HP 5961-0509 manual Class 1 Bitmap Data, Class 2 Compressed Bitmap Data, 11-54Soft Font Creation

Class 1 - Bitmap Data

Class 1 or bitmap (raster) character data is a string of bytes containing the dot-per-bit image of the character, no data compression. If a bit is set to one, the corresponding dot is printed. The data is grouped in dot rows. A row describes a one-dot-high strip of the character from left to right, in the direction of the printer’s raster scan (see the Portrait Bitmap Character Data Example, at the end of this chapter). Zeroed bits must be added to the end of each row to make it contain an integral number of bytes. The dot rows are organized from top to bottom of the character. For example, the first dot row of data corresponds to the top dot row of the character.

The number of bytes of the character data should be exactly Character Width (in bytes) times Character Height. If more

data is received, it is discarded; if less data is received, the character consists of only the data downloaded.

Class 2 - Compressed Bitmap Data

For a compressed bitmap character, the data is composed of a string of bytes using a run-length encoding with line repetition compressed format (see Figure 11-2). The first byte indicates the number of times the first raster row is repeated after its initial occurrence. It is assumed that the first pixel in a row is white, hence the second byte indicates how many white pixels start the row. The third byte indicates how many black pixels, the fourth byte indicates the number of white pixels again, etc. If the first pixel in a row is black, the white pixel indicator (the second byte) is 0. If there are more than 255 pixels in a row of the same type, there is a byte containing 255, followed by a 0 byte, followed by a byte containing the count of remaining pixels of the current type.

The width of each row is determined by the character width (in dots) as specified in the character descriptor for the character. The pixel count (number of 1’s and 0’s bits) for each row in the character cell must equal the character width. For example, in Figure 11-5, the cell width is 20, thus each row (excluding the repetition count byte) adds up to 20.

Once the row has been filled, the row is duplicated as indicated in its first byte, then a new row is started.

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