OpenGL: When and how often am I supposed to call glLineWidth, glPointSize, glLineStyle?

As far as I know it is common practice to call glColor4f or the like each time before drawing an object or primitive.开发者_开发问答

But what about point and line style properties?

• Is it normal to call glLineSize and glPointSize very often?

• Should I store a backup of the current point size and set it back after drawing, or simply call glPointSize before drawing any point, even ones which use the default size?

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Unless you are drawing tens to hundreds of thousands of lines, it really won't matter. And even then, you should profile and verify that this actually matters to performance. But let's assume you did that.

Minimizing the number of state changes could improve your performance. This means that you should sort your lines by line size and your points by point size. That way, lines that are all the same size can be drawn at the same time. This of course assumes that you could draw the lines in any order. If you need the lines to be drawn in a certain order, then you will have to live with the state changes.

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Avoid glGet** functions to determine current line width / point size. It is a big performance eater.

Instead store current property localy and update when necessary (preferred), or use glPushAttrib(GL_LINE_BIT) / glPopAttrib.

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OpenGL is a state machine, so the only important rule is, that you set state when you need it and whenever it changes. You need a certain line width? Set it and be done width. You need a number of different line widths in a single code segment: Sort by line width and set once for every line width.

Some states are expensive to switch, so it's a good idea to keep track of those; in particular the states in question are anything related to texture binding (either to a texture unit or as FBO attachment) and shaders. Everything else is actually quite cheap to change.

In general it's a good idea to set OpenGL state explicitly and don't assume certain states being preset from earlier. This also covers the transformation matrices and setup: Do a full viewport and projection setup at every beginning of the display function; advanced applications will have to change those multiple times drawing a single frame anyway (so no glViewport, glMatrixMode(GL_PROJECTION), ... in a reshape handler).