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Allegro Manual: Converting between color formats
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<h1><a name="Converting between color formats">Converting between color formats</a></h1>
<ul>
<li><a href="#bestfit_color">bestfit_color</a> — Finds a palette color fitting the requested RGB values.
<li><a href="#create_rgb_table">create_rgb_table</a> — Generates an RGB mapping table with lookup data for a palette.
<li><a href="#hsv_to_rgb">hsv_to_rgb</a> — Converts color values between the HSV and RGB color spaces.
<li><a href="#rgb_map">rgb_map</a> — Look up table to speed up reducing RGB values to palette colors.
<li><a href="#rgb_to_hsv">rgb_to_hsv</a> — Converts color values between the HSV and RGB color spaces.
</ul>
<p>
In general, Allegro is designed to be used in only one color depth at a
time, so you will call set_color_depth() once and then store all your
bitmaps in the same format. If you want to mix several different pixel
formats, you can use create_bitmap_ex() in place of create_bitmap(), and
call bitmap_color_depth() to query the format of a specific image. Most of
the graphics routines require all their input parameters to be in the same
format (eg. you cannot stretch a 15-bit source bitmap onto a 24-bit
destination), but there are some exceptions:
<ul><li>
blit() and the rotation routines can copy between bitmaps of any format,
converting the data as required.
<li>
draw_sprite() can draw 256-color source images onto destinations of any
format.
<li>
draw_character_ex() _always_ uses a 256-color source bitmap, whatever the
format of the destination.
<li>
The draw_trans_sprite() and draw_trans_rle_sprite() functions are able to
draw 32-bit RGBA images onto any hicolor or truecolor destination, as long
as you call set_alpha_blender() first.
<li>
The draw_trans_sprite() function is able to draw an 8-bit alpha channel
image over the top of an existing 32-bit image, as long as you call
set_write_alpha_blender() first.
</ul>
Expanding a 256-color source onto a truecolor destination is fairly fast
(obviously you must set the correct palette before doing this conversion!).
Converting between different truecolor formats is slightly slower, and
reducing truecolor images to a 256-color destination is very slow (it can be
sped up significantly if you set up the global rgb_map table before doing
the conversion).
<p><br>
<div class="al-api"><b>int <a name="bestfit_color">bestfit_color</a>(const <a class="autotype" href="alleg001.html#PALETTE" title="Stores palette information.">PALETTE</a> pal, int r, int g, int b);</b></div><br>
Searches the specified palette for the closest match to the requested
color, which are specified in the VGA hardware 0-63 format. Normally you
should call makecol8() instead, but this lower level function may be
useful if you need to use a palette other than the currently selected
one, or specifically don't want to use the rgb_map lookup table.
<p><b>Return value:</b>
Returns the index of the palette for the closest match to the requested
color.
<blockquote class="xref"><em><b>See also:</b></em>
<a class="xref" href="alleg012.html#makecol8" title="Converts an RGB value into a display dependent pixel format.">makecol8</a>.</blockquote>
<div class="al-api"><b>extern <a class="autotype" href="alleg001.html#RGB_MAP" title="Stores an rgb map to accelerate conversions.">RGB_MAP</a> *<a name="rgb_map">rgb_map</a>;</b></div><br>
To speed up reducing RGB values to 8-bit paletted colors, Allegro uses a
32k lookup table (5 bits for each color component). You must set up this
table before using the gouraud shading routines, and if present the table
will also vastly accelerate the makecol8() and some create_*_table()
functions. RGB tables can be precalculated with the rgbmap utility, or
generated at runtime with create_rgb_table().
