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PODepth test inverted by default in OpenGL, or did I get it wrong?
 

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  1. PODepth test inverted by default in OpenGL, or did I get it wrong?
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    <p>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 <a href="http://johnnylee.net/projects/wii/" rel="nofollow noreferrer">http://johnnylee.net/projects/wii/</a>.<br> I've had a hard time making everything work properly with textures and depth.</p> <p>I've had recently 2 problems that have somewhat visually the same impact :</p> <ul> <li>One with textures that do not blend well in 3D using the techniques I've found for 2D.</li> <li>One with objects appearing bottom above top. Like the problem exposed here: <a href="https://stackoverflow.com/questions/1046029/depth-buffer-in-opengl">Depth Buffer in OpenGL</a></li> </ul> <p><strong>I've solved both problem, but I would like to know if I get things right</strong>, <em>especially for the second point</em>.</p> <p><br/></p> <p><strong>For the first one</strong>, 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.</p> <p>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 <code>glEnable(GL_ALPHA_TEST)</code> with an test for <code>glAlphaFunc(GL_GREATER, 0.5f)</code> 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).</p> <p>Supplementary question: <em>By the way, is there a mean of specifying a different source for the alpha test than the alpha layer used for blending?</em></p> <p><br/></p> <p><strong>Second</strong>, I've found a fix to my problem. Before clearing the color and depth buffer I've set the default depth to 0 <code>glClearDepth(0.0f)</code> and and I've make use of "greater" depth function <code>glDepthFunc(GL_GREATER)</code>.</p> <p>What looks strange to me is that depth is 1.0 and the depth function is "less" <code>GL_LESS</code> by default. I'm basically inverting that so that my objects don't get displayed inverted...</p> <p>I've seen nowhere such a hack, but in the other hand I've seen nowhere objects getting drawn systematically in the wrong order, <em>regardless of which order I draw them</em>!</p> <p><br/></p> <p>OK, here's the bit of code (stripped down, not too much I hope) that is <em>now</em> working as I want:</p> <pre><code> int main(int argc, char** argv) { glutInit(&amp;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-&gt;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-&gt;getGLInternalFormat(), pi-&gt;getWidth(), pi-&gt;getHeight(), 0, pi-&gt;getGLFormat(), GL_UNSIGNED_BYTE, pi-&gt;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(); } </code></pre>
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