Documentation: SkCanvas API

site/user/api/skpaint.md

Index: site/user/api/skpaint.md
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+SkPaint
+=======
+
+*color, stroke, font, effects*
+
+<!-- Updated Jan 17, 2013 by humper@google.com -->
+
+Anytime you draw something in Skia, and want to specify what color it
+is, or how it blends with the background, or what style or font to
+draw it in, you specify those attributes in a paint.
+
+Unlike `SkCanvas`, paints do not maintain an internal stack of state
+(i.e. there is no save/restore on a paint). However, paints are
+relatively light-weight, so the client may create and maintain any
+number of paint objects, each setup for a particular use. Factoring
+all of these color and stylistic attribute out of the canvas state,
+and into (multiple) paint objects, allows canvas' save/restore to be
+that much more efficient, as all they have to do is maintain the stack
+of matrix and clip settings.
+
+<!--?prettify lang=cc?-->
+
+    SkPaint paint1, paint2, paint3;
+
+    paint1.setColor(0xFFFF0000:
+    paint1.setStyle(SkPaint::kFill_Style);
+
+    paint2.setColor(0x8000FF00);
+    paint2.setStyle(SkPaint::kStroke_Style);
+    paint2.setStrokeWidth(SkIntToScalar(3));
+
+    paint3.setColor(0xFF888888);
+    paint3.setTextSize(SkIntToScalar(24));
+    paint3.setTextScaleX(SkFloatToScalar(0.75f));
+
+
+This shows three different paints, each setup to draw in a different
+style. Now the caller can intermix these paints freely, either using
+them as is, or modifying them as the drawing proceeds.
+
+<!--?prettify lang=cc?-->
+
+    canvas->drawRect(..., paint1);
+    canvas->drawRect(..., paint2);
+
+    paint2.setStrokeWidth(SkIntToScalar(5));
+    canvas->drawOval(..., paint2);
+
+    canvas->drawText(..., paint3);
+    paint3.setColor(0xFF0000FF);
+    canvas->drawText(..., paint3);
+
+
+Beyond simple attributes such as color, strokes, and text values,
+paints support effects. These are subclasses of different aspects of
+the drawing pipeline, that when referenced by a paint (each of them is
+reference-counted), are called to override some part of the drawing
+pipeline.
+
+For example, to draw using a gradient instead of a single color,
+assign a SkShader to the paint.
+
+<!--?prettify lang=cc?-->
+
+    SkShader* shader = SkGradientShader::CreateLinear(...);
+    paint.setShader(shader);
+    shader->unref();
+
+Now, anything drawn with that paint will be drawn with the gradient
+specified in the call to CreateLinear(). The shader object that is
+returned is reference-counted. Whenever any effects object, like a
+shader, is assigned to a paint, its reference-count is increased by
+the paint. To balance this, the caller in the above example calls
+unref() on the shader once it has assigned it to the paint. Now the
+paint is the only "owner" of that shader, and it will automatically
+call unref() on the shader when either the paint goes out of scope, or
+if another shader (or null) is assigned to it.
+
+There are 6 types of effects that can be assigned to a paint:
+
+*   **SkPathEffect** - modifications to the geometry (path) before it
+    generates an alpha mask (e.g. dashing)
+*   **SkRasterizer** - composing custom mask layers (e.g. shadows)
+*   **SkMaskFilter** - modifications to the alpha mask before it is
+    colorized and drawn (e.g. blur, emboss)
+*   **SkShader** - e.g. gradients (linear, radial, sweep), bitmap patterns
+    (clamp, repeat, mirror)
+*   **SkColorFilter** - modify the source color(s) before applying the
+    xfermode (e.g. color matrix)
+*   **SkXfermode** - e.g. porter-duff transfermodes, blend modes
+
+Paints also hold a reference to a SkTypeface. The typeface represents
+a specific font style, to be used for measuring and drawing
+text. Speaking of which, paints are used not only for drawing text,
+but also for measuring it.
+
+<!--?prettify lang=cc?-->
+
+    paint.measureText(...);
+    paint.getTextBounds(...);
+    paint.textToGlyphs(...);
+    paint.getFontMetrics(...);