site/docs/user/coordinates.md

---
title: 'Skia Coordinate Spaces'
linkTitle: 'Coordinates'
---

## <span id="overview">Overview</span>

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.

---

## <span>Local Coordinates</span>

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:

<fiddle-embed name='96f782b723c5240aab440242f4c7cbfb'></fiddle-embed>

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`):

<fiddle-embed name='97cf81a465fdeff01d2298e07a0802a3'></fiddle-embed>

---

## <span>Shaders Do Not Move With Geometry</span>

Now, let's try to draw the gradient-filled square at `(100, 50)`:

<fiddle-embed name='3adc73d23d57084f954f52c6b14c8772'></fiddle-embed>

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.

---

## <span>Transforming Local Coordinate Space</span>

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:

<fiddle-embed name='ce89b326b2bbe41587eec738706bf155'></fiddle-embed>

---

## <span>Transforming Shader Coordinate Space</span>

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:

<fiddle-embed name='d4b52d94342f1b55900d489c7ba8fd21'></fiddle-embed>

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:

<fiddle-embed name='886fa46943b67e0d6aa78486dcfbcc2c'></fiddle-embed>