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79 lines
2.2 KiB
79 lines
2.2 KiB
7 months ago
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---
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title: "SkCanvas Overview"
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linkTitle: "SkCanvas Overview"
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weight: 240
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---
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*The drawing context*
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<!-- Updated Mar 4, 2011 -->
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Preview
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-------
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Here is an example of a set of drawing commands to draw a filled
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heptagram. This function can be cut and pasted into
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[fiddle.skia.org](https://fiddle.skia.org/).
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<fiddle-embed name='@skcanvas_star'></fiddle-embed>
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Details
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-------
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SkCanvas is the drawing context for Skia. It knows where to direct the
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drawing (i.e. where the screen of offscreen pixels are), and maintains
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a stack of matrices and clips. Note however, that unlike similar
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contexts in other APIs like postscript, cairo, or awt, Skia does not
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store any other drawing attributes in the context (e.g. color, pen
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size). Rather, these are specified explicitly in each draw call, via a
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SkPaint.
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<fiddle-embed name='@skcanvas_square'></fiddle-embed>
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The code above will draw a rectangle rotated by 45 degrees. Exactly
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what color and style the rect will be drawn in is described by the
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paint, not the canvas.
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Check out more detailed info on [creating a SkCanvas object](../skcanvas_creation).
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To begin with, we might want to erase the entire canvas. We can do
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this by drawing an enormous rectangle, but there are easier ways to do
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it.
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<!--?prettify lang=cc?-->
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void draw(SkCanvas* canvas) {
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SkPaint paint;
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paint.setColor(SK_ColorWHITE);
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canvas->drawPaint(paint);
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}
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This fills the entire canvas (though respecting the current clip of
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course) with whatever color or shader (and xfermode) is specified by
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the paint. If there is a shader in the paint, then it will respect the
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current matrix on the canvas as well (see SkShader). If you just want
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to draw a color (with an optional xfermode), you can just call
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drawColor(), and save yourself having to allocate a paint.
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<!--?prettify lang=cc?-->
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void draw(SkCanvas* canvas) {
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canvas->drawColor(SK_ColorWHITE);
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}
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All of the other draw APIs are similar, each one ending with a paint
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parameter.
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<fiddle-embed name='@skcanvas_paint'></fiddle-embed>
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In some of the calls, we pass a pointer, rather than a reference, to
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the paint. In those instances, the paint parameter may be null. In all
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other cases the paint parameter is required.
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Next: [SkPaint](../skpaint_overview)
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