Uniform Buffer Objects: Passing Data to Shaders

Constants Shared Across a Draw Call

Shaders need data constant for all vertices or fragments in a draw call: MVP matrices, light positions, material properties, elapsed time. Early OpenGL passed these one at a time via glUniform. Modern OpenGL and Vulkan use Uniform Buffer Objects (UBOs) for one bind instead of dozens of API calls.

A UBO is a GPU memory block bound to a shader binding point. The shader declares a uniform block and reads fields by name.

std140 Layout Rules

UBOs follow std140 packing rules for predictable alignment:

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For example, a vec3 followed by a float in the same struct may waste padding because vec3 already occupies 16 bytes in std140. Misaligned CPU structs cause shaders to read garbage silently. Engines like Unreal auto-generate reflection metadata to avoid this.

• One bind replaces many glUniform calls
• Reusable across multiple shader programs
• Vulkan equivalent: uniform buffer in a descriptor set

For This Exercise

You will define a UBO struct in C with correct std140 padding and print each field's byte offset. This task asks you to compute alignment for matrices, vec3/float pairs, and arrays. Getting struct layout wrong is a classic bug where the GPU reads garbage because CPU-side packing does not match the shader's expected layout.