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Add a description document for tight encoding

tight-png
Andri Yngvason 2019-12-29 21:35:38 +00:00
parent 52f5510467
commit 8ec33ab775
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doc/vnc-enc-tight.h 100644
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/*
* QEMU VNC display driver: tight encoding
*
* From libvncserver/rfb/rfbproto.h
* Copyright (C) 2005 Rohit Kumar, Johannes E. Schindelin
* Copyright (C) 2000-2002 Constantin Kaplinsky. All Rights Reserved.
* Copyright (C) 2000 Tridia Corporation. All Rights Reserved.
* Copyright (C) 1999 AT&T Laboratories Cambridge. All Rights Reserved.
*
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
*/
#ifndef VNC_ENC_TIGHT_H
#define VNC_ENC_TIGHT_H
/*- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
* Tight Encoding.
*
*-- The first byte of each Tight-encoded rectangle is a "compression control
* byte". Its format is as follows (bit 0 is the least significant one):
*
* bit 0: if 1, then compression stream 0 should be reset;
* bit 1: if 1, then compression stream 1 should be reset;
* bit 2: if 1, then compression stream 2 should be reset;
* bit 3: if 1, then compression stream 3 should be reset;
* bits 7-4: if 1000 (0x08), then the compression type is "fill",
* if 1001 (0x09), then the compression type is "jpeg",
* if 1010 (0x0A), then the compression type is "png",
* if 0xxx, then the compression type is "basic",
* values greater than 1010 are not valid.
*
* If the compression type is "basic", then bits 6..4 of the
* compression control byte (those xxx in 0xxx) specify the following:
*
* bits 5-4: decimal representation is the index of a particular zlib
* stream which should be used for decompressing the data;
* bit 6: if 1, then a "filter id" byte is following this byte.
*
*-- The data that follows after the compression control byte described
* above depends on the compression type ("fill", "jpeg", "png" or "basic").
*
*-- If the compression type is "fill", then the only pixel value follows, in
* client pixel format (see NOTE 1). This value applies to all pixels of the
* rectangle.
*
*-- If the compression type is "jpeg" or "png", the following data stream
* looks like this:
*
* 1..3 bytes: data size (N) in compact representation;
* N bytes: JPEG or PNG image.
*
* Data size is compactly represented in one, two or three bytes, according
* to the following scheme:
*
* 0xxxxxxx (for values 0..127)
* 1xxxxxxx 0yyyyyyy (for values 128..16383)
* 1xxxxxxx 1yyyyyyy zzzzzzzz (for values 16384..4194303)
*
* Here each character denotes one bit, xxxxxxx are the least significant 7
* bits of the value (bits 0-6), yyyyyyy are bits 7-13, and zzzzzzzz are the
* most significant 8 bits (bits 14-21). For example, decimal value 10000
* should be represented as two bytes: binary 10010000 01001110, or
* hexadecimal 90 4E.
*
*-- If the compression type is "basic" and bit 6 of the compression control
* byte was set to 1, then the next (second) byte specifies "filter id" which
* tells the decoder what filter type was used by the encoder to pre-process
* pixel data before the compression. The "filter id" byte can be one of the
* following:
*
* 0: no filter ("copy" filter);
* 1: "palette" filter;
* 2: "gradient" filter.
*
*-- If bit 6 of the compression control byte is set to 0 (no "filter id"
* byte), or if the filter id is 0, then raw pixel values in the client
* format (see NOTE 1) will be compressed. See below details on the
* compression.
*
*-- The "gradient" filter pre-processes pixel data with a simple algorithm
* which converts each color component to a difference between a "predicted"
* intensity and the actual intensity. Such a technique does not affect
* uncompressed data size, but helps to compress photo-like images better.
* Pseudo-code for converting intensities to differences is the following:
*
* P[i,j] := V[i-1,j] + V[i,j-1] - V[i-1,j-1];
* if (P[i,j] < 0) then P[i,j] := 0;
* if (P[i,j] > MAX) then P[i,j] := MAX;
* D[i,j] := V[i,j] - P[i,j];
*
* Here V[i,j] is the intensity of a color component for a pixel at
* coordinates (i,j). MAX is the maximum value of intensity for a color
* component.
*
*-- The "palette" filter converts true-color pixel data to indexed colors
* and a palette which can consist of 2..256 colors. If the number of colors
* is 2, then each pixel is encoded in 1 bit, otherwise 8 bits is used to
* encode one pixel. 1-bit encoding is performed such way that the most
* significant bits correspond to the leftmost pixels, and each raw of pixels
* is aligned to the byte boundary. When "palette" filter is used, the
* palette is sent before the pixel data. The palette begins with an unsigned
* byte which value is the number of colors in the palette minus 1 (i.e. 1
* means 2 colors, 255 means 256 colors in the palette). Then follows the
* palette itself which consist of pixel values in client pixel format (see
* NOTE 1).
*
*-- The pixel data is compressed using the zlib library. But if the data
* size after applying the filter but before the compression is less then 12,
* then the data is sent as is, uncompressed. Four separate zlib streams
* (0..3) can be used and the decoder should read the actual stream id from
* the compression control byte (see NOTE 2).
*
* If the compression is not used, then the pixel data is sent as is,
* otherwise the data stream looks like this:
*
* 1..3 bytes: data size (N) in compact representation;
* N bytes: zlib-compressed data.
*
* Data size is compactly represented in one, two or three bytes, just like
* in the "jpeg" compression method (see above).
*
*-- NOTE 1. If the color depth is 24, and all three color components are
* 8-bit wide, then one pixel in Tight encoding is always represented by
* three bytes, where the first byte is red component, the second byte is
* green component, and the third byte is blue component of the pixel color
* value. This applies to colors in palettes as well.
*
*-- NOTE 2. The decoder must reset compression streams' states before
* decoding the rectangle, if some of bits 0,1,2,3 in the compression control
* byte are set to 1. Note that the decoder must reset zlib streams even if
* the compression type is "fill", "jpeg" or "png".
*
*-- NOTE 3. The "gradient" filter and "jpeg" compression may be used only
* when bits-per-pixel value is either 16 or 32, not 8.
*
*-- NOTE 4. The width of any Tight-encoded rectangle cannot exceed 2048
* pixels. If a rectangle is wider, it must be split into several rectangles
* and each one should be encoded separately.
*
*/
#define VNC_TIGHT_EXPLICIT_FILTER 0x04
#define VNC_TIGHT_FILL 0x08
#define VNC_TIGHT_JPEG 0x09
#define VNC_TIGHT_PNG 0x0A
#define VNC_TIGHT_MAX_SUBENCODING 0x0A
/* Filters to improve compression efficiency */
#define VNC_TIGHT_FILTER_COPY 0x00
#define VNC_TIGHT_FILTER_PALETTE 0x01
#define VNC_TIGHT_FILTER_GRADIENT 0x02
/* Note: The following constant should not be changed. */
#define VNC_TIGHT_MIN_TO_COMPRESS 12
/* The parameters below may be adjusted. */
#define VNC_TIGHT_MIN_SPLIT_RECT_SIZE 4096
#define VNC_TIGHT_MIN_SOLID_SUBRECT_SIZE 2048
#define VNC_TIGHT_MAX_SPLIT_TILE_SIZE 16
#define VNC_TIGHT_JPEG_MIN_RECT_SIZE 4096
#define VNC_TIGHT_DETECT_SUBROW_WIDTH 7
#define VNC_TIGHT_DETECT_MIN_WIDTH 8
#define VNC_TIGHT_DETECT_MIN_HEIGHT 8
#endif /* VNC_ENC_TIGHT_H */