alpha | (As, As, As, As) |
| bm_inv_src_alpha | (1-As, 1-As, 1-As, 1-As) |
| bm_dest_alpha | (Ad, Ad, Ad, Ad) |
| bm_inv_dest_alpha | (1-Ad, 1-Ad, 1-Ad, 1-Ad) |
| bm_dest_colour | (Rd, Gd, Bd, Ad) |
| bm_inv_dest_colour | (1-Rd, 1-Gd, 1-Bd, 1-Ad) |
| bm_src_alpha_sat | (f, f, f, 1) where f = min(As, 1-Ad) |
It can be quite difficult to visualise how these different blend
modes work, so below you can find a simple chart that shows the
basic interaction of a single sprite being drawn with different
combinations of blend modes over a single background. Please note
that the actual results of the blend mode you use will very much
depend on the colour in the sprite, the alpha value of the sprite
and the colour and alpha of the background. 
WARNING!: Not all blend modes are available on all platforms. Android, iOS, and HTML5 without WebGL enabled, will not be able to display the following modes correctly:
- bm_src_colour
- bm_inv_src_colour
- bm_dest_colour
- bm_inv_dest_colour
- bm_src_alpha_sat
draw_set_blend_mode_ext(bm_src_alpha, bm_one);
draw_circle_colour(100, 100, 50, c_white, c_black, 0);
draw_set_blend_mode(bm_normal);
This will turn the black into transparency, creating a 'glow' effect from the white being strong on the outside and gradually weakening further from the circle centre. The blend mode is reset after the circle is drawn to ensure additive blending is not also applied to everything else after it.