Editing the pixels of a rendered image

Question

I want to render a simple OpenGL scene as usual, but then I want to superimpose a small image of my own (such as from a bitmap file) on top of the render, such that this image always shows. For example, this could be thought of as showing a logo in the corner of the screen for a 3D game, where the logo is always displayed on top of the rendered scene.

Please could somebody start me off in the right direction? What should I be looking into? I am rather a novice at OpenGL...

Let us suppose that I have the following code:

#include <GL/glut.h>

void renderScene(void)
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glBegin(GL_TRIANGLES);
        glVertex3f(-0.5,-0.5,0.0);
        glVertex3f(0.5,0.0,0.0);
        glVertex3f(0.0,0.5,0.0);
    glEnd();

    glutSwapBuffers();
}

int main(int argc, char **argv)
{
    // init GLUT and create Window
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DEPTH | GLUT_DOUBLE | GLUT_RGBA);
    glutInitWindowPosition(100,100);
    glutInitWindowSize(320,320);
    glutCreateWindow("GLUT Triangles");

    // register callbacks
    glutDisplayFunc(renderScene);

    // enter GLUT event processing cycle
    glutMainLoop();

    return 1;
}

This renders a triangle on the screen. How, for example, would I now render a 10-by-10 bitmap from file, at location (100, 100) on the screen? If the viewpoint was static, I could just calculate its 3D location and render it. However, I want the bitmap image to always be displayed in this location, even when the viewpoint changes.

Thanks :)

Answer 1

You question is a little unclear, but if you asked how to render, for instance a GUI, I have an answer.

Basically you need to render a textured 2d quad. Note that it should be rendered last.

Heres a video tutorial: https://www.youtube.com/watch?v=vOmJ1lyiJ4A&list=PLRIWtICgwaX0u7Rf9zkZhLoLuZVfUksDP&index=24

Answer 2

You need to use a textured quad with an orthographic projection matrix. Using alpha blending will make the image partially transparent allowing for the image to not appear square.

Answer 3

The title of your question is different from what you're actually asking. I assume you are asking how to render a textured quad like a part of your game's HUD.

Here's what you can do to have an object rendered on top of your other objects:

void renderScene(void)
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    // Set up matrixes in order to draw your scene
    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    glOrtho2d( -1.0, 1.0,
               -1.0, 1.0 );

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();
    gluLookAt( 0.0f, 1.0f, 0.0f,
               0.0f, 0.0f, 0.0f,
               0.0f, 1.0f, 0.0f );

    // Render your scene/triangle
    /*(...)*/

    // Set up your matrixes in order to draw your HUD over the scene
    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    glOrtho2d( 0, windowWidth, 0, windowHeight );

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();

    // Disable Depth test so that the HUD will overwrite what was rendered before
    glDisable( GL_DEPTH_TEST );

    // Render your HUD elements
    /*(...)*/
    glPushMatrix();

        glBindTexture( GL_TEXTURE_2D, &someTextureId );

        glTranslate3f( 0, windowHeight - 100, 0 );
        // For example, scale your quad/texture to 100x100 pixels
        glScalef( 100.0f, 100.0f, 0.0f );

        glBegin(GL_QUADS);
            glTexCoord2f( 0.0f, 0.0f ); 
            glVertex3f( 0.0f, 0.0f, 0.0f );

            glTexCoord2f( 1.0f, 0.0f );
            glVertex3f( 1.0f, 0.0f, 0.0f );

            glTexCoord2f( 1.0f, 1.0f );
            glVertex3f( 1.0f, 1.0f, 0.0f );

            glTexCoord2f( 0.0f, 1.0f );
            glVertex3f( 0.0f, 1.0f, 0.0f );
        glEnd();

        // Remove texture
        glBindTexture( GL_TEXTURE_2D, 0 );

   glPopMatrix();

   glEnable( GL_DEPTH_TEST );

   glutSwapBuffers();
}

I've only indented the glPushMatrix and glPopMatrix to make it easier to read, it's not absolutely necessary.

If there's something you don't understand in the code sample feel free to ask.

---

Here's a series of tutorials that I recommend to get you started in OpenGL: http://lazyfoo.net/tutorials/OpenGL/index.php

There's also how to load and use textures.