Includes a general explanation of device vs. local, how shaders don't move with geometry, and how to use canvas transforms. Also shows localMatrix on an SkShader. Finally, links to that from the SkSL page, and adds SkSL-specific notes about coordinates. Bug: skia:11763 Change-Id: I31462aff66ff8514392098b972a10e7c96e4f254 Reviewed-on: https://skia-review.googlesource.com/c/skia/+/393516 Commit-Queue: Brian Osman <brianosman@google.com> Reviewed-by: Michael Ludwig <michaelludwig@google.com>
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Skia Coordinate Spaces | Coordinates |
Overview
Skia generally refers to two different coordinate spaces: device and local. Device
coordinates are defined by the surface (or other device) that you're rendering to. They range from
(0, 0)
in the upper-left corner of the surface, to (w, h)
in the bottom-right corner - they are
effectively measured in pixels.
Local Coordinates
The local coordinate space is how all geometry and shaders are supplied to the SkCanvas
. By
default, the local and device coordinate systems are the same. This means that geometry is
typically specified in pixel units. Here, we position a rectangle at (100, 50)
, and specify that
it is 50
units wide and tall:
Local coordinates are also used to define and evaluate any SkShader
on the paint. Here, we define
a linear gradient shader that goes from green (when x == 0
) to blue (when x == 50
):
Shaders Do Not Move With Geometry
Now, let's try to draw the gradient-filled square at (100, 50)
:
What happened? Remember, the local coordinate space has not changed. The origin is still in the
upper-left corner of the surface. We have specified that the geometry should be positioned at
(100, 50)
, but the SkShader
is still producing a gradient as x
goes from 0
to 50
. We have
slid the rectangle across the gradient defined by the SkShader
. Shaders do not move with the
geometry.
Transforming Local Coordinate Space
To get the desired effect, we could create a new gradient shader, with the positions moved to
100
and 150
. That makes our shaders difficult to reuse. Instead, we can use methods on
SkCanvas
to change the local coordinate space. This causes all local coordinates (geometry
and shaders) to be evaluated in the new space defined by the canvas' transformation matrix:
Transforming Shader Coordinate Space
Finally, it is possible to transform the coordinate space of the SkShader
, relative to the canvas
local coordinate space. To do this, you supply a localMatrix
parameter when creating the
SkShader
. In this situation, the geometry is transformed by the SkCanvas
matrix. The SkShader
is transformed by the SkCanvas
matrix and the localMatrix
for that shader. The other way to
think about this: The localMatrix
defines a transform that maps the shader's coordinates to the
coordinate space of the geometry.
To help illustrate the difference, here's our gradient-filled box. It's first been translated 50
units over and down. Then, we apply a 45
degree rotation (pivoting on the center of the box) to
the canvas. This rotates the geometry of the box, and the gradient inside it:
Compare that to the second example. We still translate 50
units over and down. Here, though, we
apply the 45
degree rotation only to the shader, by specifying it as a localMatrix
to the
SkGradientShader::MakeLinear
function. Now, the box remains un-rotated, but the gradient rotates
inside the box: