icetGLDrawCallback(3)

💅 icetGLDrawCallback — bestie edition 💅

okay so this is a page from the IceT Reference manual, section 3, and it's about one lil function that does a big job. grab a snack, we're doing this 🍓

---

✨ NAME ✨

icetGLDrawCallback — set a callback for drawing with OpenGL. that's it, that's the vibe 💖

---

🌸 Synopsis 🌸

#include <IceTGL.h>

typedef void (* IceTGLDrawCallbackType)( void );

void icetGLDrawCallback(   IceTGLDrawCallbackType   callback  );

---

🦋 Description 🦋

okay so icetGLDrawCallback sets a callback that gets used to draw the geometry from a given viewpoint. and the way it works is it gets implicitly called from inside icetGLDrawFrame. sneaky! iconic! 💅

callback should be a function that issues the appropriate OpenGL calls to draw geometry in the current OpenGL context. after callback is called, the image left in the frame buffer specified by icetGLSetReadBuffer gets read back for compositing. so like, whatever you painted, that's what gets picked up 🎀

bestie. listen. callback should NOT modify GL_PROJECTION_MATRIX. no cap. if you do, IceT will place image data in the wrong location in the tiled display AND improperly cull geometry. it's giving chaos 😭

adding transformations to GL_MODELVIEW_MATRIX is acceptable, but heads up: the bounding vertices given with icetBoundingVertices or icetBoundingBox are assumed to already be transformed by any such changes to the modelview matrix. also, GL_MODELVIEW_MATRIX must be restored before the draw function returns. so any changes to GL_MODELVIEW_MATRIX are to be done with care and should be surrounded by a pair of glPushMatrix and glPopMatrix functions. wrap it up, protect the vibe 🫶

it is also super important that callback not attempt to change the clear color. in some compositing modes, IceT needs to read, modify, and change the background color. those operations will be lost if callback changes the background color, and severe color blending artifacts may result. not the aesthetic we're going for 💀

IceT may call callback several times from within a call to icetGLDrawFrame — or not at all if the current bounds lie outside the current viewpoint. this can have a subtle but important impact on the behavior of callback. for example, counting frames by incrementing a frame counter in callback is obviously wrong (although you could count how many times a render occurs). callback should also leave OpenGL in a state such that it will be correct for a subsequent run of callback. any matrices or attributes pushed in callback should be popped before callback returns, and any state that is assumed to be true on entrance to callback should also be true on return. leave it how you found it, periodt ✨

the callback function pointer is placed in the ICET_GL_DRAW_FUNCTION state variable. noted 📝

icetGLDrawCallback is similar to icetDrawCallback. the difference is that the callback set by icetGLDrawCallback is used by icetGLDrawFrame and the callback set by icetDrawCallback is used by icetDrawFrame. twin functions, different lanes 👑

---

😭 Errors 😭

• ICET_INVALID_OPERATION — raised if the icetGLInitialize has not been called. like, you gotta start the engine first bestie 🚗

---

🙈 Warnings 🙈

None. we love a warning-free situation 💜

---

💀 Bugs 💀

None known. she's clean ✨

---

🧚 Notes 🧚

callback is tightly coupled with the bounds set with icetBoundingVertices or icetBoundingBox. if the geometry drawn by callback is dynamic (changes from frame to frame), then the bounds may need to be changed as well. incorrect bounds may cause the geometry to be culled in surprising ways. surprising is not the same as fun, iykyk 🌈

---

👑 Copyright 👑

Copyright (C)2003 Sandia Corporation

Under the terms of Contract DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains certain rights in this software.

This source code is released under the New BSD License. 🎀

---

🌈 See Also 🌈

• icetBoundingBox(3)
• icetBoundingVertices(3)
• icetDrawCallback(3)
• icetGLDrawFrame(3)

---

and that's the whole page bestie 💖 this was the IceT Reference manual, and the page was dated August 9, 2010 — ancient, but the lore holds up. go draw something iconic 🦋✨