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PDF Spec 10 Scan Conversion
Part of: PDF-Spec-10-Rendering
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For clipping, the clipping region consists of the set of pixels that would be included by a fill operation. Subsequent painting operations shall affect a region that is the intersection of the set of pixels defined by the clipping region with the set of pixels for the region to be painted.
Scan conversion of character glyphs may be performed by a different algorithm from the preceding one.
- NOTE 3 That font rendering algorithm uses hints in the glyph descriptions and techniques that are specialized to glyph rasterization.
When a stroke is drawn along a path, the scan conversion algorithm may produce lines of nonuniform thickness because of rasterization effects. In general, the line width and the coordinates of the endpoints, transformed into device space, are arbitrary real numbers not quantized to device pixels. A line of a given width can intersect with different numbers of device pixels, depending on where it is positioned. Figure 55 illustrates this effect.
For best results, it is important to compensate for the rasterization effects to produce strokes of uniform thickness. This is especially important in low-resolution display applications. To meet this need, PDF 1.2 provides an optional automatic stroke adjustment feature. When stroke adjustment is enabled, the line width and the coordinates of a stroke shall automatically be adjusted as necessary to produce lines of uniform thickness. The thickness shall be as near as possible to the requested line width-no more than half a pixel different.
Figure 55 - Rasterization without stroke adjustment
If stroke adjustment is enabled and the requested line width, transformed into device space, is less than half a pixel, the stroke shall be rendered as a single-pixel line.
NOTE This is the thinnest line that can be rendered at device resolution. It is equivalent to the effect produced by setting the line width to 0 (see 10.6.4, "Scan Conversion Rules").
Because automatic stroke adjustment can have a substantial effect on the appearance of lines, PDF provides means to control whether the adjustment shall be performed. This may be specified with the stroke adjustment parameter in the graphics state, set by means of the SA entry in a graphics state parameter dictionary (see 8.4.5, "Graphics State Parameter Dictionaries").
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