Debugging OpenGL Applications

• Novice graphics programming experiences
• Common problems
• How these problems are solved
• Available graphics debugging tools
• How the tools work
• What you need to know to use the tools properly
• Examples

First graphics programming experiences

Getting started with writing grahics applications can be painful…

• I did everything exactly like the tutorial said and there’s nothing on the screen
• I completely rewrote the shaders and the rendered image still looks the same
• I did the math a hundred times and the result is exactly the opposite of what I expected

Why this happens

• The graphics pipeline can be seen as one big state machine
• Large number of bugs result from:
  • State not being set
  • State not being set correctly
  • State changes not being tracked properly

How these problems are solved

• Trial and error
• Shooting in the dark
• Plain guessing

How to solve these problems without pulling your hair out

• Check ALL API errors and warnings
• Use the right tools

OpenGL debugging tools

• GDEBugger
• NVidia NSight
• VOGL
• GLSLDevil
• BuGLe
• APITrace
• GLIntercept

OpenGL debugging tools

Tools let you:

• Keep track of and record API interaction
• Take pipeline state snapshots
  • Inspect state
  • Inspect graphics memory

What you need to know

For these debugging tools to be usefull it’s important to understand all the phases of the graphics pipeline

• High level understanding of what each phase does
• What are the inputs and outputs of each phase
• Which states affect which phases of the pipeline

OpenGL Pipeline

OpenGL Pipeline

• Pipeline input
  • VBO - Vertex Buffer Object
    • Actual vertex data
  • VAO - Vertex Array Object
    • Vertex attributes that defines structure of vertex data

OpenGL Pipeline

OpenGL Pipeline

• Vertex processing
  • Programmable via Vertex shader
  • Receives vertices as input and outputs transformed vertices
  • Typically applies world, view and projection matrices to transform vertices into clip-space

OpenGL Pipeline

• Primitive assembly
  • Collects output of vertex shaders and compose into primitive shapes

OpenGL Pipeline

OpenGL Pipeline

• Tessellation (Optional phase)
  • Programmable via Tessellation Control and Evaluation shaders
  • Tessellation Control defines how much new geometry is injected
  • Tessellation Evaluation transforms newly injected geometry

(Gif from The Little Grasshopper blog)

OpenGL Pipeline

OpenGL Pipeline

• Primitive processing (Optional phase)
  • Programmable via Geometry shader
  • Receives input from primitive assembly as input and outputs primitive
  • Can output zero or more primitives
  • Can ouput completely different type of primitives

OpenGL Pipeline

OpenGL Pipeline

• Transform Feedback (Optional opt-out)
  • Skips rest of the pipeline and returns output of geometry shader
  • Useful for GPU particle systems

OpenGL Pipeline

OpenGL Pipeline

• Clipping and Culling
  • Primitives outside fixed viewing volume are discarded
  • Custom clipping planes can be provided for additional clipping
  • Face culling (e.g. back facing primitives are discarded)
  • Viewport transform - primitive vertices are transformed from clip-space to window space

OpenGL Pipeline

OpenGL Pipeline

• Rasterization
  • Receives primitives as input and outputs sequence of fragments
  • Fragment data is interpolated from vertex data

OpenGL Pipeline

OpenGL Pipeline

• Fragment processing
  • Programmable via Fragment shader
  • Receives input from rasterizer and outputs colours, depth values and stencil values
  • Only colour and depth output is programmable
  • Different colours can be output to different buffers simultaneously
  • Depth and stencil values can only be written to a single depth buffer at a time

OpenGL Pipeline

OpenGL Pipeline

• Per-Sample Operations
  • Receives input from fragment shaders and outputs final, if any, pixel to framebuffer
  • Performs stencil test
  • Performs depth test
  • Performs blending operation
  • Performs masking operations - colour, depth and stencil values can be masked
  • Blended fragments that pass all tests and aren’t masked are written to the framebuffer

OpenGL Pipeline

Demo time!