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170 lines
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HTML
170 lines
8 KiB
HTML
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<title>color(7) - Plan 9 from User Space</title>
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<meta content="text/html; charset=utf-8" http-equiv=Content-Type>
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</head>
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<body bgcolor=#ffffff>
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<table border=0 cellpadding=0 cellspacing=0 width=100%>
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<tr height=10><td>
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<tr><td width=20><td>
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<tr><td width=20><td><b>COLOR(7)</b><td align=right><b>COLOR(7)</b>
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<tr><td width=20><td colspan=2>
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<br>
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<p><font size=+1><b>NAME </b></font><br>
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<table border=0 cellpadding=0 cellspacing=0><tr height=2><td><tr><td width=20><td>
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color – representation of pixels and colors<br>
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</table>
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<p><font size=+1><b>DESCRIPTION </b></font><br>
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<table border=0 cellpadding=0 cellspacing=0><tr height=2><td><tr><td width=20><td>
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To address problems of consistency and portability among applications,
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Plan 9 uses a fixed color map, called <tt><font size=+1>rgbv</font></tt>, on 8-bit-per-pixel
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displays. Although this avoids problems caused by multiplexing
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color maps between applications, it requires that the color map
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chosen be suitable for most purposes and usable for
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all. Other systems that use fixed color maps tend to sample the
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color cube uniformly, which has advantages--mapping from a (red,
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green, blue) triple to the color map and back again is easy--but
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ignores an important property of the human visual system: eyes
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are much more sensitive to small changes in intensity than
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to changes in hue. Sampling the color cube uniformly gives a color
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map with many different hues, but only a few shades of each. Continuous
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tone images converted into such maps demonstrate conspicuous artifacts.
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<table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table>
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Rather than dice the color cube into subregions of size 6×6×6 (as
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in Netscape Navigator) or 8×8×4 (as in previous releases of Plan
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9), picking 1 color in each, the <tt><font size=+1>rgbv</font></tt> color map uses a 4×4×4 subdivision,
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with 4 shades in each subcube. The idea is to reduce the color
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resolution by dicing the color cube into fewer
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cells, and to use the extra space to increase the intensity resolution.
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This results in 16 grey shades (4 grey subcubes with 4 samples
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in each), 13 shades of each primary and secondary color (3 subcubes
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with 4 samples plus black) and a reasonable selection of colors
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covering the rest of the color cube. The advantage is
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better representation of continuous tones.
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<table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table>
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The following function computes the 256 3-byte entries in the
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color map:<br>
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<table border=0 cellpadding=0 cellspacing=0><tr height=2><td><tr><td width=20><td>
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<tt><font size=+1>void<br>
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setmaprgbv(uchar cmap[256][3])<br>
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{<br>
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<table border=0 cellpadding=0 cellspacing=0><tr height=2><td><tr><td width=20><td>
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uchar *c;<br>
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int r, g, b, v;<br>
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int num, den;<br>
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int i, j;<br>
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for(r=0,i=0; r!=4; r++)<br>
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for(v=0; v!=4; v++,i+=16)<br>
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for(g=0,j=v−r; g!=4; g++)<br>
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for(b=0; b!=4; b++,j++){<br>
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c = cmap[i+(j&15)];<br>
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den = r;<br>
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if(g > den)<br>
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den = g;<br>
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if(b > den)<br>
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den = b;<br>
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if(den == 0) /* would divide check; pick grey shades */<br>
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c[0] = c[1] = c[2] = 17*v;<br>
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else{<br>
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num = 17*(4*den+v);<br>
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c[0] = r*num/den;<br>
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c[1] = g*num/den;<br>
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c[2] = b*num/den;<br>
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}<br>
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}<br>
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</table>
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}<br>
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<table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table>
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</font></tt>
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</table>
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There are 4 nested loops to pick the (red,green,blue) coordinates
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of the subcube, and the value (intensity) within the subcube,
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indexed by <tt><font size=+1>r</font></tt>, <tt><font size=+1>g</font></tt>, <tt><font size=+1>b</font></tt>, and <tt><font size=+1>v</font></tt>, whence the name <i>rgbv</i>. The peculiar
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order in which the color map is indexed is designed to distribute
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the grey shades uniformly through the map--the <i>i</i>’th grey
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shade, 0<=<i>i</i><=15 has index <i>i</i>x17, with black going to 0 and white to
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255. Therefore, when a call to <tt><font size=+1>draw</font></tt> converts a 1, 2 or 4 bit-per-pixel
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picture to 8 bits per pixel (which it does by replicating the
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pixels’ bits), the converted pixel values are the appropriate
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grey shades.
