How to rotate an image in openGL?

Question

I have a code below in C++ OpenGL. It has six triangles that form hexagonal. However, I need to be able to rotate it in vertically. Can someone help? TNX

Details: I have six independent triangles with vertices. In addition, there is two-dimensional array that is used for colors. There is a loop starts at line [here] two keep windows rendering until it is exited. Another line at line [here-two] that is used to show all the triangles with their color.

    //coordinates of triangle
    float triangle[6][9] = {
        {
            0.0, 0.0, 0.0,
            -0.5, 0.87, 0.0,
            0.5, 0.87, 0.0
        },
        {
            0.0, 0.0, 0.0,
            -0.5, -0.87, 0.0,
            0.5, -0.87, 0.0
        },
        {
            0.0, 0.0, 0.0,
            0.5, 0.87, 0.0,
            1.0, 0.0, 0.0
        },
        {
            0.0, 0.0, 0.0,
            0.5, -0.87, 0.0,
            1.0, 0.0, 0.0
        },
        {
            0.0, 0.0, 0.0,
            -0.5, 0.87, 0.0,
            -1.0, 0.0, 0.0
        },
        {
            0.0, 0.0, 0.0,
            -0.5, -0.87, 0.0,
            -1.0, 0.0, 0.0
        }

    };

    float color[][9]{
        {
            255, 0, 0,
            255, 0, 0,
            255, 0, 0
        },
        {
            0, 255, 0,
            0, 255, 0,
            0, 255, 0
        },
        {
            0, 0, 255,
            0, 0, 255,
            0, 0, 255
        }
    };
    int count = 0;

    /* Loop until the user closes the window */ [here]      while (!glfwWindowShouldClose(window))
    {
        glClear(GL_COLOR_BUFFER_BIT);
 [here-two]             for (int i = 0; i < 6; i++)
        {
            //Render OpenGL here
            glEnableClientState(GL_VERTEX_ARRAY);
            glEnableClientState(GL_COLOR_ARRAY);
            glVertexPointer(3, GL_FLOAT, 0, triangle[i]);
            glColorPointer(3, GL_FLOAT, 0, color[count]);
            glDrawArrays(GL_TRIANGLES, 0, 3);
            glDisableClientState(GL_COLOR_ARRAY);
            glDisableClientState(GL_VERTEX_ARRAY);
            count++;
            if (count > 2) count = 0;
        }
        //Swap front and back buffers
        glfwSwapBuffers(window);

        //Poll for and process events
        glfwPollEvents();

        // Poll for and process events
        glfwPollEvents();
    }

Answer 1

Read up on the use of matrices. What most games do in this case is they apply a matrix in the shader (as a uniform variable) that will rotate the object. In this case, you would create a rotation matrix of angle x, pass it to the shader, and then every new frame increment x and pass it to the shader again.

For more information on the specifics of the implementation read these:

• https://www.opengl.org/wiki/Uniform_(GLSL) - Creating uniform variables in a shader and updating them

• http://inside.mines.edu/fs_home/gmurray/ArbitraryAxisRotation/ -
  Creating a matrix that will rotate a vertex.

And a tip with matrix operations: remember to apply them in the right order. If you want to get the object to rotate around it's centre, make sure the rotation matrix is applied first and that the origin of your mesh is it's centre.