Add a description document for tight encoding
Andri Yngvason
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/*
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* QEMU VNC display driver: tight encoding
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*
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* From libvncserver/rfb/rfbproto.h
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* Copyright (C) 2005 Rohit Kumar, Johannes E. Schindelin
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* Copyright (C) 2000-2002 Constantin Kaplinsky. All Rights Reserved.
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* Copyright (C) 2000 Tridia Corporation. All Rights Reserved.
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* Copyright (C) 1999 AT&T Laboratories Cambridge. All Rights Reserved.
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*
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
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* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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#ifndef VNC_ENC_TIGHT_H
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#define VNC_ENC_TIGHT_H
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/*- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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* Tight Encoding.
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*
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*-- The first byte of each Tight-encoded rectangle is a "compression control
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* byte". Its format is as follows (bit 0 is the least significant one):
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*
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* bit 0: if 1, then compression stream 0 should be reset;
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* bit 1: if 1, then compression stream 1 should be reset;
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* bit 2: if 1, then compression stream 2 should be reset;
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* bit 3: if 1, then compression stream 3 should be reset;
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* bits 7-4: if 1000 (0x08), then the compression type is "fill",
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* if 1001 (0x09), then the compression type is "jpeg",
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* if 1010 (0x0A), then the compression type is "png",
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* if 0xxx, then the compression type is "basic",
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* values greater than 1010 are not valid.
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*
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* If the compression type is "basic", then bits 6..4 of the
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* compression control byte (those xxx in 0xxx) specify the following:
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*
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* bits 5-4: decimal representation is the index of a particular zlib
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* stream which should be used for decompressing the data;
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* bit 6: if 1, then a "filter id" byte is following this byte.
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*
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*-- The data that follows after the compression control byte described
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* above depends on the compression type ("fill", "jpeg", "png" or "basic").
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*
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*-- If the compression type is "fill", then the only pixel value follows, in
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* client pixel format (see NOTE 1). This value applies to all pixels of the
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* rectangle.
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*
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*-- If the compression type is "jpeg" or "png", the following data stream
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* looks like this:
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*
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* 1..3 bytes: data size (N) in compact representation;
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* N bytes: JPEG or PNG image.
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*
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* Data size is compactly represented in one, two or three bytes, according
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* to the following scheme:
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*
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* 0xxxxxxx (for values 0..127)
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* 1xxxxxxx 0yyyyyyy (for values 128..16383)
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* 1xxxxxxx 1yyyyyyy zzzzzzzz (for values 16384..4194303)
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*
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* Here each character denotes one bit, xxxxxxx are the least significant 7
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* bits of the value (bits 0-6), yyyyyyy are bits 7-13, and zzzzzzzz are the
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* most significant 8 bits (bits 14-21). For example, decimal value 10000
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* should be represented as two bytes: binary 10010000 01001110, or
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* hexadecimal 90 4E.
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*
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*-- If the compression type is "basic" and bit 6 of the compression control
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* byte was set to 1, then the next (second) byte specifies "filter id" which
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* tells the decoder what filter type was used by the encoder to pre-process
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* pixel data before the compression. The "filter id" byte can be one of the
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* following:
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*
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* 0: no filter ("copy" filter);
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* 1: "palette" filter;
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* 2: "gradient" filter.
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*
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*-- If bit 6 of the compression control byte is set to 0 (no "filter id"
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* byte), or if the filter id is 0, then raw pixel values in the client
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* format (see NOTE 1) will be compressed. See below details on the
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* compression.
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*
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*-- The "gradient" filter pre-processes pixel data with a simple algorithm
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* which converts each color component to a difference between a "predicted"
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* intensity and the actual intensity. Such a technique does not affect
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* uncompressed data size, but helps to compress photo-like images better.
