doc(tutorial): made a tutorial for unity developer a better understan…

…ding about the module

docs/tutorial-for-unity.md

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+# Terminology for Unity Developers
+
+## What is an FBO?
+
+FBO stands for frame object buffer. It's equal to the renderer target that we generated.  
+
+## What is a node?
+
+You can regard a node as a "Render Pass" in Unity.  
+A node is custom code that will be applied to the render result.  
+For example, you can change the output of **InitialPostProcessingNode** to achieve a tinting effect.  
+You can also change the input of "HighPassNode" to apply a "blooming" effect or create a node rendering only a character's outline and combine it with the output of **InitialPostProcessingNode** to create an XRay node.  
+_Note: This requires technology for a rendering-specific layer. Currently, it's unclear, whether Terasology supports this kind of  "layer" concept._
+
+## How nodes work
+
+a node has Inputs and Outputs, it's input are result from previous node, and it's output can be next node's input.  
+
+For example, a highPass node uses `_SceneColor` as input, then a bloom node uses both `_SceneColor` and the highpass node's output as input.  
+
+The Unity equivalent of `_SceneColor` is the scene texture.  
+The highpass node is used to grab a light area and send it to the blur node to achieve the bright area of a bloom effect.  
+It's being used for **InitialPostProcessingNode** as the input for the final FBO.  
+
+## How to create a FBO
+
+Setting up FBOs in Terasology translates to Unity as setting up rendering targets:
+
+```C#
+RenderTexture  computeTex = new RenderTexture (IMG_WIDTH, IMG_HEIGHT, 24);
+
+```
+
+In Terasology, FBOs can be set up using setDependencies(Context context):
+
+```java
+//the root Fbo for everything
+DisplayResolutionDependentFbo displayResolutionDependentFBOs = context.get(DisplayResolutionDependentFbo.class);
+//create the fbo with scale
+FBO someFbo = requiresFbo(new FboConfig(FBO_URI, HALF_SCALE, FBO.Type.DEFAULT), displayResolutionDependentFBOs);
+//create the fbo with width and height
+FBO someFbo2 = requiresFbo(new FboConfig(FBO_URI, IMG_WIDTH, IMG_HEIGHT, FBO.Type.NO_COLOR), displayResolutionDependentFBOs);
+```
+
+## Where are FBO being processed?
+
+The FBO is being process inside the process method of a node.
+1. Fetch the material by `var mat = getMaterial(MATERIAL_URN)`
+2. Set up material properties at `process()`
+3. Render the quad
+
+### What is the quad? Where Did i used the material?
+
+Well, a quad is a quad that you created at ctor
+and you need to activate the material in `setDependencies` like this:
+
+```java
+addDesiredStateChange(new EnableMaterial(MATERIAL_URN));
+```
+
+If you study the code deeply, you'll find that, `setDependencies(Content ctx)` is setting up OpenGL contents and then `process()` is the actual rendering logic: they are just OpenGL code wrapped into java code.
+
+## Common Issues
+
+### Unrecongnized Shader
+
+If you wrote a shader, but Terasology doesn't recognize it, you are probablly forget to create a material.
+
+1. create a material called "id.mat".  
+
+```
+{
+  "shader": "MODUEL:ID",
+  "params": {}
+}
+```
+
+2. write 2 shaders `ID_vert.glsl` and `ID_frag.glsl`.
+3. now Terasology will recognized.