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Editorial Team
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Asked: 2026-05-23T02:45:50+00:00

I have a Java OpenGL (JOGL) app, and I’m trying to create a texture

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I have a Java OpenGL (JOGL) app, and I’m trying to create a texture mapped quad that covers the entire screen. In draw some pixels to a buffer and then I want to read those pixels into a texture and redraw them on screen (with a fragment shader applied). My code for mapping the texture to the viewport is:

        gl.glMatrixMode(GL.GL_PROJECTION);                                        
        gl.glPushMatrix();                                                 
        gl.glLoadIdentity();                                               
        gl.glOrtho( 0, width, height, 0, -1, 1 );                          
        gl.glMatrixMode(GL.GL_MODELVIEW);                                  
        gl.glPushMatrix();                                                 
        gl.glLoadIdentity();
        IntBuffer ib = IntBuffer.allocate(1);
        gl.glEnable(GL.GL_TEXTURE_2D);
        gl.glGenTextures(1, ib);
        gl.glPixelStorei(GL.GL_PACK_ALIGNMENT, 1);
        //buff contains pixels read from glReadPixels
        gl.glTexImage2D(GL.GL_TEXTURE_2D, 0, GL.GL_RGBA, width, height, 0, GL.GL_RGBA, GL.GL_UNSIGNED_BYTE, buff);
        gl.glBindTexture(GL.GL_TEXTURE_2D, ib.get(0));
        gl.glBegin(GL.GL_QUADS);
        gl.glTexCoord2f(0,1);
        gl.glVertex2f(0,0); 
        gl.glTexCoord2f(0,0);
        gl.glVertex2f(0,height);
        gl.glTexCoord2f(1,0);   
        gl.glVertex2f(width,height);
        gl.glTexCoord2f(1,1);      
        gl.glVertex2f(width,0);
        gl.glEnd();
        gl.glBindTexture(GL.GL_TEXTURE_2D, 0);
        gl.glPopMatrix();
        gl.glPopMatrix();

The end result is a quad that is not covering the whole viewport (it’s partially on) and that does not contain the pixels from the buffer. What am I doing incorrectly here?

thanks,
Jeff

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1 Answer

  1. Editorial Team

    First, you should only create the texture in your initialization code. You should not be calling glTexImage2D every frame. Only call glTexImage2D again if the size of the texture changes; glTexSubImage2D can be used to upload data to the texture. Think of glTexImage2D as “new”, while glTexSubImage2D as a memory copy.

    Do this once, after initializing OpenGL.

    IntBuffer ib = IntBuffer.allocate(1);  //Store this in your object
gl.glGenTextures(1, ib);
gl.glPixelStorei(GL.GL_PACK_ALIGNMENT, 1);
//buff contains pixels read from glReadPixels
gl.glBindTexture(GL.GL_TEXTURE_2D, ib.get(0));
gl.glTexImage2D(GL.GL_TEXTURE_2D, 0, GL.GL_RGBA, width, height, 0, GL.GL_RGBA, GL.GL_UNSIGNED_BYTE, 0);
gl.glBindTexture(GL.GL_TEXTURE_2D, 0);

    Then, each frame, do this:

    gl.glMatrixMode(GL.GL_PROJECTION);                                        
gl.glPushMatrix();                                                 
gl.glLoadIdentity();                                               
gl.glMatrixMode(GL.GL_MODELVIEW);                                  
gl.glPushMatrix();                                                 
gl.glLoadIdentity();

gl.glBindTexture(GL.GL_TEXTURE_2D, ib.get(0));  //Retrieved from your object
gl.glEnable(GL.GL_TEXTURE_2D);
gl.glTexSubImage2D(GL.GL_TEXTURE_2D, 0, 0, 0, width, height, 0, GL.GL_RGBA, GL.GL_UNSIGNED_BYTE, buff);

gl.glBegin(GL.GL_QUADS);
    gl.glTexCoord2f(0,1);
    gl.glVertex2f(-1, -1); 
    gl.glTexCoord2f(0, 0);
    gl.glVertex2f(-1, 1);
    gl.glTexCoord2f(1, 0);   
    gl.glVertex2f(1, 1);
    gl.glTexCoord2f(1, 1);      
    gl.glVertex2f(1, -1);
gl.glEnd();

gl.glMatrixMode(GL.GL_MODELVIEW);                                  
gl.glPopMatrix();   
gl.glMatrixMode(GL.GL_PROJECTION);                                        
gl.glPopMatrix();   
gl.glMatrixMode(GL.GL_MODELVIEW);

    By using identity for projection and modelview, we are able to supply vertex coordinates directly in clip-space. The [-1, 1] range in clip-space maps to [0, width/height] in window space. So we don’t have to know or care about how big the window is; as long as the glViewport was set up correctly, this should work.

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