# Skia Coordinate Spaces

## 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: