Trying to call glGenerateMipmap on a texture with the internal format of GL_RED in an OpenGL ES 3.x context produces a GL_INVALID_OPERATION error. Explicitly sized formats such as GL_R8 work, and so do other formats with no size specified, such as GL_RGBA. Core context is not affected. Tested with the radeonsi and llvmpipe drivers.
For me it seems this behavior is according to the OpenGL ES spec: "An INVALID_OPERATION error is generated if the level base array was not specified with an unsized internal format from table 8.3 or a sized internal for- mat that is both color-renderable and texture-filterable according to table 8.10." Table 8.3 does not list GL_RED.
@Tapani Pälli: yes, you seem to be right. I somehow missed this bit. Sorry for the false alarm. I guess it's ANGLE exhibiting non-conformant behavior, then (unless it's covered by some extension that I'm not aware of).
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