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How to draw single pixel in GL 3+ (without fixed pipeline)?


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#1 junkyska   Members   -  Reputation: 143

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Posted 03 February 2012 - 07:09 AM

Hi everyone,

I'm integrating Gwen (Gui system) in my engine. I'm using Open GL 3+ (without fixed pipeline, all in the shaders).
I have a problem drawing a single pixel because it's really SLOW. What I'm doing now is drawing a rectangle with startX = endX and startY = endY, but when the Gui have to render many single pixels (ex: Color pick up box) the frame rate drops down.
Is there any efficent way of doing this?

Sorry for my English, it's not very good.

Thanks.

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#2 Brother Bob   Moderators   -  Reputation: 7870

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Posted 03 February 2012 - 07:16 AM

Don't render pixel by pixel. At the very least, make a buffer containing all pixels, and draw them all in one go.

But a better approach is not to ask how to draw a pixel as efficiently as possible. Your problem is not that drawing an individual pixel is slow, but that you're drawing individual pixels in the first place. Instead, describe what you want to achieve, and we can come up with a much better solution that perhaps doesn't involve pixel drawings at all.

#3 junkyska   Members   -  Reputation: 143

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Posted 03 February 2012 - 07:31 AM

First of all thanks to answer.

I'm integrating Gwen (http://code.google.com/p/gwen/) a gui library. To achive this, I have to implement my custom UI render. This is mostly done.
Gwen define his own render interface, so I have to implement this interface.
One of this functions is draw colored rectangle. This function works well.
Another function is draw a single pixel. To achive this I call the function that draws a colored rectangle but with the rectangle bounds like that: rect.x = pixel.x, rect.y = pixel.y, rect.heigh = 1, rect.width = 1.
The problem comes when the Gui system tries to perform many draw pixel operations. This drops down the frame rate.
I attach an image showing that problem (Color pick up), the frame rates drops down to 10 fps.

draw_pixel_problem.jpg

#4 Brother Bob   Moderators   -  Reputation: 7870

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Posted 03 February 2012 - 08:01 AM

I don't know Gwen. Are you implementing your own color box or are you trying to improve the performance of the existing color box?

If the former, don't draw it pixel by pixel as I said. Make an image, a texture, or any kind or separate buffer that contains the entire set of pixels. Draw that buffer, and update it only when you need to change the colors within the buffer.

#5 mhagain   Crossbones+   -  Reputation: 7579

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Posted 03 February 2012 - 08:06 AM

The way I would do this is: each time DrawSinglePixel is called, add the parameters to a list (which could be stored in a VBO in a format that you can use directly for drawing with). So long as nothing else happens keep adding for each DrawSinglePixel call, but as soon as something else happens (a state change or something else needs to be drawn, or you reach the max size of your list) you flush the entire list in one go and reset it to the start. That would achieve reasonable batching without much in the way of overhead.

It appears that the gentleman thought C++ was extremely difficult and he was overjoyed that the machine was absorbing it; he understood that good C++ is difficult but the best C++ is well-nigh unintelligible.





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