How do the pieces of Android's (2D) Canvas drawing pipeline fit together?

Question

I would like to have a better understanding of how the components of Android\'s (2D) Canvas drawing pipeline fit together.

For example, how do XferMode, Shader, Mask

Answer 1

This question is difficult to answer on StackOverflow. Before I get started however, note that shapes (drawRect() for instance) do NOT have an intrinsic color. The color information always comes from the Paint object.

This erases an oval. Before I noticed this my mental-model was that drawing to a canvas (conceptually) draws to a separate "layer" and then that layer is composed with the Canvas's Bitmap using the Paint's transfer mode. If it were as simple as that then the above code would erase the entire Bitmap (within the clipping region) as CLEAR always sets the color (and alpha) to 0 regardless of the source's alpha. So this implies that there's an additional sort of masking going on to constrain the erasing to an oval.

Your model is a bit off. The oval is not drawn into a separate layer (unless you call Canvas.saveLayer()), it is drawn directly onto the Canvas' backing bitmap. The Paint's transfer mode is applied to every pixel drawn by the primitive. In this case, only the result of the rasterization of an oval affects the Bitmap. There's no special masking going on, the oval itself is the mask.

Anyhow, here is a simplified view of the pipeline:

1. Primitive (rect, oval, path, etc.)
2. PathEffect
3. Rasterization
4. MaskFilter
5. Color/Shader/ColorFilter
6. Xfermode

(I just saw your update and yes, what you found describes the stages of the pipeline in order.)

The pipeline becomes just a little bit more complicated when using layers (Canvas.saveLayer()), as the pipeline doubles. You first go through the pipeline to render your primitive(s) inside an offscreen bitmap (the layer), and the offscreen bitmap is then applied to the Canvas by going through the pipeline.