FBO woes

medevilenemy · 2012-06-15T15:40:28

I've been trying to come up with a way to draw an opaque quad over my scene through which holes would be punched to allow one to see the scene below. Through some research and with some help I've determined that the best way to do this is probably by rendering to a texture through a framebuffer object, alphablending the holes into the quad and then drawing the texture over the scene. I'm having trouble getting it to work, though, and I can't find examples of what I'm trying to do. So far I've succeeded in creating my framebuffer object and target texture based on the example block at the bottom of http://www.songho.ca...gl/gl_fbo.html. After that, I've attempted various configurations of drawing what I want to the texture, but all I can seem to manage is to draw an opaque quad to it. I can't seem to get depth, alpha blending, or even texture mapping to work properly (no matter how I map the texture coords, the result is the same). glClear(GL_COLOR_BUFFER_BIT) on the texture causes my whole scene to get messed up, as well. The following code demonstrates how I'm trying to set up rendering to the texture: glPushMatrix(); // set rendering destination to FBO glBindFramebuffer(GL_FRAMEBUFFER, 92); glViewport(0,0,screen_width, screen_height); glMatrixMode(GL_PROJECTION); glLoadIdentity(); // Clearing buffers glClearDepth(1.0f); glClearColor(0.0f, 0.0f, 0.0f, 1.0f); glClear(GL_DEPTH_BUFFER_BIT); // Clearing the GL_COLOR_BUFFER_BIT causes things to break // Set up internal rendering projection glOrtho(-0.5 * width, 0.5*width, -1, 1, 1, 0); glEnable(GL_BLEND); glEnable(GL_DEPTH_TEST); // Set blend func //glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); // Draw stuff here! At the moment just trying to draw a couple overlapping quads as a proof of concept // Unbind fbo glBindFramebuffer(GL_FRAMEBUFFER, 0); glPopMatrix(); // Set scene-relative blending function. glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); // function for blending // Draw the rendered texture on top of the underlying scene glEnable(GL_TEXTURE_2D); glBindTexture(GL_TEXTURE_2D, 90); glBegin(GL_QUADS); glTexCoord2f(0.0, 0.0f); glVertex3f(-0.1,-0.1,0.01); // Bottom-Left Vertex glTexCoord2f(1.0f, 0.0f); glVertex3f(0.1,-0.1,0.01); // Bottom-Right Vertex glTexCoord2f(1.0f, 1.0f); glVertex3f(0.1,0.1,0.01); // Top-Right Vertex glTexCoord2f(0.0f, 1.0f); glVertex3f(-0.1,0.1,0.01); // Top-Left Vertex glEnd(); glDisable(GL_TEXTURE_2D); Any help is greatly appreciated!

Graphics and GPU Programming Programming OpenGL

Started by medevilenemy May 11, 2012 05:45 AM

20 comments, last by medevilenemy 11 years, 10 months ago

medevilenemy

363

Author

June 10, 2012 10:22 PM

I've done some more tweaking, and I've managed to draw an opaque quad with a transparent window in its center. The reason one of the quads was clipping before seems to have been depth (I was drawing a couple of its vertices with 0.5 depth and for some reason it wasn't playing nice). The "window" is created by drawing a 0.0 alpha quad in the appropriate location with alpha func set to GL_ONE, GL_ZERO. The next thing I need is to be able to achieve the desired effect by subtracting the alpha of the "window" from the opaque quad (this way I could do overlapping lights and gradients). alpha func GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA doesn't seem to work for this. Any suggestions?

There was a saying we had in college: Those who walk into the engineering building are never quite the same when they walk out.

medevilenemy

363

Author

June 15, 2012 03:40 PM

After lots more tweaking and digging around, the blending proof of concept is now functional (after some fiddling and experimentation with the blend subract and minmax extensions). Thanks for your help @Ohforf sake

.

There was a saying we had in college: Those who walk into the engineering building are never quite the same when they walk out.

FBO woes

.

This topic is closed to new replies.

Popular Topics

Recommended Tutorials

FBO woes

.

This topic is closed to new replies.

Popular Topics

Recommended Tutorials

Reticulating splines