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TilingPattern: Add comment to explain the implementation

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Rob Wu 2019-03-25 01:03:26 +01:00
parent d3dc8f16b5
commit 5985d4069a
1 changed files with 20 additions and 0 deletions
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src/display/pattern_helper.js
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@ -330,6 +330,26 @@ var TilingPattern = (function TilingPatternClosure() {
info('TilingType: ' + tilingType);
// A tiling pattern as defined by PDF spec 8.7.2 is a cell whose size is
// described by bbox, and may repeat regularly by shifting the cell by
// xstep and ystep.
// Because the HTML5 canvas API does not support pattern repetition with
// gaps in between, we use the xstep/ystep instead of the bbox's size.
//
// This has the following consequences (similarly for ystep):
//
// - If xstep is the same as bbox, then there is no observable difference.
//
// - If xstep is larger than bbox, then the pattern canvas is partially
// empty: the area bounded by bbox is painted, the outside area is void.
//
// - If xstep is smaller than bbox, then the pixels between xstep and the
// bbox boundary will be missing. This is INCORRECT behavior.
// "Figures on adjacent tiles should not overlap" (PDF spec 8.7.3.1),
// but overlapping cells without common pixels are still valid.
// TODO: Fix the implementation, to allow this scenario to be painted
// correctly.
var x0 = bbox[0], y0 = bbox[1], x1 = bbox[2], y1 = bbox[3];
// Obtain scale from matrix and current transformation matrix.