Intro To Black Magic... Also Know as Shaders

Intro to Shaders

                Shaders are like mystical black magic in a text file, the visual effects they can achieve is incredible. Shaders are responsible for all most all of the graphical advancements in games. A shader manipulates data in the graphics pipeline at certain programmable steps. I cannot stress this enough: what is passed to a shader is just DATA and you can do whatever you want to DATA! There are currently 5 programmable stages in the pipeline the vertex shader, Tessellation Control & Evaluation Shaders, Geometry Shaders, Fragment Shaders and Compute Shaders. Below is a diagram of the full openGL graphics pipeline.

From http://img.hexus.net/v2/lowe/News/Khronos/OGLCS.jpg

 Most games can get away with only using the vertex and fragment shaders to create Interesting visual effects. This blog will be focused on the vertex and fragment shader as they are the easiest to understand. Below is a simplified version of the Graphics pipeline that makes it easier to understand what happens.

From http://goanna.cs.rmit.edu.au/`gl/teaching/Interactive3D/2012/images/pipline.png

                Shaders are written in a couple of programming languages such as HLSL, GLSL and CG. These languages have different syntax but they all do the same stuff. The program language I use is GLSL because I found it to be congruent with openGL, the graphics APU I use. A Vertex has a specific job to process individual vertices and send out its new position to the pipeline. Here is a simple Vertex shader in GLSL.

#version 150

uniform mat4 uni_ModelViewProjection;

in vec3 in_vertex;

void main(void)

{

                gl_Position = uni_ModelViewProjection * vec4(in_vertex, 1.0);

}

All this Shader does is takes a vertex -in_vertex- and transforms it by the modelVeiwProjection matrix which both are variables passed in from the c++ program. It then outputs the new vertex in the system variable gl_Position. A vertex Shader must set gl_Position! If it doesn’t set it the shader will not compile.

The Fragment shader is next which manipulates the pixel colour of an object per fragment. The only thing the following example does, is set every output color to red.  A fragment shader must send out a colour vec4 or vec3.

#version 150

out vec4 out_color;

void main(void)

{

                out_color =  vec4 (1.0f, 0.0f, 0.0f, 0.0f);

}

                With vertex and fragment shaders you can make amazing visual effects from the extremely easy one I showed to very nice and complex ones like these:

From http://www.userinter.net/wp-content/uploads/2011/04/Shader-Effects.jpg

From http://devmaster.net/assets/post/3021/featured_image/sized_for_post_17ced3ab-a06c-4edd-9b0d-a1ef4be7c4b7.jpg