qemu/ui/vnc-enc-tight.h

6  * Copyright (C) 2000-2002 Constantin Kaplinsky.  All Rights Reserved.
33 /*- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
36  *-- The first byte of each Tight-encoded rectangle is a "compression control
43  *   bits 7-4: if 1000 (0x08), then the compression type is "fill",
49  * If the compression type is "basic", then bits 6..4 of the
52  *   bits 5-4:  decimal representation is the index of a particular zlib
53  *              stream which should be used for decompressing the data;
56  *-- The data that follows after the compression control byte described
59  *-- If the compression type is "fill", then the only pixel value follows, in
63  *-- If the compression type is "jpeg" or "png", the following data stream
66  *   1..3 bytes:  data size (N) in compact representation;
69  * Data size is compactly represented in one, two or three bytes, according
77  * bits of the value (bits 0-6), yyyyyyy are bits 7-13, and zzzzzzzz are the
78  * most significant 8 bits (bits 14-21). For example, decimal value 10000
82  *-- If the compression type is "basic" and bit 6 of the compression control
84  * tells the decoder what filter type was used by the encoder to pre-process
85  * pixel data before the compression. The "filter id" byte can be one of the
92  *-- If bit 6 of the compression control byte is set to 0 (no "filter id"
97  *-- The "gradient" filter pre-processes pixel data with a simple algorithm
100  * uncompressed data size, but helps to compress photo-like images better.
101  * Pseudo-code for converting intensities to differences is the following:
103  *   P[i,j] := V[i-1,j] + V[i,j-1] - V[i-1,j-1];
106  *   D[i,j] := V[i,j] - P[i,j];
112  *-- The "palette" filter converts true-color pixel data to indexed colors
114  * is 2, then each pixel is encoded in 1 bit, otherwise 8 bits is used to
115  * encode one pixel. 1-bit encoding is performed such way that the most
116  * significant bits correspond to the leftmost pixels, and each raw of pixels
118  * palette is sent before the pixel data. The palette begins with an unsigned
124  *-- The pixel data is compressed using the zlib library. But if the data
126  * then the data is sent as is, uncompressed. Four separate zlib streams
130  * If the compression is not used, then the pixel data is sent as is,
131  * otherwise the data stream looks like this:
133  *   1..3 bytes:  data size (N) in compact representation;
134  *   N bytes:     zlib-compressed data.
136  * Data size is compactly represented in one, two or three bytes, just like
139  *-- NOTE 1. If the color depth is 24, and all three color components are
140  * 8-bit wide, then one pixel in Tight encoding is always represented by
145  *-- NOTE 2. The decoder must reset compression streams' states before
146  * decoding the rectangle, if some of bits 0,1,2,3 in the compression control
150  *-- NOTE 3. The "gradient" filter and "jpeg" compression may be used only
151  * when bits-per-pixel value is either 16 or 32, not 8.
153  *-- NOTE 4. The width of any Tight-encoded rectangle cannot exceed 2048