为何GLSL中常规纹理或纹理缓冲对象类型必须设为uniform？

Great question! This is a common point of confusion when starting out with OpenGL, but it all boils down to how OpenGL connects CPU-side resource management with GPU-side shader execution. Let’s break down the key reasons:

• Samplers/TBOs are resource handles, not raw data
  Unlike vertex colors or position data that gets passed per-vertex, a sampler (like sampler2D) or TBO sampler (like samplerBuffer) doesn’t store the actual texture/buffer data. Instead, it acts as a reference to a GPU resource you’ve already created and bound on the CPU side. Uniforms are the standard way to send these reference IDs to the shader—for example, when you call glUniform1i(samplerLocation, 0) on the CPU, you’re telling the shader to use the texture bound to texture unit 0.

• They’re per-draw-state data, not per-element
  Samplers and TBOs almost always stay consistent across an entire draw call (or batch of draws). You don’t need to switch textures for every vertex or fragment, so using a uniform (which is set once per draw state) is far more efficient than trying to pass this data as a vertex attribute or fragment input. Uniforms are optimized for this kind of global, infrequently changing state.

• Hardware and API specification requirements
  OpenGL’s core specification explicitly mandates that sampler types must be declared as uniforms. This ties into how GPU hardware is designed: sampler access uses dedicated texture units, and uniforms are the designated channel for communicating which texture unit the shader should use. Trying to use a sampler as a vertex attribute or local variable wouldn’t work because the hardware doesn’t support routing those inputs to the texture sampling units.

• TBOs follow the same logic
  Texture Buffer Objects (TBOs) let you treat buffer data as a texture, so their samplers work just like regular texture samplers. The samplerBuffer uniform tells the shader which buffer object to sample from—again, this is a resource reference that needs to be set once per draw state, making uniforms the perfect fit.

To put it simply: uniforms are for sending "global settings" to your shaders, and samplers/TBOs are exactly that—settings that tell the shader which GPU resource to access during execution.

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