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Scan conversion of character glyphs is performed by a different algorithm from the one above That font rendering algorithm uses hints in the glyph descriptions and techniques that are specialized to glyph rasterization
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654 Automatic Stroke Adjustment
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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 67 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 12 provides an optional automatic stroke adjustment feature When stroke adjustment is enabled, the line width and the coordinates of a stroke are automatically adjusted as necessary to produce lines of uniform thickness The thickness is as near as possible to the requested line width no more than half a pixel different
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1 pixel Resulting line Resulting line
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FIGURE 67 Rasterization without stroke adjustment
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CHA P TE R 6
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Note: If stroke adjustment is enabled and the requested line width, transformed into device space, is less than half a pixel, the stroke is rendered as a single-pixel line 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 Section 653, Scan Conversion Rules ) Because automatic stroke adjustment can have a substantial effect on the appearance of lines, a PDF document must be able to control whether the adjustment is to be performed This can be speci ed with the stroke adjustment parameter in the graphics state, set via the SA entry in a graphics state parameter dictionary (see Section 434, Graphics State Parameter Dictionaries )
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Transparency
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PDF 14 EXTENDS the Adobe imaging model to include the notion of transparency Transparent objects do not necessarily obey a strict opaque painting model, but can blend (composite) in interesting ways with other overlapping objects This chapter describes the general transparency model, but does not attempt to cover how it is to be implemented Although implementation-like descriptions are used at various points to describe how things work, this is only for the purpose of elucidating the behavior of the model; the actual implementation will almost certainly be different from what these descriptions might imply The chapter is organized as follows:
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Section 71, Overview of Transparency, introduces the basic concepts of the
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Section 72, Basic Compositing Computations, describes the mathematics
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involved in compositing a single object with its backdrop
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Section 73, Transparency Groups, introduces the concept of transparency
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Section 74, Soft Masks, covers the creation and use of masks to specify
position-dependent shape and opacity
Section 75, Specifying Transparency in PDF, describes how transparency
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Section 76, Color Space and Rendering Issues, deals with some speci c interactions between transparency and other aspects of color speci cation and rendering
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Transparency
71 Overview of Transparency
The original Adobe imaging model paints objects ( lls, strokes, text, and images), possibly clipped by a path, opaquely onto a page The color of the page at any point is that of the topmost enclosing object, disregarding any previous objects it may overlap This effect can be and often is realized simply by rendering objects directly to the page in the order in which they are speci ed, with each object completely overwriting any others that it overlaps Under the transparent imaging model, all of the objects on a page can potentially contribute to the result Objects at a given point can be thought of as forming a transparency stack (or just stack for short), arranged from bottom to top in the order in which they are speci ed The color of the page at each point is determined by combining the colors of all enclosing objects in the stack according to compositing rules de ned by the transparency model Note: The order in which objects are speci ed determines the stacking order, but not necessarily the order in which the objects are actually painted onto the page In particular, the transparency model does not require a viewer application to rasterize objects immediately or to commit to a raster representation at any time before rendering the entire stack onto the page This is important, since rasterization often causes signi cant loss of information and precision that is best avoided during intermediate stages of the transparency computation A given object is composited with a backdrop Ordinarily, the backdrop consists of the stack of all objects that have been speci ed previously; the result of compositing is then treated as the backdrop for the next object However, within certain kinds of transparency group (see below), a different backdrop is chosen When an object is composited with its backdrop, the color at each point is computed using a speci ed blend mode, which is a function of both the object s color and the backdrop color The blend mode determines how colors interact; different blend modes can be used to achieve a variety of useful effects A single blend mode is in effect for compositing all of a given object, but different blend modes can be applied to different objects Compositing of an object with its backdrop is mediated by two scalar quantities called shape and opacity Conceptually, for each object, these quantities are de ned at every point in the plane, just as if they were additional color components