## 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: