OpenGL Programming Guide
Programming Guide > Appendix g


Appendix G
Programming Tips

This appendix lists some tips and guidelines that you might find useful. Keep in mind that these tips are based on the intentions of the designers of the OpenGL, not on any experience with actual applications and implementations! This appendix has the following major sections:


OpenGL Correctness Tips

 
gluOrtho2D(0, width, 0, height);

where width and height are the dimensions of the viewport. Given this projection matrix, polygon vertices and pixel image positions should be placed at integer coordinates to rasterize predictably. For example, glRecti(0, 0, 1, 1) reliably fills the lower left pixel of the viewport, and glRasterPos2i(0, 0) reliably positions an unzoomed image at the lower left of the viewport. Point vertices, line vertices, and bitmap positions should be placed at half-integer locations, however. For example, a line drawn from (x1, 0.5) to (x2, 0.5) will be reliably rendered along the bottom row of pixels into the viewport, and a point drawn at (0.5, 0.5) will reliably fill the same pixel as glRecti(0, 0, 1, 1).

An optimum compromise that allows all primitives to be specified at integer positions, while still ensuring predictable rasterization, is to translate x and y by 0.375, as shown in the following code fragment. Such a translation keeps polygon and pixel image edges safely away from the centers of pixels, while moving line vertices close enough to the pixel centers.

glViewport(0, 0, width, height);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0, width, 0, height);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0.375, 0.375, 0.0);
/* render all primitives at integer positions */
  • Avoid using negative w vertex coordinates and negative q texture coordinates. OpenGL might not clip such coordinates correctly and might make interpolation errors when shading primitives defined by such coordinates.
  • Do not assume the precision of operations, based upon the data type of parameters to OpenGL commands. For example, if you are using glRotated(), you should not assume that geometric processing pipeline operates with double-precision floating point. It is possible that the parameters to glRotated() are converted to a different data type before processing.

OpenGL Performance Tips

  • Use glColorMaterial() when only a single material property is being varied rapidly (at each vertex, for example). Use glMaterial() for infrequent changes, or when more than a single material property is being varied rapidly.
  • Use glLoadIdentity() to initialize a matrix, rather than loading your own copy of the identity matrix.
  • Use specific matrix calls such as glRotate*(), glTranslate*(), and glScale*() rather than composing your own rotation, translation, or scale matrices and calling glMultMatrix().
  • Use query functions when your application requires just a few state values for its own computations. If your application requires several state values from the same attribute group, use glPushAttrib() and glPopAttrib() to save and restore them.
  • Use display lists to encapsulate potentially expensive state changes.
  • Use display lists to encapsulate the rendering calls of rigid objects that will be drawn repeatedly.
  • Use texture objects to encapsulate texture data. Place all the glTexImage*() calls (including mipmaps) required to completely specify a texture and the associated glTexParameter*() calls (which set texture properties) into a texture object. Bind this texture object to select the texture.
  • If the situation allows it, use gl*TexSubImage() to replace all or part of an existing texture image rather than the more costly operations of deleting and creating an entire new image.
  • If your OpenGL implementation supports a high-performance working set of resident textures, try to make all your textures resident; that is, make them fit into the high-performance texture memory. If necessary, reduce the size or internal format resolution of your textures until they all fit into memory. If such a reduction creates intolerably fuzzy textured objects, you may give some textures lower priority, which will, when push comes to shove, leave them out of the working set.
  • Use evaluators even for simple surface tessellations to minimize network bandwidth in client-server environments.
  • Provide unit-length normals if it's possible to do so, and avoid the overhead of GL_NORMALIZE. Avoid using glScale*() when doing lighting because it almost always requires that GL_NORMALIZE be enabled.
  • Set glShadeModel() to GL_FLAT if smooth shading isn't required.
  • Use a single glClear() call per frame if possible. Do not use glClear() to clear small subregions of the buffers; use it only for complete or near-complete clears.
  • Use a single call to glBegin(GL_TRIANGLES) to draw multiple independent triangles rather than calling glBegin(GL_TRIANGLES) multiple times, or calling glBegin(GL_POLYGON). Even if only a single triangle is to be drawn, use GL_TRIANGLES rather than GL_POLYGON. Use a single call to glBegin(GL_QUADS) in the same manner rather than calling glBegin(GL_POLYGON) repeatedly. Likewise, use a single call to glBegin(GL_LINES) to draw multiple independent line segments rather than calling glBegin(GL_LINES) multiple times.
  • Some OpenGL implementations benefit from storing vertex data in vertex arrays. Use of vertex arrays reduces function call overhead. Some implementations can improve performance by batch processing or reusing processed vertices.
  • In general, use the vector forms of commands to pass precomputed data, and use the scalar forms of commands to pass values that are computed near call time.
  • Avoid making redundant mode changes, such as setting the color to the same value between each vertex of a flat-shaded polygon.
  • Be sure to disable expensive rasterization and per-fragment operations when drawing or copying images. OpenGL will even apply textures to pixel images if asked to!
  • Unless absolutely needed, avoid having different front and back polygon modes.

GLX Tips

  • Use glXWaitGL() rather than glFinish() to force X rendering commands to follow GL rendering commands.
  • Likewise, use glXWaitX() rather than XSync() to force GL rendering commands to follow X rendering commands.
  • Be careful when using glXChooseVisual(), because boolean selections are matched exactly. Since some implementations won't export visuals with all combinations of boolean capabilities, you should call glXChooseVisual() several times with different boolean values before you give up. For example, if no single-buffered visual with the required characteristics is available, check for a double-buffered visual with the same capabilities. It might be available, and it's easy to use.