I've been playing around with OpenGL for a full week or equivalent.
After 2D I'm now trying 3D. I want to reproduce the 3D scene you can see in the third video on http://johnnylee.net/projects/wii/.
I've had a hard time making everything work properly with textures and depth.
I've had recently 2 problems that have somewhat visually the same impact :
- One with textures that do not blend well in 3D using the techniques I've found for 2D.
- One with objects appearing bottom above top. Like the problem exposed here: Depth Buffer in OpenGL
I've solved both problem, but I would like to know if I get things right, especially for the second point.
For the first one, I think I've got it. I have an image of a round target, with alpha for anything outside the disc. It's loaded fine inside OpenGL. Some (due to my z-ordering problem) other targets behind it suffered from being hidden by the transparent regions of the naturally square quad I used to paint it.
The reason for that was that every part of the texture is assumed to be full opaque with regard for the depth buffer.
Using an glEnable(GL_ALPHA_TEST) with an test for glAlphaFunc(GL_GREATER, 0.5f) makes the alpha layer of the texture act as a per pixel (boolean) opacity indicator, and thus makes the blending quite useless (because my image has boolean transparency).
Supplementary question: By the way, is there a mean of specifying a different source for the alpha test than the alpha layer used for blending?
Second, I've found a fix to my problem.
Before clearing the color and depth buffer I've set the default depth to 0 glClearDepth(0.0f) and and I've make use of "greater" depth function glDepthFunc(GL_GREATER).
What looks strange to me is that depth is 1.0 and the depth function is "less" GL_LESS by default.
I'm basically inverting that so that my objects don't get displayed inverted...
I've seen nowhere such a hack, but in the other hand I've seen nowhere objects getting drawn systematically in the wrong order, regardless of which order I draw them!
OK, here's the bit of code (stripped down, not too much I hope) that is now working as I want:
int main(int argc, char** argv) {
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH);
glutInitWindowSize(600, 600); // Size of the OpenGL window
glutCreateWindow("OpenGL - 3D Test"); // Creates OpenGL Window
glutDisplayFunc(display);
glutReshapeFunc(reshape);
PngImage* pi = new PngImage(); // custom class that reads well PNG with transparency
pi->read_from_file("target.png");
GLuint texs[1];
glGenTextures(1, texs);
target_texture = texs[0];
glBindTexture(GL_TEXTURE_2D, target_texture);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, pi->getGLInternalFormat(), pi->getWidth(), pi->getHeight(), 0, pi->getGLFormat(), GL_UNSIGNED_BYTE, pi->getTexels());
glutMainLoop(); // never returns!
return 0;
}
void reshape(int w, int h) {
glViewport(0, 0, (GLsizei) w, (GLsizei) h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-1, 1, -1, 1);
gluPerspective(45.0, w/(GLdouble)h, 0.5, 10.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
void display(void) {
// The stared *** lines in this function make the (ugly?) fix for my second problem
glClearColor(0, 0, 0, 1.00);
glClearDepth(0); // ***
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glShadeModel(GL_SMOOTH);
glEnable(GL_DEPTH_TEST);
glEnable(GL_DEPTH_FUNC); // ***
glDepthFunc(GL_GREATER); // ***
draw_scene();
glutSwapBuffers();
glutPostRedisplay();
}
void draw_scene() {
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(1.5, 0, -3, 0, 0, 1, 0, 1, 0);
glColor4f(1.0, 1.0, 1.0, 1.0);
glEnable(GL_TEXTURE_2D);
// The following 2 lines fix the first problem
glEnable(GL_ALPHA_TEST); // makes highly transparent parts
glAlphaFunc(GL_GREATER, 0.2f); // as not existent/not drawn
glBindTexture(GL_TEXTURE_2D, target_texture);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
// Drawing a textured target
float x = 0, y = 0, z = 0, size = 0.2;
glBegin(GL_QUADS);
glTexCoord2f(0.0f, 0.0f);
glVertex3f(x-size, y-size, z);
glTexCoord2f(1.0f, 0.0f);
glVertex3f(x+size, y-size, z);
glTexCoord2f(1.0f, 1.0f);
glVertex3f(x+size, y+size, z);
glTexCoord2f(0.0f, 1.0f);
glVertex3f(x-size, y+size, z);
glEnd();
// Drawing an textured target behind the other (but drawn after)
float x = 0, y = 0, z = 2, size = 0.2;
glBegin(GL_QUADS);
glTexCoord2f(0.0f, 0.0f);
glVertex3f(x-size, y-size, z);
glTexCoord2f(1.0f, 0.0f);
glVertex3f(x+size, y-size, z);
glTexCoord2f(1.0f, 1.0f);
glVertex3f(x+size, y+size, z);
glTexCoord2f(0.0f, 1.0f);
glVertex3f(x-size, y+size, z);
glEnd();
}