<blockquote class="xref"><em><b>See also:</b></em>
<a class="xref" href="#create_rgb_table" title="Generates an RGB mapping table with lookup data for a palette.">create_rgb_table</a>,
<a class="xref" href="alleg012.html#makecol8" title="Converts an RGB value into a display dependent pixel format.">makecol8</a>,
<a class="xref" href="alleg020.html#create_trans_table" title="Fills a color mapping table for translucency effects.">create_trans_table</a>,
<a class="xref" href="alleg020.html#create_light_table" title="Fills a color mapping table for lighting effects.">create_light_table</a>,
<a class="xref" href="alleg020.html#create_color_table" title="Fills a color mapping table for customised effects.">create_color_table</a>.</blockquote>
<blockquote class="eref"><em><b>Examples using this:</b></em>
<a class="eref" href="alleg045.html#ex3d" title="3d 'bouncy cubes' demo.">ex3d</a>,
<a class="eref" href="alleg045.html#excolmap" title="Creating graphical effects with color mapping tables.">excolmap</a>,
<a class="eref" href="alleg045.html#exrgbhsv" title="RGB <-> HSV color space conversions.">exrgbhsv</a>,
<a class="eref" href="alleg045.html#exshade" title="Gouraud shaded sprites.">exshade</a>,
<a class="eref" href="alleg045.html#extrans" title="Lighting and translucency effects.">extrans</a>.</blockquote>
<div class="al-api"><b>void <a name="create_rgb_table">create_rgb_table</a>(<a class="autotype" href="alleg001.html#RGB_MAP" title="Stores an rgb map to accelerate conversions.">RGB_MAP</a> *table, const <a class="autotype" href="alleg001.html#PALETTE" title="Stores palette information.">PALETTE</a> pal,
void (*callback)(int pos));</b></div><br>
Fills the specified RGB mapping table with lookup data for the specified
palette. If the callback function is not NULL, it will be called 256
times during the calculation, allowing you to display a progress
indicator. Example:
<blockquote class="code"><pre>
<a href="alleg001.html#RGB_MAP" class="autotype" title="Stores an rgb map to accelerate conversions.">RGB_MAP</a> rgb_table;
<a href="#create_rgb_table" class="autotype" title="Generates an RGB mapping table with lookup data for a palette.">create_rgb_table</a>(&rgb_table, palette, NULL);
<a href="#rgb_map" class="autotype" title="Look up table to speed up reducing RGB values to palette colors.">rgb_map</a> = &rgb_table;</pre></blockquote>
<blockquote class="xref"><em><b>See also:</b></em>
<a class="xref" href="#rgb_map" title="Look up table to speed up reducing RGB values to palette colors.">rgb_map</a>.</blockquote>
<blockquote class="eref"><em><b>Examples using this:</b></em>
<a class="eref" href="alleg045.html#ex3d" title="3d 'bouncy cubes' demo.">ex3d</a>,
<a class="eref" href="alleg045.html#excolmap" title="Creating graphical effects with color mapping tables.">excolmap</a>,
<a class="eref" href="alleg045.html#exrgbhsv" title="RGB <-> HSV color space conversions.">exrgbhsv</a>,
<a class="eref" href="alleg045.html#exshade" title="Gouraud shaded sprites.">exshade</a>,
<a class="eref" href="alleg045.html#extrans" title="Lighting and translucency effects.">extrans</a>.</blockquote>
<div class="al-api"><b>void <a name="hsv_to_rgb">hsv_to_rgb</a>(float h, float s, float v, int *r, int *g, int *b);</b></div><br>
<div class="al-api-cont"><b>void <a name="rgb_to_hsv">rgb_to_hsv</a>(int r, int g, int b, float *h, float *s, float *v);</b></div><br>
Convert color values between the HSV and RGB color spaces. The RGB values
range from 0 to 255, hue is from 0 to 360, and saturation and value are
from 0 to 1. Example:
<blockquote class="code"><pre>
int r, g, b;
float hue, saturation, value;
...
/* Convert a reddish color to HSV format. */
<a href="#rgb_to_hsv" class="autotype" title="Converts color values between the HSV and RGB color spaces.">rgb_to_hsv</a>(255, 0, 128, &hue, &saturation, &value);
/* Now put our tin foil hat, and verify that. */
<a href="#hsv_to_rgb" class="autotype" title="Converts color values between the HSV and RGB color spaces.">hsv_to_rgb</a>(hue, saturation, value, &r, &g, &b);
<a href="alleg044.html#ASSERT" class="autotype" title="Debugging helper macro to assert.">ASSERT</a>(r == 255);
<a href="alleg044.html#ASSERT" class="autotype" title="Debugging helper macro to assert.">ASSERT</a>(g == 0);
<a href="alleg044.html#ASSERT" class="autotype" title="Debugging helper macro to assert.">ASSERT</a>(b == 128);</pre></blockquote>
<blockquote class="eref"><em><b>Examples using this:</b></em>
<a class="eref" href="alleg045.html#exlights" title="One way to do colored lighting effects in a hicolor video mode.">exlights</a>,
<a class="eref" href="alleg045.html#exrgbhsv" title="RGB <-> HSV color space conversions.">exrgbhsv</a>.</blockquote>
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