1--- 2title: 'Skia Coordinate Spaces' 3linkTitle: 'Coordinates' 4--- 5 6## Overview 7 8Skia generally refers to two different coordinate spaces: **device** and 9**local**. Device coordinates are defined by the surface (or other device) that 10you're rendering to. They range from `(0, 0)` in the upper-left corner of the 11surface, to `(w, h)` in the bottom-right corner - they are effectively measured 12in pixels. 13 14--- 15 16## Local Coordinates 17 18The local coordinate space is how all geometry and shaders are supplied to the 19`SkCanvas`. By default, the local and device coordinate systems are the same. 20This means that geometry is typically specified in pixel units. Here, we 21position a rectangle at `(100, 50)`, and specify that it is `50` units wide and 22tall: 23 24<fiddle-embed name='96f782b723c5240aab440242f4c7cbfb'></fiddle-embed> 25 26Local coordinates are also used to define and evaluate any `SkShader` on the 27paint. Here, we define a linear gradient shader that goes from green (when 28`x == 0`) to blue (when `x == 50`): 29 30<fiddle-embed name='97cf81a465fdeff01d2298e07a0802a3'></fiddle-embed> 31 32--- 33 34## Shaders Do Not Move With Geometry 35 36Now, let's try to draw the gradient-filled square at `(100, 50)`: 37 38<fiddle-embed name='3adc73d23d57084f954f52c6b14c8772'></fiddle-embed> 39 40What happened? Remember, the local coordinate space has not changed. The origin 41is still in the upper-left corner of the surface. We have specified that the 42geometry should be positioned at `(100, 50)`, but the `SkShader` is still 43producing a gradient as `x` goes from `0` to `50`. We have slid the rectangle 44across the gradient defined by the `SkShader`. Shaders do not move with the 45geometry. 46 47--- 48 49## Transforming Local Coordinate Space 50 51To get the desired effect, we could create a new gradient shader, with the 52positions moved to `100` and `150`. That makes our shaders difficult to reuse. 53Instead, we can use methods on `SkCanvas` to **change the local coordinate 54space**. This causes all local coordinates (geometry and shaders) to be 55evaluated in the new space defined by the canvas' transformation matrix: 56 57<fiddle-embed name='ce89b326b2bbe41587eec738706bf155'></fiddle-embed> 58 59--- 60 61## <span>Transforming Shader Coordinate Space</span> 62 63Finally, it is possible to transform the coordinate space of the `SkShader`, 64relative to the canvas local coordinate space. To do this, you supply a 65`localMatrix` parameter when creating the `SkShader`. In this situation, the 66geometry is transformed by the `SkCanvas` matrix. The `SkShader` is transformed 67by the `SkCanvas` matrix **and** the `localMatrix` for that shader. The other 68way to think about this: The `localMatrix` defines a transform that maps the 69shader's coordinates to the coordinate space of the geometry. 70 71To help illustrate the difference, here's our gradient-filled box. It's first 72been translated `50` units over and down. Then, we apply a `45` degree rotation 73(pivoting on the center of the box) to the canvas. This rotates the geometry of 74the box, and the gradient inside it: 75 76<fiddle-embed name='d4b52d94342f1b55900d489c7ba8fd21'></fiddle-embed> 77 78Compare that to the second example. We still translate `50` units over and down. 79Here, though, we apply the `45` degree rotation _only to the shader_, by 80specifying it as a `localMatrix` to the `SkGradientShader::MakeLinear` function. 81Now, the box remains un-rotated, but the gradient rotates inside the box: 82 83<fiddle-embed name='886fa46943b67e0d6aa78486dcfbcc2c'></fiddle-embed> 84