draw rectangle on non-transparent background only

[bacteriaman]

I have an object with a DRAW event that displays a semi-opaque overlay (aka. dimmer) over the entire room. This is used when displaying a dialog in the foreground (just like a browser modal window.)

draw_set_alpha(0.6);
draw_set_color(COLOR_BROWN);
draw_rectangle(0, 0, 1024, 662, false);
draw_set_alpha(1)

However, I only want the overlay to appear on non-transparent portions of the room. Transparent portions of the room should be unaffected and display the background color.

I believe this can be accomplished with Shaders, but I am not sure how to go about it. I am prepared to do my own experimentation, but I would be grateful for any tips the community would like to share.

 

[rytan451]

If you're dimming, then you could instead draw everything you're dimming to a second surface (at the beginning of each step, clear it to black and alpha 0). Then, after everything is drawn to the surface, using a multiply blend mode gpu_set_blendmode_ext(bm_dest_color, bm_zero), draw the rectangle over it. Then, using the normal blend mode, draw that surface onto the application surface (which you need to have already cleared to the background color).

 

[bacteriaman]

@rytan451, thank you for your response.

I like your suggestion, but the solution to my particular challenge turned out to be straightforward.

I actually created a shader to achieve the desired effect, but using a shader turned out to be unnecessary.

I ended up simply using draw_sprite_ext() to draw the background image with the desired blend and alpha values.

That said, I am reminded of how cool shaders are and I look forward to using them more in the future.