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TrueType scan conversion

Scan conversion is essentially the job of coloring inside the character outlines contained in the font. According to the TrueType specification, the TrueType rasterizer generates a character bitmap from pixels which are turned on whenever their centers are inside or exactly on the character outlines. Performing this task is the job of the scan converter, a component of the rasterizer.

Scan Conversion Diagram 1

The scan converter is able to maintain the continuity of character bitmaps by performing dropout control. Dropouts occur when the space within the outlines becomes so narrow that pixel centers are missed, as illustrated below.

Scan Conversion Diagram 2
Figure 1. Character outline, before and after dropout control.

The process of scan conversion consists of four steps:

  1. Measurement - The outline of the character outline is traversed point by point and contour by contour in order to find the maximum and minimum co-ordinate values of the outline. In addition, the amount of workspace memory that will be needed to perform steps two and three is calculated.
  2. Rendering - Every contour is broken into lines and splines. Calculations are made to find the point at which each line or spline intersects with scan lines (lines passing through bitmap pixel centers). The intersections for each scan line are sorted from left to right.
  3. Filling - Using the sorted intersections, runs of pixels are set for each scan line of the bitmap from top to bottom.
  4. Dropout control - If dropout control is enabled, the intersection list is checked again looking for dropouts. If various criteria are met it is decided which dropout pixel to set, and it is set. Dropout control requires scanning in the vertical direction as well as the horizontal.

Last updated 30 June 1997.

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