3.1.9.2.3 Decoding Run-Length Sequences

Encoding of run-length sequences ensures that there is at least one subsegment per scan-line. The control byte described in section 2.2.2.5.1.2 contains all of the information necessary to decode the sequence bytes.

The process of decoding an encoded sequence of bytes for a color plane using RDP 6.0 RLE is best illustrated with a practical example. Assume that the following bytes are from a color plane in a bitmap that is 6 pixels wide by 3 pixels high.

 0x13, 0xFF, 0x20, 0xFE, 0xFD, 0x60, 0x01, 0x7D, 
 0xF5, 0xC2, 0x9A, 0x38, 0x60, 0x01, 0x67, 0x8B, 
 0xA3, 0x78, 0xAF

The first byte is the control byte of the first segment from the first scan-line.

 0x13 -> Control byte = 1 RAW value, 3-length RUN of the last raw value
 0xFF -> Raw value

Hence, decoding the first segment according to the information in the control byte yields the following:

 0xFF, 0xFF, 0xFF, 0xFF

Because these values are from the first scan-line, the values are absolute. Shown in unsigned decimal, the values are the following:

 255, 255, 255, 255

The second segment control byte is the following:

 0x20

This implies two raw values and a zero-length run, resulting in the following two unsigned decimal absolute values.

 0xFE, 0xFD

This completes the first scan-line, and all values decoded from this point are deltas and MUST be specially decoded. Before decoding, the deltas MUST be interpreted as unsigned 1-byte values. If the encoded delta value is odd, then decrement it by 1, shift it 1 bit toward the lowest bit, and subtract it from 255. This yields the original absolute value. If the encoded delta value is even, shift it 1 bit toward the lowest bit. This yields the original absolute value.

Examining the third segment yields six raw values and a zero-length run.

 0x01, 0x7D, 0xF5, 0xC2, 0x9A, 0x38

The deltas MUST be decoded. The unsigned decimal representation of the deltas is as follows.

 1, 125, 245, 194, 154, 56

Applying the delta transformation to the deltas in the second scan-line yields the following 1-byte unsigned values.

 255, 193, 133, 97, 77, 28

To compute the final absolute values for the second row, the unsigned, 8-bit delta values are added to the absolute values of the first scan-line using 1-byte arithmetic.

 Column 1: 255 + 255 = 254
 Column 2: 255 + 193 = 192
 Column 3: 255 + 133 = 132
 Column 4: 255 +  97 =  96
 Column 5: 254 +  77 =  75
 Column 6: 253 +  28 =  25

This completes the second scan-line, and the final absolute values are the following:

 254, 192, 132, 96, 75, 25

The third segment has six raw values and a zero-length run.

 0x01, 0x67, 0x8B, 0xA3, 0x78, 0xAF

The decoded unsigned 8-bit unsigned values representing the final deltas are as follows.

 255, 204, 186, 174, 60, 168 

To compute the final absolute values for the third row, the unsigned 8-bit delta values are added to the absolute values of the second scan-line using 1-byte arithmetic.

 Column 1: 254 + 255 = 253
 Column 2: 192 + 204 = 140
 Column 3: 132 + 186 =  62
 Column 4:  96 + 174 =  14
 Column 5:  75 +  60 = 135
 Column 6:  25 + 168 = 193

This completes the second scan-line, and the final absolute values are the following:

 253, 140, 62, 14, 135, 193

The fully decoded plane is the following:

 255, 255, 255, 255, 254, 253
 254, 192, 132,  96,  75,  25
 253, 140,  62,  14, 135, 193