Tiles and Scan Lines

A bitmap image in a bitmapped section consists of tiles and scan lines. A tile is a rectangular portion of an image, containing at least one scan line. An image is one or more tiles wide and one or more tiles long. A tile column is the horizontal positioning of a tile; the tiles that have their x-coordinate equal to zero belong to tile column zero, with tile column numbers incrementing in the direction of the increasing x-coordinates.

The file parser specifies its tile length in terms of scan lines. Once the length is specified, the display engine always requests bitmap data as whole tiles; that is, it tells the parser to stop only on integral multiples of the tile length. For formats that contain multiple tiles, file parsers should set the tile length to the minimum number of scan lines required for a single tile. Formats that are not stored in tiles should have the tile width set equal to the image width and the tile length set to one scan line.

The following values are expected to be valid when tiles are created.

TILESACROSS = (ImageWidth+TileWidth-1)/TileWidth
TILESDOWN = (ImageLength+TileLength-1)/TileLength
TILESPERIMAGE=TILESACROSS*TILESDOWN

To output bitmap data, the file parser outputs a scan line at a time, in sequential order, by using the SOPutScanLineData function. All of the scan line must belong to the same tile column. After each scan line, the file parser calls the SOPutBreak function with the SO_SCANLINEBREAK value. As is normally the case, the return value from SOPutBreak indicates whether the file parser should return from the VwStreamReadFunc function.

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