1 SkCanvas 2 ======== 3 4 *The drawing context* 5 6 <!-- Updated Mar 4, 2011 --> 7 8 Preview 9 ------- 10 11 Here is an example of a set of drawing commands to draw a filled 12 heptagram. This function can be cut and pasted into 13 [fiddle.skia.org](https://fiddle.skia.org/). 14 15 <fiddle-embed name='@skcanvas_star'></fiddle-embed> 16 17 Details 18 ------- 19 20 SkCanvas is the drawing context for Skia. It knows where to direct the 21 drawing (i.e. where the screen of offscreen pixels are), and maintains 22 a stack of matrices and clips. Note however, that unlike similar 23 contexts in other APIs like postscript, cairo, or awt, Skia does not 24 store any other drawing attributes in the context (e.g. color, pen 25 size). Rather, these are specified explicitly in each draw call, via a 26 SkPaint. 27 28 <fiddle-embed name='@skcanvas_square'></fiddle-embed> 29 30 The code above will draw a rectangle rotated by 45 degrees. Exactly 31 what color and style the rect will be drawn in is described by the 32 paint, not the canvas. 33 34 Check out more detailed info on [creating a SkCanvas object](canvas). 35 36 To begin with, we might want to erase the entire canvas. We can do 37 this by drawing an enormous rectangle, but there are easier ways to do 38 it. 39 40 <!--?prettify lang=cc?--> 41 42 void draw(SkCanvas* canvas) { 43 SkPaint paint; 44 paint.setColor(SK_ColorWHITE); 45 canvas->drawPaint(paint); 46 } 47 48 This fills the entire canvas (though respecting the current clip of 49 course) with whatever color or shader (and xfermode) is specified by 50 the paint. If there is a shader in the paint, then it will respect the 51 current matrix on the canvas as well (see SkShader). If you just want 52 to draw a color (with an optional xfermode), you can just call 53 drawColor(), and save yourself having to allocate a paint. 54 55 <!--?prettify lang=cc?--> 56 57 void draw(SkCanvas* canvas) { 58 canvas->drawColor(SK_ColorWHITE); 59 } 60 61 All of the other draw APIs are similar, each one ending with a paint 62 parameter. 63 64 <fiddle-embed name='@skcanvas_paint'></fiddle-embed> 65 66 In some of the calls, we pass a pointer, rather than a reference, to 67 the paint. In those instances, the paint parameter may be null. In all 68 other cases the paint parameter is required. 69 70 Next: [SkPaint](/user/api/skpaint) 71
