USplitAlpha

Split Alpha channel for Android and iOS

According to a survey (Unity Texture compression and optimization) about various different texture formats in Unity, ETC1 and Split Alpha channel provides the best build size and memory footprint. As mentioned here, Unity currently does not support SplitAlpha on iOS and there was a bug that makes it not work on UI in older versions.

USplitAlpha is a library that helps you apply Split Alpha texture to both Android and iOS easily. On Android it uses ETC1 RGB4bit both main texture and alpha texture. On iOS is uses PVRTC RGB4bit.

The inspiration for this library is drawn from https://github.com/keijiro/unity-alphamask and Unity、Mobile端末向けの画一的なテクスチャ圧縮方法

Usage

To apply USplitAlpha into your project, there are 3 main steps:

Create Split Alpha textures from the original textures
Create materials that uses Split Alpha
Apply materials to UI Image, UI Raw Image or Sprite.

Step 1: Create Split Alpha textures from the original textures

Preconditions:
- The texture that you wish to create alpha channel for must have width and height being power of two and multiple of 4. Note that the texture does NOT have to be squared.
- The texture must also be png. If you have a psd, please convert it to png first.
- The texture must not be packed by Unity SpritePacker, i.e. no packing tag
In the Project view, right-click on the texture. Select USplitAlpha>Apply. This will generate an alpha channel texture and put it in a folder called _alpha in the same folder as the original texture After that, both the original texture and the alpha texture import settings will be changed to using ETC1 RGB4bit and PVRTC RGB4bit on Android and iOS, respectively.

Step 2: Create Material that uses Split Alpha

Create a new material
Set the material's shader to one of the shaders in Plugins/USplitAlpha/Shaders. For instance, use SplitAlpha/Sprites/Default shader if you want to apply to Sprites.
The material's shader will have 2 texture fields: Sprite Texture and External Alpha (the names might vary slightly according to shaders). Set the original texture to Sprite Texture and the alpha mask texture External Alpha

Step 3: Apply material to UI Image, UI Raw Image or Sprite

Apply the material created in step 2 to the appropriate elements. For UI Image and UI Raw Image, use SplitAlpha/UI/Default. For Sprites, use SplitAlpha/Sprites/Default. For mobile particles, use SplitAlpha/Mobile/Particles/Additive or SplitAlpha/Mobile/Particles/Multiply or SplitAlpha/Mobile/Particles/Alpha Blended
Set the correct sprite. For RawImage you don't have to set anything. For UI Image and Sprite, you have to choose a sprite inside the original texture. Most of the time, you would want to create the material from an Atlas so that you can batch multiple UI Image and Sprites to save draw call.

Bonus 1: Batch create split alpha texture

You can actually select several textures and/or folders then choose USplitAlpha>Apply. This will create split alpha textures for all of the png files in all subfolders recursively.

Bonus 2: Revert split alpha textures

You can actually select several textures and/or folders then choose USplitAlpha>Revert to revert the textures to their original settings without split alpha. All of the split alpha textures will be removed and folders _alpha as well if they are empty.

Bonus 3: Create texture Atlas

I use the plugin SimpleSpritePacker to create texture atlas from NPOT sprites. You can also use other external tools such as TexturePacker to create atlases.

Bonus 4: Loading from AssetBundle

You can simply put all the textures and materials in AssetBundle and load them from server. Only thing to note is after you load the material, you will have to re-set the shader of the material like so:

    material.shader = Shader.Find (material.shader.name);

Bonus 5: Loading atlas from AssetBundle

You cannot put Atlas packed by Unity's SpritePacker and load it back, that's why we have to use manual Atlas. Then, loading sprites by name from an Atlas loaded from AssetBundle is trivial You can use a solution such as UBootstrap.SpriteCollection to do it.