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* Pseudo-code for converting intensities to differences is the following:
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*
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* P[i,j] := V[i-1,j] + V[i,j-1] - V[i-1,j-1];
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* if (P[i,j] < 0) then P[i,j] := 0;
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* if (P[i,j] > MAX) then P[i,j] := MAX;
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* D[i,j] := V[i,j] - P[i,j];
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*
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* Here V[i,j] is the intensity of a color component for a pixel at
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* coordinates (i,j). MAX is the maximum value of intensity for a color
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* component.
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*
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*-- The "palette" filter converts true-color pixel data to indexed colors
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* and a palette which can consist of 2..256 colors. If the number of colors
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* is 2, then each pixel is encoded in 1 bit, otherwise 8 bits is used to
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* encode one pixel. 1-bit encoding is performed such way that the most
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* significant bits correspond to the leftmost pixels, and each raw of pixels
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* is aligned to the byte boundary. When "palette" filter is used, the
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* palette is sent before the pixel data. The palette begins with an unsigned
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* byte which value is the number of colors in the palette minus 1 (i.e. 1
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* means 2 colors, 255 means 256 colors in the palette). Then follows the
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* palette itself which consist of pixel values in client pixel format (see
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* NOTE 1).
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*
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*-- The pixel data is compressed using the zlib library. But if the data
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* size after applying the filter but before the compression is less then 12,
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* then the data is sent as is, uncompressed. Four separate zlib streams
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* (0..3) can be used and the decoder should read the actual stream id from
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* the compression control byte (see NOTE 2).
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*
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* If the compression is not used, then the pixel data is sent as is,
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* otherwise the data stream looks like this:
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*
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* 1..3 bytes: data size (N) in compact representation;
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* N bytes: zlib-compressed data.
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*
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* Data size is compactly represented in one, two or three bytes, just like
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* in the "jpeg" compression method (see above).
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*
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*-- NOTE 1. If the color depth is 24, and all three color components are
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* 8-bit wide, then one pixel in Tight encoding is always represented by
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* three bytes, where the first byte is red component, the second byte is
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* green component, and the third byte is blue component of the pixel color
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* value. This applies to colors in palettes as well.
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*
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*-- NOTE 2. The decoder must reset compression streams' states before
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* decoding the rectangle, if some of bits 0,1,2,3 in the compression control
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* byte are set to 1. Note that the decoder must reset zlib streams even if
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* the compression type is "fill", "jpeg" or "png".
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*
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*-- NOTE 3. The "gradient" filter and "jpeg" compression may be used only
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* when bits-per-pixel value is either 16 or 32, not 8.
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*
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*-- NOTE 4. The width of any Tight-encoded rectangle cannot exceed 2048
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* pixels. If a rectangle is wider, it must be split into several rectangles
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* and each one should be encoded separately.
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*
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*/
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#define VNC_TIGHT_EXPLICIT_FILTER 0x04
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#define VNC_TIGHT_FILL 0x08
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#define VNC_TIGHT_JPEG 0x09
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#define VNC_TIGHT_PNG 0x0A
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#define VNC_TIGHT_MAX_SUBENCODING 0x0A
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/* Filters to improve compression efficiency */
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#define VNC_TIGHT_FILTER_COPY 0x00
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#define VNC_TIGHT_FILTER_PALETTE 0x01
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#define VNC_TIGHT_FILTER_GRADIENT 0x02
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/* Note: The following constant should not be changed. */
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#define VNC_TIGHT_MIN_TO_COMPRESS 12
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/* The parameters below may be adjusted. */
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#define VNC_TIGHT_MIN_SPLIT_RECT_SIZE 4096
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#define VNC_TIGHT_MIN_SOLID_SUBRECT_SIZE 2048
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#define VNC_TIGHT_MAX_SPLIT_TILE_SIZE 16
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#define VNC_TIGHT_JPEG_MIN_RECT_SIZE 4096
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#define VNC_TIGHT_DETECT_SUBROW_WIDTH 7
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#define VNC_TIGHT_DETECT_MIN_WIDTH 8
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#define VNC_TIGHT_DETECT_MIN_HEIGHT 8
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#endif /* VNC_ENC_TIGHT_H */
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