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programmer_tom

Trouble transforming vertices in simple shader.

7 posts in this topic

Hey all,

 

Having some trouble with what should be a simple thing to do...

I've successfully compiled and linked very simple vertex and fragment shaders.

Having trouble seeing my red triangle though.

 

This stuff all goes fine:

 

GLint ProgramObject = glCreateProgram();
glAttachShader ( ProgramObject, VertexShaderObject ); 
glAttachShader ( ProgramObject, PixelShaderObject );
glLinkProgram ( ProgramObject );
glUseProgram ( ProgramObject );

 

 

VBO creation:

 

GLuint vbo;
glGenBuffers ( 1, &vbo );
glBindBuffer ( GL_ARRAY_BUFFER, vbo );
    
GLfloat pos[] = { -0.5, -0.5, 0.5,
                           0.5, -0.5, 0.5,
                           0.0,  0.5, 0.5 };
                                     
glBufferData, GL_ARRAY_BUFFER, sizeof(pos), &pos[0], GL_STATIC_DRAW;

 

 

// Something is wrong here i am guessing...

 

int nPositionID = glGetAttribLocation ( ProgramObject, "s_vPosition" );
glEnableVertexAttribArray ( nPositionID );
glVertexAttribPointer ( nPositionID, 3, GL_FLOAT, FALSE, 0, 0 );
           
   /*
   glBegin(GL_TRIANGLES);
   glVertexAttrib3f ( nPositionID, -0.5, -0.5, 0.0 );  // If i do this, i see my red triangle (correct fragment color).
   glVertexAttrib3f ( nPositionID,  0.5, -0.5, 0.0 );  // Makes me believe context is ok.
   glVertexAttrib3f ( nPositionID,  0.0,  0.5, 0.0 );
   glEnd();
   */
   
glDrawArrays ( GL_TRIANGLES, 0, 3 );
    
SwapBuffers ( m_DeviceContext );

 

 

vertex shader looks like this:

 

#version 130\n
in vec4 s_vPosition;
void main()
{
gl_Position = vec4 ( s_vPosition.x, s_vPosition.y, s_vPosition.z, 1.0 );
}

 

 

Any ideas?

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You are using 0.5 as z in your vbo, and in the commented code you are using 0.0. Maybe that's the reason?

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OpenGL by default looks down the -z axis. Setting your triangle z coords to -1 should help

GLfloat pos[] = { -0.5, -0.5, -1.0,
                           0.5, -0.5, -1.0,
                           0.0,  0.5, -1.0 };
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Actually the OpenGL clip space is from -1 to 1 so 0.5 should be ok. One problem could be, that in the shader you specify the position as a 4 component vector, whereas you only hand in 3 component vectors with glVertexAttribPointer.

 

Also make sure you disabled the depth test.

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It looks like your vertices are not been transform by any perspective nor ortho matrix.

You don't need to.
 
The following code works for me (Linux, NVidia drivers):
        glewInit();


        const char *vertexShader ="#version 130\n"
                "in vec3 s_vPosition;"
                "void main()"
                "{"
                "    gl_Position = vec4 ( s_vPosition.x, s_vPosition.y, s_vPosition.z, 1.0 );"
                "}";

        const char *pixelShader ="#version 130\n"
                "void main()"
                "{"
                "    gl_FragColor = vec4(1.0, 0.0, 0.0, 0.0);"
                "}";


        GLint VertexShaderObject = glCreateShader(GL_VERTEX_SHADER);
        glShaderSource(VertexShaderObject, 1, &vertexShader, NULL);
        glCompileShader(VertexShaderObject);

        GLint PixelShaderObject = glCreateShader(GL_FRAGMENT_SHADER);
        glShaderSource(PixelShaderObject, 1, &pixelShader, NULL);
        glCompileShader(PixelShaderObject);

        GLint ProgramObject = glCreateProgram();
        glAttachShader ( ProgramObject, VertexShaderObject );
        glAttachShader ( ProgramObject, PixelShaderObject );
        glLinkProgram ( ProgramObject );

        int success;
        glGetProgramiv(ProgramObject, GL_LINK_STATUS, &success);
        if (success != GL_TRUE)
            std::cout << "Linking failed!" << std::endl;


        GLuint vbo;
        glGenBuffers ( 1, &vbo );
        glBindBuffer ( GL_ARRAY_BUFFER, vbo );

        GLfloat pos[] = { -0.5, -0.5, 0.5,
                                   0.5, -0.5, 0.5,
                                   0.0,  0.5, 0.5 };

        glBufferData(GL_ARRAY_BUFFER, sizeof(pos), &pos[0], GL_STATIC_DRAW);

        while (!window.getState().shutdown) {

            glClear(GL_COLOR_BUFFER_BIT);

            glDisable(GL_DEPTH_TEST);
            glUseProgram ( ProgramObject );
            int nPositionID = glGetAttribLocation ( ProgramObject, "s_vPosition" );

            glBindBuffer ( GL_ARRAY_BUFFER, vbo );
            glEnableVertexAttribArray ( nPositionID );
            glVertexAttribPointer ( nPositionID, 3, GL_FLOAT, GL_FALSE, 0, 0 );

            glDrawArrays ( GL_TRIANGLES, 0, 3 );


            Engine::Platform::BaseWindow::Event event;
            while (window.getNextEvent(event)) {
            }
            window.getContext()->swap();
        }
Can you give a minimal not working example?
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