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Mantear

GL_SCISSOR_TEST

7 posts in this topic

Relative to most state changes, how expensive is it so glEnable/Disable GL_SCISSOR_TEST? I have some lines of text I want to draw, but I want to constrain them to certain areas. I would also like to have pixel-based boundaries instead of character based. So, I plan on use the scissor test to crop the window where the text is drawn. My question is, should I [enable -> set scissor window for text -> disable] for each line of text, or enable once and then [set scissor window for text -> set scissor window to entire window] for each line of text? Thanks.
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Scissor testing is, on the whole, just as expensive as other fixed function state changes (found in the dear old FAQ). If this is related to your GUI text boxes, then why not just enable/set the scissor window once per text box? There's no need to do it on a line by line basis, unless the text box changes shape to accommodate each line of text. So,:
//  when rendering text box
draw any boundary/border of the text box
enable scissor testing
set scissor window
render line 1
render line 2
...
disable scissor testing
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I ran into problems creating the TextWindow class and decided I need to use scissoring. So, I've gone back and am working first on the TextBox class and getting scissoring to work there first. The TextBox class only has a single line of text. I fully intend to do what you suggested, and only perform one set of scissor instructions for the entire TextWindow draw method.

I guess my question comes down to this. Do I want to actually enable/disable scissoring on a per widget basis (TextBox, TextWindow, etc), or should I enable it once and always keep it on, and at the end of drawing each widget that uses scissoring, simply expand the scissor window to coveer the entire screen? In other words:

for (all widgets)
draw textbox
enable scissoring
set scissoring window for text
disable scissoring

or:
enable scissoring
set scissoring to cover entire window
for (all widgets)
draw textbox
set scissoring window for text
set scissoring window to cover entire window
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Aha. My guess is that you'll want to enable/disable at the coarsest granularity you can. Setting the scissor window to the size of the entire screen/window very well could result in the exact same clipping tests getting applied to the same geometry twice. So, enable/disable on a per TextBox and TextWindow (that has more than one line of text?) basis. If you're able, you may try rendering scissored things and non-scissored things separately, and dis/en-abling once. You may not be able to do so for transparency (draw order)/texturing (switching textures is more expensive) reasons.

Of course, quick tests very well may shoot this right down, which is why you should do them if you find this is a bottleneck. My bet is that it's not.
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It's not a bottle-neck as far as I can tell, so this could very well fall under micro-optimizations except for the fact that it is somewhat of a design decision.

Sorting based upon scissoring is out of the question, I think, since there are so many other things I could sort by that would make a larger impact.

I see your point about creating an additional clipping check for everything drawn if I simply leave it enabled. That sounds bad to me in the long run, since there should be many more things drawn than text boxes/windows (yes, a text window has multiple lines of text, scroll bars, etc, that a text box does not). I think I'm going to go the route of enabling/disabling the scissors test for each text widget.

Thanks for the input!
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On all modern hardware, the scissor test is always enabled. So disabling it in OpenGL actually just makes the driver set the scissor rect to the whole viewport. It's best to leave the scissor test enabled in OpenGL, and change the scissor rect as needed.
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Do you have a reference for that? What is considered 'modern'?

I guess I need to change my plans and always have scissoring enabled. Thank you for the information.
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