5c036433b7
No-Try: true Docs-Preview: https://skia.org/user/api?cl=108502 Change-Id: I17547b53b50a00e4e9ad1704a032b5b0d14ea436 Reviewed-on: https://skia-review.googlesource.com/108502 Reviewed-by: Cary Clark <caryclark@google.com> Commit-Queue: Hal Canary <halcanary@google.com>
71 lines
2.2 KiB
Markdown
71 lines
2.2 KiB
Markdown
SkCanvas Overview
|
|
========
|
|
|
|
*The drawing context*
|
|
|
|
<!-- Updated Mar 4, 2011 -->
|
|
|
|
Preview
|
|
-------
|
|
|
|
Here is an example of a set of drawing commands to draw a filled
|
|
heptagram. This function can be cut and pasted into
|
|
[fiddle.skia.org](https://fiddle.skia.org/).
|
|
|
|
<fiddle-embed name='@skcanvas_star'></fiddle-embed>
|
|
|
|
Details
|
|
-------
|
|
|
|
SkCanvas is the drawing context for Skia. It knows where to direct the
|
|
drawing (i.e. where the screen of offscreen pixels are), and maintains
|
|
a stack of matrices and clips. Note however, that unlike similar
|
|
contexts in other APIs like postscript, cairo, or awt, Skia does not
|
|
store any other drawing attributes in the context (e.g. color, pen
|
|
size). Rather, these are specified explicitly in each draw call, via a
|
|
SkPaint.
|
|
|
|
<fiddle-embed name='@skcanvas_square'></fiddle-embed>
|
|
|
|
The code above will draw a rectangle rotated by 45 degrees. Exactly
|
|
what color and style the rect will be drawn in is described by the
|
|
paint, not the canvas.
|
|
|
|
Check out more detailed info on [creating a SkCanvas object](canvas).
|
|
|
|
To begin with, we might want to erase the entire canvas. We can do
|
|
this by drawing an enormous rectangle, but there are easier ways to do
|
|
it.
|
|
|
|
<!--?prettify lang=cc?-->
|
|
|
|
void draw(SkCanvas* canvas) {
|
|
SkPaint paint;
|
|
paint.setColor(SK_ColorWHITE);
|
|
canvas->drawPaint(paint);
|
|
}
|
|
|
|
This fills the entire canvas (though respecting the current clip of
|
|
course) with whatever color or shader (and xfermode) is specified by
|
|
the paint. If there is a shader in the paint, then it will respect the
|
|
current matrix on the canvas as well (see SkShader). If you just want
|
|
to draw a color (with an optional xfermode), you can just call
|
|
drawColor(), and save yourself having to allocate a paint.
|
|
|
|
<!--?prettify lang=cc?-->
|
|
|
|
void draw(SkCanvas* canvas) {
|
|
canvas->drawColor(SK_ColorWHITE);
|
|
}
|
|
|
|
All of the other draw APIs are similar, each one ending with a paint
|
|
parameter.
|
|
|
|
<fiddle-embed name='@skcanvas_paint'></fiddle-embed>
|
|
|
|
In some of the calls, we pass a pointer, rather than a reference, to
|
|
the paint. In those instances, the paint parameter may be null. In all
|
|
other cases the paint parameter is required.
|
|
|
|
Next: [SkPaint](/user/api/skpaint_overview)
|