PatternBrush Class

Tile. PatternBrush.Tile is the rectangle that repeats, in the coordinates of the pattern, and PatternBrush.Content draws it, in those coordinates: with a tile at (0, 0) of 16 by 16, the callback draws between 0 and 16, and the same drawing is at (16, 0), (0, 16) and so on. What the callback draws outside the tile is not painted. PatternBrush.Transform places the pattern on the canvas, as it places a gradient or an image; without it, the coordinates of the pattern are those of the canvas, whatever the shape that is painted.

Rendering. When a shape is painted, the tile is drawn once, on an image of the size the transformation (the one of the brush, then the one of the canvas) gives it, with the spacing (PatternBrush.SpacingX, PatternBrush.SpacingY) on its right and below it, transparent, then that image is laid over the shape like the image of an ImageBrush with the wrap modes of the pattern (ImageWrapMode.Repeat by default): a pattern that is enlarged stays as sharp as its drawing. The image has the width ceil(w - 1/1024), where w is the width of the tile and of its spacing on the canvas (and the same for its height), so that a tile whose size is a whole number of pixels is drawn on exactly that many: its pixels are then those of the drawing, exactly, when it is placed on whole pixels. Otherwise the image is interpolated (ImageSampling.Bilinear), also across the edges of the tiles. The image has colors and alpha, with 8-bit samples on an 8-bit color canvas and 16-bit samples otherwise, as a layer has, or 8-bit samples on every canvas with PatternBrush.TilePrecision. The canvas the callback receives is that image: its DrawingCanvas.Width and DrawingCanvas.Height are its size in pixels, not the size of the tile, and filling it fills the tile.

A tile whose image would have more than four times the pixels the clip can show (and 65,536), or would exceed the limits of the image, is drawn where the canvas shows it: along each axis where the canvas shows less than a tile, the image holds that part of the pattern, at the same scale, and the callback runs once for each tile it shows (two at most, mirrored for a tile that ImageWrapMode.Reflect turns), clipped to its tile. Under a projective transformation, the image of the tile is drawn at the resolution those limits allow instead.

Callback. The callback runs while a shape is drawn, before any pixel of that shape changes, and may run once for each shape: it must draw the same thing every time, must not be asynchronous and must not keep the canvas it receives. It can use any brush except, directly or not, the pattern itself. A brush that is kept and reused keeps its callback, and everything the callback refers to.