I can't reply to my previous thread ("Allegro 4 game - Steam overlay") as I was the last poster on it, and the forum software won't allow two posts in a row from the same person.

I got my game working, using AllegroGL to copy the back buffer to the screen - but I can't change video mode. Doing this in windowed results in a transparent window (see attachment), in fullscreen I get a black screen. Also the music stops playing, although that is easily dealt with.

Fullscreen at desktop resolution (1080p) always results in a black screen, even if that's the starting resolution.

If I change back to the original screen resolution, the game continues as normal.

I'm guessing this is a case of AllegroGL being partly unfinished (as I found out after hours of trying to work out why stretch_blit didn't work - was never implemented) and not having had the same level of testing as Allegro as a whole.

Well, it works in that I can run it - but I can't release my game without the ability to change resolution or run at desktop resolution.

I use allegro_gl_make_texture_ex to turn the back buffer into a texture, then draw a single quad.

640x480 to 800x600 and back doesn't work either (or 512/1024).

In 1080p fullscreen, the game is still running, I just can't see anything. allegro_gl_make_texture_ex does not produce an error code.

I will look into Allegro-Legacy next week.

GLuint tex = allegro_gl_make_texture_ex(0, active_page, GL_RGB8);
if (tex == 0)
	end_it_all("Error: allegro_gl_make_texture_ex failed. Please report this error to psector@arganoid.com, or try"
		" disabling OpenGL rendering in AppData\\psector\\psector.cfg");

glBindTexture(GL_TEXTURE_2D, tex);
		
glClearColor(1.0f, 0.0f, 0.0f, 1.f);

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);

glEnable(GL_TEXTURE_2D);

glBegin(GL_QUADS);

glTexCoord2f(0, 1);
glVertex2f(-1, -1);

glTexCoord2f(1, 1);
glVertex2f(1, -1);

glTexCoord2f(1, 0);
glVertex2f(1, 1);

glTexCoord2f(0, 0);
glVertex2f(-1, 1);

glEnd();

allegro_gl_flip();

GLuint textures[] = { tex };
glDeleteTextures(1, textures);

Your source looks OK to me - which now that I think again it should since it works before the resolution change. Possibly try adding glClear command before drawing the quad and clear both color and depth? Also, possibly allegro_error can provide you with more details:

https://www.allegro.cc/manual/4/api/using-allegro/allegro_error

Also, if there is 'allegro.log' file in your app directory that might contain more details (though that might only appear with debug version of the library).

Sorry I can't help in a better way.