OpenGL texture inverted - c++

Question

I'm trying to map the input from my webcam to a plane in OpenGL. I'm using OpenCV to get the images from the webcam.
The problem I have is that the texture is vertically inverted, if my texture is "v", the current result is "^".
I want to fit the image taken from the webcam to my plane (2x2). Its lower left corner is -1, -1 and the upper right corner is 1,1.
The code is:
const int VIEWPORT_WIDTH = 640;
const int VIEWPORT_HEIGHT = 480;
const int KEY_ESCAPE = 27;
CvCapture* g_Capture;
IplImage* image;
GLint g_hWindow;
GLvoid InitGL();
GLvoid OnDisplay();
GLvoid OnReshape(GLint w, GLint h);
GLvoid OnKeyPress (unsigned char key, GLint x, GLint y);
int main(int argc, char* argv[])
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE | GLUT_DEPTH);
glutInitWindowPosition(100, 100);
glutInitWindowSize(VIEWPORT_WIDTH, VIEWPORT_HEIGHT);
g_hWindow = glutCreateWindow("Image");
image = cvLoadImage("average.jpg", 1);
gluBuild2DMipmaps(GL_TEXTURE_2D, GL_RGB, image->width, image->height, GL_RGB, GL_UNSIGNED_BYTE, image->imageData);
InitGL();
glutMainLoop();
return 0;
}
GLvoid InitGL()
{
glClearColor (0.0, 0.0, 0.0, 0.0);
glutDisplayFunc(OnDisplay);
glutReshapeFunc(OnReshape);
glutKeyboardFunc(OnKeyPress);
}
GLvoid OnDisplay(void)
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glEnable(GL_TEXTURE_2D);
glPushMatrix();
glTranslatef(0.0f, 0.0f, -2.5f);
glBegin(GL_QUADS);
glTexCoord2f(0.0f, 0.0f);
glVertex3f(-1.0f,-1.0f, 1.0f);
glTexCoord2f(1.0f, 0.0f);
glVertex3f( 1.0f,-1.0f, 1.0f);
glTexCoord2f(1.0f, 1.0f);
glVertex3f( 1.0f, 1.0f, 1.0f);
glTexCoord2f(0.0f, 1.0f);
glVertex3f(-1.0f, 1.0f, 1.0f);
glEnd();
glPopMatrix();
glutSwapBuffers();
}
GLvoid OnReshape(GLint width, GLint height)
{
if (height==0)
height=1;
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glViewport(0, 0, width, height);
gluPerspective(45.0f,(GLfloat)width/(GLfloat)height, 1.0f, 10.0f);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt( 0.0, 0.0, 0.0, 0.0, 0.0, -6.0, 0.0f, 1.0f, 0.0f);
}
GLvoid OnKeyPress(unsigned char key, int x, int y)
{
switch (key) {
case KEY_ESCAPE:
cvReleaseImage(&image);
glutDestroyWindow(g_hWindow);
exit(0);
break;
}
glutPostRedisplay();
}
BTW. In this code I'm loading an image instead of getting from the webcam.
Any suggestion?

Answer 1

Flip the texture coordinates on the quad you're rendering. OpenGL does store texture "upside-down" from how most people assume they'd be, so the simplest way to handle it is just work with OGL.
glTexCoord2f(0.0f, 1.0f);
glVertex3f(-1.0f,-1.0f, 1.0f);
glTexCoord2f(1.0f, 1.0f);
glVertex3f( 1.0f,-1.0f, 1.0f);
glTexCoord2f(1.0f, 0.0f);
glVertex3f( 1.0f, 1.0f, 1.0f);
glTexCoord2f(0.0f, 0.0f);
glVertex3f(-1.0f, 1.0f, 1.0f);
should do it, I think.