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Grazer2

OpenGL render to texture question

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hey there, i got a small question concerning render to texture in openGL. is it possible to have two pbuffers, render something into the first one, bind this pbuffer to a texture, then use this texture to render into the second pbuffer? in the end i wanna have a loop of always rendering into one pbuffer using the other one bound to a texture. i know its not possible to do this with just one pbuffer because you cant render into it while it's still bound to a texture. but is this also an invalid operation with 2 pbuffers? basically what i'm trying to implement is a motion blur for just a specific object in the scene. so i wanna render this object into a texture, do some image processing (run a gaussian filter over it) on this texture, update the object position, then first draw that pbuffer texture into another pbuffer, so after that i can draw the updated object into this other pbuffer and do the image processing on that new texture and so on... oh and why i'm asking this is because i can't seem to get it to work. it doesn't seem to be using the old pbuffer texture for rendering into the new pbuffer.

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The simple answer is yes you can use two Pbuffers to image process a image N times.

1 . Render into A, bind it to a texture.
2 . Render into B using A as a texture.
3 . Unbind A, Render into A using B as a texture.
4 . Unbind B, Render into B using A as a texture.

Keep doing 3 and 4 till you're done.

Though you'd have to make sure the PBuffers shared contexts, else they wouldn't know about each others textures.

GOCP

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You must declare the pixelformat for the pbuffer context with flags allowing it to be bound to a texture; are you doing that?

I'm pretty sure you can ping pong a texture between the back and front buffer of 1 pbuffer context as well, thus avoiding the N context switches that would be necessary with the 2 pbuffer approach.

I can tell you that I've rendered to texture on the back buffer and then used it as a texture on the front buffer of the same pbuffer context...

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Quote:
Original post by Aeluned
I can tell you that I've rendered to texture on the back buffer and then used it as a texture on the front buffer of the same pbuffer context...


Oh really? I thought there was no way this could be possible, because i've read about this restriction of not being able to use the same pbuffer as texture for another render into itself. That's the only reason why i've created a second pbuffer, otherwhise i would've gone with front and back of one pbuffer since all i need is 2 of these. I'm definately gonna go with your solution then.

Anyways, thanks a lot to you both. Now at least I know that i've got some error in my code somewhere and it's not a hardware restriction. I already got to work plain render-to-texture with one target-texture. It's just the ping-pong thing i couldn't get to work yet. Hopefully i can find the error myself now.

thanks for now.

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Guest Anonymous Poster
nope, i just can't seem to get it to work. i guess its one of my last resorts to just dump some source code and wait and pray that someone can be arsed to read through it and maybe spot a logical problem. so that's just what im gonna do now. the most important parts of my ping pong loop in a mixture of pseudo and real code look as follows:


//==============start============================
//
//my 2 pbuffer objects:
//pbuffer_[0]
//pbuffer_[1]
//int new = 0;
//int old = 1;

flip(new, old); //i.e.: iteration 1: (new = 1, old = 0), iteration 2: (new = 0, old = 1), ...

wglMakeCurrent(pbuffer_[new]);

glClear(...);

//draw old pbuffer to new one
glBindTexture(GL_TEXTURE_2D, render_texture);
glEnable(GL_TEXTURE_2D);
-> render a plane with texture of old scene to fill the screen with the previous scene (using glOrtho etc...)
glDisable(GL_TEXTURE_2D);

wglMakeCurrent(regular_display);

wglReleaseTexImageARB(pbuffer_[old]);
glBindTexture(GL_TEXTURE_2D, render_texture);
wglBindTexImageARB(pbuffer_[new]);

//==============end============================

it might look odd that i put the ReleaseTex function at the end of the loop, but this should be alright. at the next iteration, after the "flip(new, old)" the texture should be bound to the old pbuffer and the rendering should be performed in the new one which was released at the end of the last iteration.

to see something on the screen i just started out with an object in one of the buffers, then go through this function and afterwards draw the contents of the current pbuffer to the screen. then loop these last 2 steps. in my opinion i should get a static picture of my initial object (which gets swapped as a texture on a plane between the 2 buffers with render to texture, but not explicitely re-drawn). instead i just get a black screen.

again, with just one pbuffer the render to texture thing works (tested in a slightly altered version of the apllication).

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