Re: OpenGL in draw callback

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Tom McReynolds (tomcat++at++proxima.asd.sgi.com)
Sat, 15 Jun 1996 21:48:50 -0700

On Jun 14, 8:46am, Jeannie Burleson wrote:
> Subject: OpenGL in draw callback
>
> Hello,
>
> I am very new to OpenGL -- but I think that my question
> is a pretty *basic* one. I am trying to do some pixel
> manipulation (OpenGL) in my draw callback function (using
> perfly).
>
> Since, I've been having problems all I would like to
> do is read in the frame buffer & write it back out again.
> No changes, nothing. The code I've added in my callback
> is:
>
> ...
> PreDraw() ;
>
> // my predraw stuff
>
> glReadPixels ( 0, 0, x_window_size, y_window_size,
> GL_RGBA, GL_BYTE, buffer ) ;
>
> // this is where I'll add in my manipulation stuff
>
> glRasterPos2i ( 0, 0 ) ;
> glDrawPixels ( 0, 0, x_window_size, y_window_size,
> GL_RGBA, GL_BYTE, buffer ) ;
>
> pfDraw() ;
> PostDraw() ;
>
> Definitions:
> struct PixelType
> {
> unsigned char r, g, b, a ;
> } ;
> int x_window_size = 1280 ;
> int y_window_size = 1024 ;
> PixelType *buffer = new PixelType [ x_window_size * y_window_size ] ;
>
>
> I would have thought that these lines of code would do
> essentially nothing. BUT, I end up with 2 images on the screen.
> The first starts at the lower left hand corner, and the second
> image starts to the right & above the middle point in the screen.
> Most (3/4) of the second image isn't displayed since it falls
> off the screen. I thought that the glRasterPos2i would force
> the new image (glDrawPixels) to be drawn starting at the lower
> left hand of the frame buffer.
>

I think your problem is that the current raster position is described in object
coordinates, not window coordinates. You'll need to figure out where your
lower left corner of the screen is defined in object space. I don't know
what's on the matrix stack at the start of the draw callback off hand, although
I suspect it's the identity matrix for MODELVIEW and PROJECTION. Even if it
isn't, you can push the matricies, set them as you want, then pop them
when your through.

It's sounds to me like the following is happening:

You're saving what was in the previous frame (no clear was done since you
set a draw callback; that's now up to you)

Now your redrawing the saved image (after your processing) using glDrawPixels
with the lower left corner of the image at the center of the screen (the
location of the origin in default NDC coordinates, which is what you get with
no transforms)

now you call pfDraw and draw over all or part of the image you rendered
with glDrawPixels

Try glRasterPos2i(-1, -1) and see if that fixes it.

                        -Tom

> I figure I must be using these routines incorrectly, but I
> can't figure it out from the man pages. I would appreciate any
> help!! Thanks in advance.
>
> Jeannie
>
> ********************************************************
> Jeannie M. Burleson email: jeannie++at++cambertx.com
> Camber Corporation voice: (214) 991-5322 x160
> 4885 Alpha Rd. Suite 110 fax: (214) 991-5352
> Dallas, TX 75244
> ********************************************************
>
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>-- End of excerpt from Jeannie Burleson

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