I'm actually scaling up from 480x270 to 960x540 on my mac that has a display size of 1440x900. I guess I need to create an allegro display equal to my physical display and just center the main bitmap on the allegro display.
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1 al_set_target_bitmap (draw_buffer
); //draw buffer is 480x270
2
3/* draw some primitives and bitmaps */
4
5
6 al_set_target_bitmap ( al_get_backbuffer ( main_display
) );//display is 960 x 540
7
8#if 0
9 al_draw_scaled_bitmap ( draw_buffer,
10 0,
11 0,
12 SCREEN_WIDTH,
//480
13 SCREEN_HEIGHT,
//270
14 0,
15 0,
16 MAIN_DISPLAY_WIDTH,
//960
17 MAIN_DISPLAY_HEIGHT,
//540
18 0);
19#else
20// calculate scaling factor
21 sx
= MAIN_DISPLAY_WIDTH
/ SCREEN_WIDTH
;
22 sy
= MAIN_DISPLAY_HEIGHT
/ SCREEN_HEIGHT
;
23 scale
= G_MIN
(sx, sy
);
24
25 // calculate how much the buffer should be scaled
26 scaleW
= SCREEN_WIDTH
* scale
;
27 scaleH
= SCREEN_HEIGHT
* scale
;
28 scaleX
= (MAIN_DISPLAY_WIDTH
- scaleW
) / 2;
29 scaleY
= (MAIN_DISPLAY_HEIGHT
- scaleH
) / 2;
30
31 al_draw_scaled_bitmap(draw_buffer,
32 0,
33 0,
34 SCREEN_WIDTH,
//480
35 SCREEN_HEIGHT,
//270
36 scaleX,
//0
37 scaleY,
//0
38 scaleW,
//960
39 scaleH,
//540
40 0);
41#endif
42 al_flip_display ();
The key thing I was missing earlier was that I was not matching the allegro display size with my physical screen size. With the code below I can center the final bitmap in the display with a letterbox effect.
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1 num_full_displays
= al_get_num_display_modes();
2 main_display_return
= al_get_display_mode(0,
&main_display_mode
);
3 main_display
= al_create_display ( main_display_mode.width,
4 main_display_mode.height
);
5
6
7 display_width
= al_get_display_width(main_display
);
8 display_height
= al_get_display_height(main_display
);
9
10 // calculate scaling factor
11 sx
= display_width
/ SCREEN_WIDTH
;
12 sy
= display_height
/ SCREEN_HEIGHT
;
13 scale
= G_MIN
(sx, sy
);
14
15 // calculate how much the buffer should be scaled
16 scaleW
= SCREEN_WIDTH
* scale
;
17 scaleH
= SCREEN_HEIGHT
* scale
;
18 scaleX
= (display_width
- scaleW
) / 2;
19 scaleY
= (display_height
- scaleH
) / 2;
]]>
. An odd thing occurs when I use the ALLEGRO_FULLSCREEN_WINDOW option on OSX. I'm drawing on a bitmap that is 480 x 270 which is then scaled up to an Allegro display that is 1920 x 1080. It looks like some additional space is added to the bottom of the display which flickers between black and white. I have some code that displays the mouse position and it is able to go beyond what I specified for the vertical size of the display. I'll try to post some pictures later of the problem.