how to combine jpg + separate alpha in png?

http://www.tasharen.com/forum/index.php?topic=4018.msg19784#msg19784

I have tons of large sprites, I need to reduce the build size. The ideal is to have the color image in a jpg and the alpha information in a separate 8-bit png.

I am aware, this is not how Unity works (you import the texture into a format, Unity compresses the asset package at build time etc.) Still, I found out I can load jpg and png trough Texture2D.LoadImage, but I have no idea how to put them together. I guess a shader could do that, but I have no clue how to set them up in Unity and even less how to use them in code to call something like Finale = Merge(jpg, png).

void Start () { //load the prepared image data TextAsset colortxt; TextAsset alphatxt; Texture2D colorjpg = new Texture2D(4, 4); Texture2D alphapng = new Texture2D(4, 4); colortxt = Resources.Load("1024cloak01") as TextAsset; alphatxt = Resources.Load ("1024cloak01A") as TextAsset; colorjpg.LoadImage(colortxt.bytes); alphapng.LoadImage(alphatxt.bytes); //load the prepared Shader asset Shader rgbplusa; rgbplusa = Shader.Find("RGBplusA"); //use NGUI to display the image pixel-perfect, make use of the camera setup created with NGUI > Open the UI Wizard UITexture perfect = NGUITools.AddWidget<UITexture>(GameObject.Find ("Panel")); perfect.material =  new Material (rgbplusa); perfect.material.SetTexture("_MainTex", colorjpg); perfect.material.SetTexture("_Mask", alphapng); perfect.MakePixelPerfect(); }

so I figured out a shader that does what I need, see below. If there are some errors or unnecessary stuff, please let me know. It takes one RGB 24 bit Unity texture and one Alpha 8 Unity Texture (with Alpha from Greyscale ON). I tested it in the editor with a simple scene. I created a material, changed the shader to the custom one, added the textures. Then I added the material onto a plane and it works.

I also figured out how to avoid Unity importing my .jpg and .png as textures once I add them in the Resource folder, as I want them added to the build in their compressed form. I change the extension to .bytes and then use Resource.Load("name") as TextAsset and then create a texture and add the image data from the TextAsset.bytes with Texture2D.LoadImage.

So I have a shader and the two textures loaded from the image files in code. I can create a plane, but I am not sure how to scale and position it so that the image is displayed pixel perfect. Is there an easy way how to do it? Is there a way how to use the shader without a plane/mesh?

1. Shader "RGBplusA" {
2. Properties {
3. _MainTex ("Main Texture", 2D) = "white" {}
4. _Mask ("Mask Texture", 2D) = "white" {}
5. }
6. SubShader {
7. Blend SrcAlpha OneMinusSrcAlpha
9. Pass
10. {
11. SetTexture [_Mask] {combine texture}
12. SetTexture [_MainTex] {combine texture, previous}
13. }
14. }
15. }