From 78e51a8c6678b6e3dff3d619aa786669f531f4bc Mon Sep 17 00:00:00 2001 From: rsc Date: Fri, 14 Jan 2005 03:45:44 +0000 Subject: checkpoint --- man/man7/color.html | 169 ++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 169 insertions(+) create mode 100644 man/man7/color.html (limited to 'man/man7/color.html') 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 @@ + +color(7) - Plan 9 from User Space + + + + +
+
+
COLOR(7)COLOR(7) +
+
+

NAME
+ +
+ + color – representation of pixels and colors
+ +
+

DESCRIPTION
+ +
+ + To address problems of consistency and portability among applications, + Plan 9 uses a fixed color map, called rgbv, 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. + +
+ + 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 rgbv 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. +
+ + The following function computes the 256 3-byte entries in the + color map:
+ +
+ + void
+ setmaprgbv(uchar cmap[256][3])
+ {
+ +
+ + uchar *c;
+ int r, g, b, v;
+ int num, den;
+ int i, j;
+ for(r=0,i=0; r!=4; r++)
+ for(v=0; v!=4; v++,i+=16)
+ for(g=0,j=v−r; g!=4; g++)
+ for(b=0; b!=4; b++,j++){
+ c = cmap[i+(j&15)];
+ den = r;
+ if(g > den)
+ den = g;
+ if(b > den)
+ den = b;
+ if(den == 0) /* would divide check; pick grey shades */
+ c[0] = c[1] = c[2] = 17*v;
+ else{
+ num = 17*(4*den+v);
+ c[0] = r*num/den;
+ c[1] = g*num/den;
+ c[2] = b*num/den;
+ }
+ }
+ +
+ }
+ +
+ + +
+ There are 4 nested loops to pick the (red,green,blue) coordinates + of the subcube, and the value (intensity) within the subcube, + indexed by r, g, b, and v, whence the name rgbv. The peculiar + order in which the color map is indexed is designed to distribute + the grey shades uniformly through the map--the i’th grey + shade, 0<=i<=15 has index ix17, with black going to 0 and white to + 255. Therefore, when a call to draw 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. +
+ + The rgbv 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. +
+ + 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. +
+ + Colors may also be created with an opacity factor called alpha, + which is scaled so 0 represents fully transparent and 255 represents + opaque color. The alpha is premultiplied into the other channels, + as described in the paper by Porter and Duff cited in draw(3). + The function setalpha (see allocimage(3)) aids the + initialization of color values with non-trivial alpha. +
+ + 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. +
+ + To maintain a constant external representation, the draw(3) interface + as well as the various graphics libraries represent colors by + 32-bit numbers, as described in color(3).
+ +
+

SEE ALSO
+ +
+ + color(3), graphics(3), draw(3)
+ +
+ +

 +
+
+ + +
+
+
+Space Glenda +
+
+ + -- cgit v1.2.3