GLSL program fails to link without errors

OpenGL
1

Hi all,

I’m trying to use a Geometry shader.
My OpenGL program is compossed by this 3 shaders:


//#shader vertex
#version 330 core
layout(location=0) in vec4 position;
layout(location=1) in float type;

uniform vec4 u_colors[4];
out vec4 fragmentColor;
uniform mat4 u_view;
uniform mat4 u_model;
uniform mat4 u_proj;
void main() {
    gl_Position = u_proj * u_view * u_model * position;
    fragmentColor = u_colors[int(type)];
}

//#shader fragment
#version 330 core
in vec4 fragmentColor;
out vec4 color;
void main() {
    color = fragmentColor;
}

//#shader geometry
#version 330
layout (lines) in;
layout (line_strip, max_vertices = 2) out;
in vec4 vFragColorVs[2];
out vec4 vFragColor;
void main(void) {
     int i;
     for ( i=0; i < gl_in.length(); i++) {
          vFragColor = vFragColorVs[i];
          gl_Position = gl_in[i].gl_Position;
          EmitVertex();
     }
     EndPrimitive();
}

When I try to compile it there are no compilation errors.
Thats the code that I use to link:


glLinkProgram(rendererID);

    GLint isLinked = 0;
    glGetProgramiv(rendererID, GL_LINK_STATUS, &isLinked);
    if (isLinked == GL_FALSE)
    {
        int length;
        std::string log;
        glGetProgramiv(rendererID, GL_INFO_LOG_LENGTH, &length);
        log.reserve(static_cast<unsigned long>(length));
        glGetProgramInfoLog(rendererID, length, &length, &log[0]);

        // The program is useless now. So delete it.
        glDeleteProgram(rendererID);
        std::cerr << "cannot Link: " << log << std::endl;
        throw std::runtime_error("cannot link opengl program");
    }

The output is only cannot link: without a log message.
The program without the Geometry shader links and works well.

Where is the mistake?

2

I believe what you’re doing with trying to reading the link log into a std::string isn’t portable.

Try this: Shader_Compilation#Error_handling. That is, read it into a vector of chars, or into a raw char array.

3

string::reserve() only reserves space; the length will still be zero. Use e.g.


log.assign(length, '\0');

to enlarge the string. Or just create the string at the appropriate size:


std::string log(length, '\0');

Note that a vector has essentially the same issue; vector::reserve() will only reserve space, it won’t change the length.

4

The fragmentColor is the issue. You need to pass the colour from the vertex shader, to the geom shader, and out to the fragment shader. e.g.

// in vertex shader:
out vec4 gs_fragmentColor;

// in geom shader:
in vec4 gs_fragmentColor[];
out vec4 fs_fragmentColor;

// in frag shader:
in vec4 fs_fragmentColor;

Currently you’re assuming fragmentColor will go directly from the vertex shader to the fragment shader (it won’t!), and the vFragColorVs/vFragColor vars in your geom shader don’t match the vertex/fragment shader inputs/outputs.

5

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