Create SplitAlpha Materials

Select one or more Textures in the Project view
Right click and select USplitAlpha > Create ... Materials
You can create 3 basic particle materials: Additive, Alpha Blended and Multiply or SpriteDefault, or UIDefault

Examples

You can find all examples in the scene Assets/Tests/Test

Test A with alpha uses a material with a single sprite texture
Test B with alpha UI RawImage uses a material with a raw texture
Test B with alpha UI uses the same material as Test B with alpha UI RawImage
Batchable UI Image 1, ..2, etc. uses ItemAtlasUI material created from ItemAtlas. This allows to use the same material for many UI Images and allow them to batch
BatchableSprite 1, ..2, etc. is similar to Batchable UI Image 1, except that the shader is replaced by SplitAlpha/Sprites/Default

Support Shaders

Currently, there are 5 Split Alpha shaders corresponding to 5 original shaders:

UI/Default: SplitAlpha/UI/Default
Sprites/Default: SplitAlpha/Sprites/Default
Mobile/Particles/Additive: SplitAlpha/Mobile/Particles/Additive
Mobile/Particles/Multiply: SplitAlpha/Mobile/Particles/Multiply
Mobile/Particles/Alpha Blended: SplitAlpha/Mobile/Particles/Alpha Blended

The code for the ogiginal shaders was taken from Unity then modified to support Split Alpha. Extending SplitAlpha support to other shaders is straightforward.

Future Improvements

Obviously, the workflow so far is a little laborious. You have to create split alpha textures by hand, create materials by hand, apply material to UI elements by hand. There are so many improvements that can be made to automate all of these tasks better. In fact, all of these improvements have been implemented and applied to production but I cannot share the code.

Improvement 1: Put hard-coded configurable parameters in global settings

Currently, the compression quality is hard-coded. We should put all configurable parameters in a single ScriptableObject.

Improvement 2: Make it work with Unity SpritePacker

This is quite straightforward. When you create split alpha texture, you have to copy all settings from the original texture TextureImporter, except for the packing tag. You will add a prefix or suffix to the original packing tag instead. Then you have to change PackerPolicy by creating a custom PackerPolicy, so that the generated Atlas will use ETC1 RGB4bit or PVRTC RGB4bit compression.

Improvement 3: Automatic material creation

This is how you implement:

Create a lookup table to replace normal shader by a split alpha one. For instance, UI/Default will be replaced by SplitAlpha/UI/Default, and so on.
Create Helper Component (MonoBehaviour) for different visual elements, such as UIRawImageSplitAlpha, UIImageSplitAlpha, SpriteRendererSplitAlpha, MeshRendererSplitAlpha and so on.
Helper Components will operate in 2 modes: Editor mode and Play Mode.
In Editor Mode, Helper Component will create a dummy material, set the correct shader by searching for matching shader in the lookup table in step 1. Then Helper Component will set the original texture and alpha texture correspondingly. Finally, Helper Component will set the dummy material so that the element is displying correctly in Edit Mode.
In Play Mode, Helper Component will create shared static material instead of dummy material. This is done so that multiple component using the same original texture will also share the same Split Alpha material. One thing to note is that sprites that are packed with Unity's SpritePacker using packing tag can also work.

Now you can simply assign sprites to different visual elements, then set all of those sprites to use the same packing tag and voila, all your visual elements can batch and use Split Alpha. However, this approach can NOT work with AssetBundle.

Improvement 4: Add Helper Component to GameObject automatically

There is still one manual step that you have to do in Improvement 1, that is adding the correct MonoBehaviour to the correct GameObject. We can automate this step like so:

Create a tool that allows you to select several objects, prefabs, or scenes, recursively.
Make an Apply function that search for certain Components in those GameObjects, Prefabs, or Scenes. The Components can be UIImage, UIRawImage, SpriteRenderer, MeshRenderer or other custom renderer.
Add the corresponding UIImageSplitAlpha, UIRawImageSplitAlpha, SpriteRendererSplitAlpha, MeshRendererSplitAlpha or other custom components.

Also add a Revert function to revert the change easily.

Improvement 5: Create Split Alpha material automatically

Create a tool so that users only have to set the main texture, the alpha texture will be filled in automatically by searching in _alpha folder. This is DONE.

Improvement 6: Reduce the size of Alpha mask

If the alpha mask of a texture is very simple, for instance, it's mostly white pixels or smooth gradation, then it can be downsized to 1/4 or 1/16 of the area of original texture. Then it can be used by expanding it with a bilinear filter.

Install

To include USplitAlpha into your project, you can copy Assets/Plugins/USplitAlpha to your project folder. Alternatively, you can use npm method of package management described here.

Changelog

0.0.5

Add create materials from selected textures

0.0.4

Add Apply function for Material

0.0.3

Add SplitAlphaReplacer for Material

0.0.2

Create namespace USplitAlpha
Improve readme

0.0.1

Initial commit

minhhh/USplitAlpha