1ex12bit(3)                      Allegro manual                      ex12bit(3)
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NAME

6       ex12bit - How to fake a 12-bit truecolor mode on an 8-bit card. Allegro
7       game programming library.
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SYNOPSIS

10       #include <allegro.h>
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13       Example ex12bit
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DESCRIPTION

16       This program sets up a 12-bit mode on any 8-bit card, by setting  up  a
17       256-colour  palette that will fool the eye into grouping two 8-bit pix‐
18       els into one  12-bit  pixel.  In  order  to  do  this,  you  make  your
19       256-colour  palette with all the combinations of blue and green, assum‐
20       ing green ranges from 0-15 and blue from 0-14. This takes up  16x15=240
21       colours.  This  leaves 16 colours to use as red (red ranges from 0-15).
22       Then you put your green/blue in one pixel, and your red  in  the  pixel
23       next to it. The eye gets fooled into thinking it's all one pixel.
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25       The  example  starts  setting  a normal 256 color mode, and construct a
26       special palette for it. But then comes the trick: you need to write  to
27       a set of two adjacent pixels to form a single 12 bit dot. Two eight bit
28       pixels is the same as one 16 bit pixel, so after setting the video mode
29       you  need to hack the screen bitmap about, halving the width and chang‐
30       ing it to use the 16 bit drawing code. Then, once  you  have  packed  a
31       color  into  the correct format (using the makecol12() function below),
32       any of the normal Allegro drawing functions can be used  with  this  12
33       bit display!
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35       Things to note:
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37       The  horizontal  width  is halved, so you get resolutions like 320x480,
38       400x600, and 512x768.
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40       Because each dot is spread over two actual pixels, the display will  be
41       darker than in a normal video mode.
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43       Any  bitmap  data  will obviously need converting to the correct 12 bit
44       format: regular 15 or 16 bit images won't display correctly...
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46       Although this works like a truecolor mode, it is actually using  a  256
47       color palette, so palette fades are still possible!
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49       This code only works in linear screen modes (don't try Mode-X).
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SEE ALSO

53       BITMAP(3),   END_OF_MAIN(3),   MATRIX(3),   PALETTE(3),  RGB(3),  alle‐
54       gro_error(3),  allegro_init(3),  allegro_message(3),   apply_matrix(3),
55       blit(3),       circle(3),       clear_bitmap(3),       clear_keybuf(3),
56       clear_to_color(3), create_bitmap(3), create_bitmap_ex(3),  destroy_bit‐
57       map(3),  ellipsefill(3),  fade_out(3),  fixcos(3), fixed(3), fixsin(3),
58       fixtoi(3), font(3), get_rotation_matrix(3),  getpixel(3),  install_key‐
59       board(3),     itofix(3),     keypressed(3),     line(3),    makecol(3),
60       masked_blit(3),  putpixel(3),  rest(3),  screen(3),   set_clip_rect(3),
61       set_color(3),    set_gfx_mode(3),    set_palette(3),    text_height(3),
62       text_length(3), textout_ex(3), textprintf_ex(3)
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66Allegro                          version 4.4.3                      ex12bit(3)
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