Microsoft RLE

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This is a description of Microsoft's run length encoding (RLE) data format, as excerpted from their official documentation. MS RLE data can occur in both static BMP images and AVI animations.

Bitmap Compression

Windows versions 3.0 and later support run-length encoded (RLE) formats for compressing bitmaps that use 4 bits per pixel and 8 bits per pixel. Compression reduces the disk and memory storage required for a bitmap.

Compression of 8-Bits-per-Pixel Bitmaps

When the biCompression member of the BITMAPINFOHEADER structure is set to BI_RLE8, the DIB is compressed using a run-length encoded format for a 256-color bitmap. This format uses two modes: encoded mode and absolute mode. Both modes can occur anywhere throughout a single bitmap.

Encoded Mode

A unit of information in encoded mode consists of two bytes. The first byte specifies the number of consecutive pixels to be drawn using the color index contained in the second byte. The first byte of the pair can be set to zero to indicate an escape that denotes the end of a line, the end of the bitmap, or a delta. The interpretation of the escape depends on the value of the second byte of the pair, which must be in the range 0x00 through 0x02. Following are the meanings of the escape values that can be used in the second byte:

Second byte     Meaning

        0       End of line.
        1       End of bitmap.
        2       Delta. The two bytes following the escape contain unsigned
                values indicating the horizontal and vertical offsets of 
                the next pixel from the current position.

Absolute Mode

Absolute mode is signaled by the first byte in the pair being set to zero and the second byte to a value between 0x03 and 0xFF. The second byte represents the number of bytes that follow, each of which contains the color index of a single pixel. Each run must be aligned on a word boundary. Following is an example of an 8-bit RLE bitmap (the two-digit hexadecimal values in the second column represent a color index for a single pixel):

Compressed data         Expanded data

03 04                   04 04 04
05 06                   06 06 06 06 06
00 03 45 56 67 00       45 56 67
02 78                   78 78
00 02 05 01             Move 5 right and 1 down
02 78                   78 78
00 00                   End of line
09 1E                   1E 1E 1E 1E 1E 1E 1E 1E 1E
00 01                   End of RLE bitmap

Compression of 4-Bits-per-Pixel Bitmaps

When the biCompression member of the BITMAPINFOHEADER structure is set to BI_RLE4, the DIB is compressed using a run-length encoded format for a 16-color bitmap. This format uses two modes: encoded mode and absolute mode.

Encoded Mode

A unit of information in encoded mode consists of two bytes. The first byte of the pair contains the number of pixels to be drawn using the color indexes in the second byte.

The second byte contains two color indexes, one in its high-order nibble (that is, its low-order 4 bits) and one in its low-order nibble.

The first pixel is drawn using the color specified by the high-order nibble, the second is drawn using the color in the low-order nibble, the third is drawn with the color in the high-order nibble, and so on, until all the pixels specified by the first byte have been drawn.

The first byte of the pair can be set to zero to indicate an escape that denotes the end of a line, the end of the bitmap, or a delta. The interpretation of the escape depends on the value of the second byte of the pair. In encoded mode, the second byte has a value in the range 0x00 through 0x02. The meaning of these values is the same as for a DIB with 8 bits per pixel.

Absolute Mode

In absolute mode, the first byte contains zero, the second byte contains the number of color indexes that follow, and subsequent bytes contain color indexes in their high- and low-order nibbles, one color index for each pixel. Each run must be aligned on a word boundary. Following is an example of a 4-bit RLE bitmap (the one-digit hexadecimal values in the second column represent a color index for a single pixel):

Compressed data         Expanded data

03 04                   0 4 0
05 06                   0 6 0 6 0
00 06 45 56 67 00       4 5 5 6 6 7
04 78                   7 8 7 8
00 02 05 01             Move 5 right and 1 down
04 78                   7 8 7 8
00 00                   End of line
09 1E                   1 E 1 E 1 E 1 E 1
00 01                   End of RLE bitmap