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diff --git a/man/man7/color.html b/man/man7/color.html new file mode 100644 index 00000000..262e4633 --- /dev/null +++ b/man/man7/color.html @@ -0,0 +1,169 @@ +<head> +<title>color(7) - Plan 9 from User Space</title> +<meta content="text/html; charset=utf-8" http-equiv=Content-Type> +</head> +<body bgcolor=#ffffff> +<table border=0 cellpadding=0 cellspacing=0 width=100%> +<tr height=10><td> +<tr><td width=20><td> +<tr><td width=20><td><b>COLOR(7)</b><td align=right><b>COLOR(7)</b> +<tr><td width=20><td colspan=2> + <br> +<p><font size=+1><b>NAME </b></font><br> + +<table border=0 cellpadding=0 cellspacing=0><tr height=2><td><tr><td width=20><td> + + color – representation of pixels and colors<br> + +</table> +<p><font size=+1><b>DESCRIPTION </b></font><br> + +<table border=0 cellpadding=0 cellspacing=0><tr height=2><td><tr><td width=20><td> + + To address problems of consistency and portability among applications, + Plan 9 uses a fixed color map, called <tt><font size=+1>rgbv</font></tt>, on 8-bit-per-pixel + displays. Although this avoids problems caused by multiplexing + color maps between applications, it requires that the color map + chosen be suitable for most purposes and usable for + all. Other systems that use fixed color maps tend to sample the + color cube uniformly, which has advantages--mapping from a (red, + green, blue) triple to the color map and back again is easy--but + ignores an important property of the human visual system: eyes + are much more sensitive to small changes in intensity than + to changes in hue. Sampling the color cube uniformly gives a color + map with many different hues, but only a few shades of each. Continuous + tone images converted into such maps demonstrate conspicuous artifacts. + + <table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table> + + Rather than dice the color cube into subregions of size 6×6×6 (as + in Netscape Navigator) or 8×8×4 (as in previous releases of Plan + 9), picking 1 color in each, the <tt><font size=+1>rgbv</font></tt> color map uses a 4×4×4 subdivision, + with 4 shades in each subcube. The idea is to reduce the color + resolution by dicing the color cube into fewer + cells, and to use the extra space to increase the intensity resolution. + This results in 16 grey shades (4 grey subcubes with 4 samples + in each), 13 shades of each primary and secondary color (3 subcubes + with 4 samples plus black) and a reasonable selection of colors + covering the rest of the color cube. The advantage is + better representation of continuous tones. + <table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table> + + The following function computes the 256 3-byte entries in the + color map:<br> + + <table border=0 cellpadding=0 cellspacing=0><tr height=2><td><tr><td width=20><td> + + <tt><font size=+1>void<br> + setmaprgbv(uchar cmap[256][3])<br> + {<br> + + <table border=0 cellpadding=0 cellspacing=0><tr height=2><td><tr><td width=20><td> + + uchar *c;<br> + int r, g, b, v;<br> + int num, den;<br> + int i, j;<br> + for(r=0,i=0; r!=4; r++)<br> + for(v=0; v!=4; v++,i+=16)<br> + for(g=0,j=v−r; g!=4; g++)<br> + for(b=0; b!=4; b++,j++){<br> + c = cmap[i+(j&15)];<br> + den = r;<br> + if(g > den)<br> + den = g;<br> + if(b > den)<br> + den = b;<br> + if(den == 0) /* would divide check; pick grey shades */<br> + c[0] = c[1] = c[2] = 17*v;<br> + else{<br> + num = 17*(4*den+v);<br> + c[0] = r*num/den;<br> + c[1] = g*num/den;<br> + c[2] = b*num/den;<br> + }<br> + }<br> + + </table> + }<br> + + <table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table> + </font></tt> + + </table> + There are 4 nested loops to pick the (red,green,blue) coordinates + of the subcube, and the value (intensity) within the subcube, + 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 + order in which the color map is indexed is designed to distribute + the grey shades uniformly through the map--the <i>i</i>’th grey + shade, 0<=<i>i</i><=15 has index <i>i</i>x17, with black going to 0 and white to + 255. Therefore, when a call to <tt><font size=+1>draw</font></tt> converts a 1, 2 or 4 bit-per-pixel + picture to 8 bits per pixel (which it does by replicating the + pixels’ bits), the converted pixel values are the appropriate + grey shades. + <table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table> + + The <tt><font size=+1>rgbv</font></tt> map is not gamma-corrected, for two reasons. First, photographic + film and television are both normally under-corrected, the former + by an accident of physics and the latter by NTSC’s design. Second, + we require extra color resolution at low intensities because of + the non-linear response and adaptation of + the human visual system. Properly gamma-corrected displays with + adequate low-intensity resolution pack the high-intensity parts + of the color cube with colors whose differences are almost imperceptible. + Either reason suggests concentrating the available intensities + at the low end of the range. + <table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table> + + On ‘true-color’ displays with separate values for the red, green, + and blue components of a pixel, the values are chosen so 0 represents + no intensity (black) and the maximum value (255 for an 8-bit-per-color + display) represents full intensity (e.g., full red). Common display + depths are 24 bits per pixel, with 8 bits per + color in order red, green, blue, and 16 bits per pixel, with 5 + bits of red, 6 bits of green, and 5 bits of blue. + <table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table> + + Colors may also be created with an opacity factor called <tt><font size=+1>alpha</font></tt>, + which is scaled so 0 represents fully transparent and 255 represents + opaque color. The alpha is <i>premultiplied</i> into the other channels, + as described in the paper by Porter and Duff cited in <a href="../man3/draw.html"><i>draw</i>(3)</a>. + The function <tt><font size=+1>setalpha</font></tt> (see <a href="../man3/allocimage.html"><i>allocimage</i>(3)</a>) aids the + initialization of color values with non-trivial alpha. + <table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table> + + The packing of pixels into bytes and words is odd. For compatibility + with VGA frame buffers, the bits within a pixel byte are in big-endian + order (leftmost pixel is most significant bits in byte), while + bytes within a pixel are packed in little-endian order. Pixels + are stored in contiguous bytes. This results in unintuitive + pixel formats. For example, for the RGB24 format, the byte ordering + is blue, green, red. + <table border=0 cellpadding=0 cellspacing=0><tr height=5><td></table> + + To maintain a constant external representation, the <a href="../man3/draw.html"><i>draw</i>(3)</a> interface + as well as the various graphics libraries represent colors by + 32-bit numbers, as described in <a href="../man3/color.html"><i>color</i>(3)</a>.<br> + +</table> +<p><font size=+1><b>SEE ALSO </b></font><br> + +<table border=0 cellpadding=0 cellspacing=0><tr height=2><td><tr><td width=20><td> + + <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> + +</table> + +<td width=20> +<tr height=20><td> +</table> +<!-- TRAILER --> +<table border=0 cellpadding=0 cellspacing=0 width=100%> +<tr height=15><td width=10><td><td width=10> +<tr><td><td> +<center> +<a href="../../"><img src="../../dist/spaceglenda100.png" alt="Space Glenda" border=1></a> +</center> +</table> +<!-- TRAILER --> +</body></html> |