Admin at 15:57, 20 September 2015

2015-09-20T15:57:15Z

← Older revision		Revision as of 17:57, 20 September 2015
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			* http://forum.doom9.org/showthread.php?p=1509153#post1509153
			* http://www.virtualdub.org/blog/pivot/entry.php?id=151
			* https://engineering.purdue.edu/~bouman/ece637/notes/pdf/Halftoning.pdf (page 11)

			It is ordered input dithering with a 02/31 recursive Bayer pattern (contrast normal 13/42 pattern) modified for equal sums in both rows and columns. Avery

			Lee described the recursive generation in his blog a while ago on Dithering. I modified the resultant pattern for equal summing to eliminate an obvious

			pattern visible in 16x16 cells.

			The dithering is added as an extra lower 8 bits (4bits for chroma) on the input pixels, making 16bit data. This is then used as an index into a 65K LUT to

			get the output 8 bit pixel. The dither pattern effectively replaces the 0.5 rounding term in generating the LUT.

			without dithering:

			i = 0..255
			mapR[i] = int(min(max(i * r/255.0, 0.0), 1.0) * 255.0 + 0.5);
			example: r=2, i=16 => mapR[16] = int(32/255.0 * 255.0 + 0.5) = 32

			for (int y=0; y<vi.height; ++y) {
			for (int x=0; x<vi.width; ++x) {
			p[x] = map[p[x]];
			}
			p += pitch;
			}

			with dithering:

			i = 0..256*256-1
			mapR[i] = int(min(max(i * r - 127.5)/(255.0256), 0.0), 1.0) 255.0 + 0.5);
			example: r=2, i=16256 => mapR[16256] = int((32256 - 127.5)/(255.0256) * 255.0 + 0.5) = 32
			bias = -127.5 ??

			for (int y=0; y<vi.height; ++y) {
			const int _y = (y << 4) & 0xf0;
			for (int x=0; x<vi.width; ++x) {
			p[x] = map[ p[x]<<8 \| ditherMap[(x&0x0f)\|_y] ];
			}
			p += pitch;
			}

			// 16x16 dither table:
			_y = (y << 4) & 0xf0 = ...
			ditherMap[(x&0x0f)\|_y] = ...

			y=15+3 => _y = (y << 4) & 0xf0 = 18/2^4 & (f16 + 01) = 18/16 & 15*16 = 1 & 1111 0000 = 0
			y=1516 => _y = (y << 4) & 0xf0 = 1516/2^4 & (f16 + 01) = 15 & 15*16 = 1111 & 1111 0000 = 0
			ditherMap[(x&0x0f)\|_y] = ditherMap[(x & 1111) \| 0] = ditherMap[x & 1111]
			y=1616 => _y = (y << 4) & 0xf0 = 1616/2^4 & (f16 + 01) = 16 & 15*16 = 1 0000 & 1111 0000 = 1
			ditherMap[(x&0x0f)\|_y] = ditherMap[(x & 1111) \| 1]
			* so each 256 pixels, the offset in ditherMap is shifted by one.

			// 4x4 dither table:
			const int _y = (y << 2) & 0xC;
			ditherMap4[(x&0x3)\|_y];

			http://web.archive.org/web/20130512190753/http://white.stanford.edu/~brian/psy221/reader/Bayer.1973.pdf

	[[Category:Glossary]]		[[Category:Glossary]]

Admin at 22:00, 23 June 2014

2014-06-23T22:00:59Z

← Older revision		Revision as of 00:00, 24 June 2014
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	todo		todo

			[[Category:Glossary]]

Admin: Created page with "todo"

2014-06-23T21:59:15Z

Created page with "todo"

New page

todo

Ordered dithering - Revision history

Admin at 15:57, 20 September 2015

Admin at 22:00, 23 June 2014

Admin: Created page with "todo"