Q:
How does one share textures between mulitple OpenGL contexts?
A:
Textures can be easily shared between OpenGL contexts on
Mac OS X and Mac OS 9.
// From the "Carbon SetupGL" sample code (Carbon SetupGL.c line 733):
*paglContext = aglCreateContext (pcontextInfo->fmt, aglShareContext);
if (aglShareContext == NULL)
aglShareContext = *paglContext;
---
// From the "Carbon SetupGL" sample code (Carbon SetupGL Test.c line 576):
// gTexture is the global texturing flag.
// structWindowInfo contains the agl context for each window
// (stored in the window's refcon)
// LoadTextureRes just loads a buffer from a resource file
long i = 0;
GLbyte * pBuffer = NULL;
short width = 0, height = 0;
gTexture = 1 - gTexture;
if (gTexture)
{
if (gpWindowList[0] != NULL) // load the texture for the first window
{
structWindowInfoPtr pWindowInfo =
(structWindowInfoPtr) GetWRefCon (gpWindowList [i]);
aglSetCurrentContext(pWindowInfo->aglContext);
glEnable (GL_TEXTURE_2D);
if (nameTexture)
glDeleteTextures (1, &nameTexture);
glGenTextures (1, &nameTexture);
glBindTexture(GL_TEXTURE_2D, nameTexture);
LoadTextureRes (1000, &pBuffer, &width, &height);
glTexImage2D (GL_TEXTURE_2D, 0, 3, width, height, 0,
GL_RGB, GL_UNSIGNED_BYTE, pBuffer);
}
}
// enable texturing for all windows but do not load texture again
for (i = 0; kMaxWindows > i; i++)
{
if (gpWindowList[i] != NULL)
{
structWindowInfoPtr pWindowInfo =
(structWindowInfoPtr) GetWRefCon (gpWindowList [i]);
aglSetCurrentContext(pWindowInfo->aglContext);
if (gTexture)
{
glEnable (GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, nameTexture);
}
else
glDisable (GL_TEXTURE_2D);
}
}
if (pBuffer)
DisposePtr ((Ptr) pBuffer);
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Listing 1. Creating shared contexts and textures
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First, create all contexts as shared contexts (see below).
The requirement to create all contexts first maybe lifted
in future versions of Mac OS X. Once all contexts are created,
create the texture(s) and texture to any one context. All
other contexts will share the texture that is current or
one that you can bind via glBindTexture.
The specifics:
- Create shared contexts:
The first part of the code in listing 1 shows the creation
of shared contexts. Where the global/static variable aglShareContext
is the context to share. aglShareContext is initially set
globally to NULL and then set to the first context created
for all other contexts. This will, in effect, share all
contexts.
- Load and share textures:
The second half of the code in listing 1 shows how to load
a texture for the first context and then just bind this for
use with the other contexts.
This technique is illustrated completely in the "Carbon
SetupGL" sample on the sample code web site at <http://developer.apple.com/samplecode/>.
[May 01 2001]
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