I am working on a 3D volume rendering application, which download volume data to the graphic card as 3D textures and do ray casting. To maximize performance I would like to make my 3D textures resident as much as possible. So I create all the textures with TexImage3D(), sample them in my shader, and let OpenGL do all the texture management. This works fine. As a result if I am rendering a 600MB dataset on a card with 512MB VRAM, my program will take all video memory available.
The problem arises when I want to start a second instance of my program to render another dataset at the same time. I am unable to allocate any textures because there is no VRAM available. This supprised me a little bit because I would expect the display driver to do something smart like swapping out textures from the first program and make room for the new textures from the second program. But this doesn’t seem to be the case. So my question is, is this really how it’s supposed to work? If one greedy program grabs all the video memory then all other programs would fail to allocate any textures? If so, is there any workaround I can use to solve my problem? Is this controlled by OpenGL? the display driver? or the operating system?
I wrote a standalone test program to show the issue. Running the program will allocate 100 2D textures (8MB each). The second instance (or the third, depending on the amount of VRAM on your card) will fail to run, even after the first instance delete all its textures. However if the first instance delete its GL context, the second instance will be able to allocate textures again.
#include "windows.h"
#include <iostream>
#include "gl/glew.h"
#include "gl/glut.h"
using namespace std;
int main(int argc, char* argv[])
{
// create gl context
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH);
glutCreateWindow("Test");
GLenum err = glewInit();
if (GLEW_OK != err)
{
cout << "Error: " << glewGetErrorString(err) << endl;
}
int nTextures = 100;
GLuint* textures = new GLuint[nTextures];
glGenTextures(nTextures, textures);
// create FBO
GLuint fbo;
glGenFramebuffersEXT(1, &fbo);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo);
for (int i = 0; i < nTextures; ++i)
{
// create texture
glBindTexture(GL_TEXTURE_2D, textures[i]);
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA16F_ARB,
1024, 1024, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
// attach texture to fbo
glFramebufferTexture2DEXT( GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT,
GL_TEXTURE_2D, textures[i], 0);
glClear(GL_COLOR_BUFFER_BIT);
GLenum status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
if (status != GL_FRAMEBUFFER_COMPLETE_EXT)
{
cout << "Error found (fbo): " << status << endl;
cout << "press Enter to exit." << endl;
getchar();
return 0;
}
GLenum error = glGetError();
if (error != GL_NO_ERROR)
{
cout << "Error found (gl): " << error << endl;
cout << "press Enter to exit." << endl;
getchar();
return 0;
}
}
glFramebufferTexture2DEXT( GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT,
GL_TEXTURE_2D, 0, 0);
cout << "press Enter to delete textures." << endl;
getchar();
glDeleteTextures(nTextures, textures);
glDeleteFramebuffersEXT(1, &fbo);
delete [] textures;
cout << "press Enter to delete gl context." << endl;
getchar();
HGLRC rc = wglGetCurrentContext();
wglMakeCurrent(NULL, NULL);
wglDeleteContext(rc);
cout << "press Enter to exit." << endl;
getchar();
return 0;
}
