opengl - GLSL Half (floating point) attribute type

Question

I have been trying to get a 16bit float (half-floating point) as an attribute into my GLSL vertex shader. It won't let me compile saying:

error C7506: OpenGL does not define the global type half

but my #version is 410 and so it should support half? Am I missing something obvious?

Answer 1

OpenGL and OpenGL ES define two concurrent types of precision.

• Storage precision in buffers
• Minimum computational precision used in shaders.

Storage precision is defined by your vertex attribute upload, such as GL_FLOAT or GL_HALF_FLOAT. This will be the precision used to store the data in memory.

Usage precision is defined in the shader as highp (at least 32-bit), mediump (at least 16-bit), and lowp (at least 9-bit). These are minimum precisions; it is perfectly legal for a shader to specify a variable as mediump and for the shader compiler to generate fp32 data types. Desktop GPUs tend to only support fp32 computation, so the use of highp, mediump and lowp all map to fp32 data types (the precision qualifiers are only included to keep compatibility with OpenGL ES shaders, and can legally be ignored by the compiler). Mobile GPUs implementing OpenGL ES tend to map highp to fp32, and mediump and lowp to fp16. Detailed information can be found in the GLSL ES 3.0 Specification, Section 4.5.1

When binding a vertex attribute in memory to an input shader variable the storage and usage precisions are not required to match; the API will include transparent attribute precision conversion. It is perfectly legal for a user to upload e.g. a GL_FLOAT and then use it in a shader as a mediump fp16 variable, although it would be a waste of memory bandwidth to do so.