Shaders

Hello and welcome people of the internet to this blog of well stuff, yeah stuff that’s the best way to describe this blog, since I talk about stuff and every once in a while I rant about stuff. SO YEAH, welcome to the blog that talks about stuff, the stuff that will be focused on in this blog will be a little something called shaders. This blog will cover a brief overview of what shaders are some of the types of shaders and examples of what shaders can do and where you can use them.

So you may be wondering what shaders are, and why the hell should you care about shaders, well I am going to answer those questions in just one moment. 

WHAT ARE SHADERS?

Now shaders, in the field of computer graphics, are the name for a computer program that are used to produce special effects and do post processing. Shaders can be used to create anything from shadows to reflective glass. They are one of the most flexible ways to calculate the rendering effects on graphics hardware, most of which are primarily coded for the GPU (Otherwise known as the graphics processing unit). Programmers use shaders in order to program the programmable GPU rendering pipeline (which you can learn more about HERE) in order to get around and exceed the fixed functionality of the pipeline. 

GPU Graphics Pipeline

By using shaders we are able to use customizable effects and alter things such as position, hue, saturation, brightness, and contrast of all pixels, vertices and textures at the drop of a hat. 

TYPE OF SHADERS

 

There are a lot of different types of shaders, however I will only be focusing on three types of shaders in this blog, which are The Vertex Shader, Pixel Shader and the Geometry shader.

VERTEX SHADER

Vertex shaders are one of the main shaders used in computer graphics which are run once per vertex depending on the graphics processor. Their purpose is to transform the 3D position of each vector into the 2D coordinate that will appear on the screen. The vertex shader is responsible for the manipulation of position, colour and texture coordinates, and can enable powerful control over the position, movement, lighting, and colour of any 3D scene. 

The following are just a few simple examples of some vertex shader code:

#version 120

void main()

{

            gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
}

This is a simple pass through shader that passes the model view projection matrix and the vertex through to a position.  The next example is a draw attribute vertex shader:

#version 120

varying vec3 passVertex;

varying vec3 passNormal;

varying vec2 passTexcoord;

void main()

{

            gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;

            passVertex = gl_Vertex.xyz;

            passNormal = gl_Normal;

            passTexcoord = gl_MultiTexCoord0.st;

}

PIXEL SHADER

The pixel shader, otherwise known as the fragment shader, computes colour and other attributes on a per pixel bases rather than on the vertices like the vertex shader does. The fragment shader is responsible for changing the look and feel of colors, applying lighting value, translucency, shadows, bump mapping, among other effects. The fragment shader also has the ability to alter the depth of the fragment (for Z-buffering) or output for multiple target rendering. Now while the pixel shader cannot produce very complex effects it does have the knowledge of the screen coordinates being drawn and can sample the screen and nearby pixels, which can enable a large variety  of 2D post processing effects such as blur, edge detection and cell shading. 

The following are just a few simple examples of some Pixel (fragment) shader code:

#version 120

void main()

{

//this sets all targets to the same colour

            gl_FragColor = vec4(1.0, 0.5, 0.0, 1.0);

}

The is a simple fragment shader pass through that sets all targets to the same color, while the following code is an example of a simple fragment shader that draws the attributes:

#version 120

varying vec3 passVertex;

varying vec3 passNormal;

varying vec2 passTexcoord;

void main()

{

            //vec3 colour = passVertex*0.5 + 0.5;

            vec3 colour = passNormal*0.5 + 0.5;

            //vec3 colour = vec3(passTexcoord, 0.0);

            gl_FragColor = vec4(colour, 1.0);

}

GEOMETRY SHADER

The last shader I will be talking briefly about in this blog will be the Geometry shader, which is a relatively new shader in the OpenGL language that has the ability to generate new graphics primitives, such as points, lines and triangles, from the ones that were sent to the beginning of the GPU pipeline. Geometry shaders are executed after the vertex shader and take the whole primitive as well as additional information as its input. This type of shader is an optional shader and does not have to be used.

Now I mentioned a few post processing techniques which I will discuss more in a later blog, my next blog will focus on lighting.