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Re: glutSwapBuffers bug?

Subject: Re: glutSwapBuffers bug?
From: Kenneth Tilton <email@hidden>
Date: Sat, 13 Nov 2004 13:53:55 -0500
Delivered-to: email@hidden
Delivered-to: email@hidden


On Nov 13, 2004, at 12:52 PM, Geoff Stahl wrote:

I woill have to inspect the code on this. Top of my head there should be no reason to do a flush then swap or a swap then flush as the swap should implicitly flush (nothing that you will in the code as this will happen in the engine) all pending commands. I will have to refresh my memory on how single buffered contexts are handled by GLUT to know if a glutSwap should automatically be a glFlush.

Feel free to write a bug with a short sample of the improper behavior that you are seeing as the bottom line is we should be in line with the latest official glut spec (not freeglut).

First of all, I understand completely that Freeglut is beside the point. I just mentioned that FWIW. Also FWIW, the Freeglut crowd is rather obsessive about being compatible right down to the bugs with the original Glut, and so they do study the original pretty closely. Anyway...

Write a bug? Uh, does Lisp count? <g> Here is the glutDisplayFunc, which sometimes works and sometimes does not, as described below:

(ccl::defcallback display-cb (:void)
  (let ((bounds #2a((0.0 0.0) (250.0 500.0) (500.0 0.0)))
	(point #(75.0 50.0)))
    (declare (ignorable bounds point))
    (#_glClearColor 0.0 1.0 0.0 1.0)
    (#_glClear #$GL_COLOR_BUFFER_BIT)

    ;; everything above always runs -------------
   ;; everything below was tried in diff combos, results below code

;; option "glBegin/glEnd" (progn (#_glBegin #$GL_POINTS) (dotimes (i 5000) (let ((j (random 3))) (setf (aref point 0) (/ (+ (aref point 0) (aref bounds j 0)) 2.0) (aref point 1) (/ (+ (aref point 1) (aref bounds j 1)) 2.0)) (#_glVertex2f (aref point 0) (aref point 1)))) (#_glEnd))

   ;; option "glFlush"
    ;; (#_glFlush)

   ;; option "glutSwapBuffers"
    ;; (#_glutSwapBuffers)
    ))

Either of these next two are sufficient, but only one is necessary to produce something other than a blank screen:

    "glBegin/End" green screen with pretty picture
    "glFlush" green screen

Bug: option "glutSwapBuffers" alone will not produce the green background.

To be clear, if none of the options runs I also do not get the green background.

I was thrown for a bit while preparing this when the glFlush call proved unnecessary as long as the big vertex sequence was operational. Perhaps glEnd does a glFlush? ie, maybe if there were some small gl calls after the glEnd they would not take effect without the glFlush. Anyway, I left that in purely in the interest of full disclosure. You can chop that whole sequence and have a simpler report.

btw, if you are curious, i can steer you to a nice downloadable which neatly installs OpenMCL along with Emacs and nice interface from Emacs to OpenMCL. With that and about three instructions you could run working/nonworking code yourself.

cheers, kenny


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References:
	>Re: glutSwapBuffers bug? (From: Geoff Stahl <email@hidden>)

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