Precision Qualifiers

One notable new feature addition to OpenGL ES that differs from desktop OpenGL is the introduction of precision qualifiers to the shading language. Precision qualifiers enable the shader author to specify the precision with which computations for a shader variable are performed. Variables can be declared to have either low, medium, or high precision. These qualifiers are used as hints to the compiler to allow it to perform computations with variables at a potentially lower range and precision. It is possible that at lower precisions, some implementations of OpenGL ES might either be able to run the shaders faster or with better power efficiency. Of course, that efficiency savings comes at the cost of precision, which can result in artifacts if precision qualifiers are not used properly. Note that there is nothing in the OpenGL ES specification that says that multiple precisions must be supported in the underlying hardware, so it is perfectly valid for an implementation of OpenGL ES to do all calculations at the highest precision and simply ignore the qualifiers. However, on some implementations using a lower precision might be an advantage.

Precision qualifiers can be used to specify the precision of any float or integer-based variable. The keywords for specifying the precision are lowp, mediump, and highp. Some examples of declarations with precision qualifiers are shown here.