MSDN Documentation

OpenGL ES Shaders: The Basics

Understand the fundamental concepts of shaders in OpenGL ES, the programmable pipeline, and how to write your first vertex and fragment shaders.

What are Shaders?

Shaders are small programs that run on the graphics processing unit (GPU). In modern graphics APIs like OpenGL ES, they are essential for rendering. Instead of fixed-function hardware, you control the rendering pipeline using programmable shaders. This gives you immense flexibility and power to create complex visual effects.

OpenGL ES uses two primary types of shaders for basic rendering:

• Vertex Shaders: These programs process individual vertices. Their main job is to transform vertex coordinates from model space to clip space.
• Fragment Shaders (or Pixel Shaders): These programs process individual fragments (potential pixels). Their main job is to determine the final color of each fragment.

The Programmable Pipeline

Traditionally, the graphics pipeline had fixed stages. With shaders, many of these stages become programmable. The basic flow looks like this:

1. Vertex Data: Your 3D model data (vertices, normals, texture coordinates) is sent to the GPU.
2. Vertex Shader: Processes each vertex, transforming its position and passing other per-vertex data (like color or texture coordinates) to the next stage.
3. Primitive Assembly: Vertices are assembled into primitives (points, lines, triangles).
4. Rasterization: Primitives are converted into fragments.
5. Fragment Shader: Processes each fragment, determining its final color.
6. Frame Buffer: The final colors are written to the screen buffer.

GLSL ES: The Shader Language

Shaders for OpenGL ES are typically written in GLSL ES (OpenGL Shading Language for Embedded Systems), a C-like language. It's designed to be efficient for the GPU.

Example Vertex Shader

This simple vertex shader takes vertex positions and transforms them by a model-view-projection matrix. It also passes the vertex color to the fragment shader.

#version 300 es

// Input vertex attributes
in vec4 a_Position; // Vertex position in model space
in vec4 a_Color;   // Vertex color

// Output to fragment shader
out vec4 v_Color;  // Interpolated color for fragment

// Uniforms (variables set by the application)
uniform mat4 u_MVPMatrix; // Model-View-Projection matrix

void main()
{
    // Transform vertex position to clip space
    gl_Position = u_MVPMatrix * a_Position;

    // Pass color to the fragment shader
    v_Color = a_Color;
}
            

Example Fragment Shader

This fragment shader simply outputs the color it receives from the vertex shader. For more complex effects, you would perform lighting calculations, texture lookups, etc., here.

#version 300 es

// Input from vertex shader
in vec4 v_Color;  // Interpolated color

// Output color
out vec4 fragColor;

void main()
{
    // Set the final fragment color
    fragColor = v_Color;
}
            

Key Concepts in GLSL ES

• #version: Specifies the GLSL ES version.
• in: Declares input variables (vertex attributes for vertex shaders, interpolated data for fragment shaders).
• out: Declares output variables (data to be passed to the next stage, or the final color for fragment shaders).
• uniform: Declares variables whose values are set by the application and are the same for all vertices/fragments in a single draw call.
• gl_Position: A built-in output variable in vertex shaders representing the vertex's clip-space coordinates.
• main(): The entry point for the shader program.

Compiling and Linking Shaders

In your application code (e.g., C++ or Java for Android), you'll need to:

1. Load the shader source code as strings.
2. Create shader objects (one for vertex, one for fragment).
3. Compile each shader object.
4. Create a program object.
5. Attach the compiled shaders to the program object.
6. Link the program object.
7. Use the program object for rendering.

The exact API calls depend on your platform and programming language.

Next Steps

Now that you understand the basics, you can explore how to use these shaders to:

• Apply transformations and projections.
• Pass and use vertex attributes like texture coordinates.
• Implement basic lighting models.
• Sample textures to add detail to your models.

Refer to the subsequent tutorials for more advanced concepts and practical examples.