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<table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table>
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The <tt><font size=+1>rgbv</font></tt> map is not gamma-corrected, for two reasons. First, photographic
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film and television are both normally under-corrected, the former
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by an accident of physics and the latter by NTSC’s design. Second,
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we require extra color resolution at low intensities because of
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the non-linear response and adaptation of
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the human visual system. Properly gamma-corrected displays with
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adequate low-intensity resolution pack the high-intensity parts
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of the color cube with colors whose differences are almost imperceptible.
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Either reason suggests concentrating the available intensities
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at the low end of the range.
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<table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table>
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On ‘true-color’ displays with separate values for the red, green,
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and blue components of a pixel, the values are chosen so 0 represents
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no intensity (black) and the maximum value (255 for an 8-bit-per-color
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display) represents full intensity (e.g., full red). Common display
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depths are 24 bits per pixel, with 8 bits per
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color in order red, green, blue, and 16 bits per pixel, with 5
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bits of red, 6 bits of green, and 5 bits of blue.
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<table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table>
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Colors may also be created with an opacity factor called <tt><font size=+1>alpha</font></tt>,
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which is scaled so 0 represents fully transparent and 255 represents
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opaque color. The alpha is <i>premultiplied</i> into the other channels,
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as described in the paper by Porter and Duff cited in <a href="../man3/draw.html"><i>draw</i>(3)</a>.
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The function <tt><font size=+1>setalpha</font></tt> (see <a href="../man3/allocimage.html"><i>allocimage</i>(3)</a>) aids the
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initialization of color values with non-trivial alpha.
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<table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table>
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The packing of pixels into bytes and words is odd. For compatibility
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with VGA frame buffers, the bits within a pixel byte are in big-endian
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order (leftmost pixel is most significant bits in byte), while
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bytes within a pixel are packed in little-endian order. Pixels
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are stored in contiguous bytes. This results in unintuitive
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pixel formats. For example, for the RGB24 format, the byte ordering
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is blue, green, red.
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<table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table>
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To maintain a constant external representation, the <a href="../man3/draw.html"><i>draw</i>(3)</a> interface
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as well as the various graphics libraries represent colors by
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32-bit numbers, as described in <a href="../man3/color.html"><i>color</i>(3)</a>.<br>
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</table>
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<p><font size=+1><b>SEE ALSO </b></font><br>
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<table border=0 cellpadding=0 cellspacing=0><tr height=2><td><tr><td width=20><td>
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<a href="../man3/color.html"><i>color</i>(3)</a>, <a href="../man3/graphics.html"><i>graphics</i>(3)</a>, <a href="../man3/draw.html"><i>draw</i>(3)</a><br>
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</table>
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<td width=20>
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<tr height=20><td>
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</table>
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<!-- TRAILER -->
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<table border=0 cellpadding=0 cellspacing=0 width=100%>
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<tr height=15><td width=10><td><td width=10>
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<tr><td><td>
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<center>
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<a href="../../"><img src="../../dist/spaceglenda100.png" alt="Space Glenda" border=1></a>
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</center>
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</table>
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<!-- TRAILER -->
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</body></html>
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