When switching to GLSL 300, met the following error

Question

when I switch to use OpenGL ES 3 with GLSL 300, I met the following error in my frag shader

undeclared identifier gl_FragColor

when using GLSL 100, everything is fine.

Answer 1

Modern versions of GLSL do fragment shader outputs simply by declaring them as out values, and gl_FragColor is no longer supported, hence your error. Try this:

out vec4 fragColor;
void main()
{
    fragColor = vec4(1.0, 0.0, 0.0, 1.0);
}

Note that gl_FragDepth hasn't changed and is still available.

For more information see https://www.opengl.org/wiki/Fragment_Shader

Answer 2

The predefined variable gl_FragColor does not exist anymore in GLSL ES 3.00. You need to define your own out variable for the output of the fragment shader. You can use any name you want, for example:

out vec4 FragColor;

void main() {
    ...
    FragColor = ...;
}

This follows the Core Profile of full OpenGL. The reason for not having a pre-defined fragment shader output is that it does not scale well for multiple render targets, and for render targets that need types other than float vectors.