I'm creating a box and placing "magnets" on the bottom. The sides are slightly see through(alpha is somewhere between .2 and .5) and the bottom is solid. I'm trying to use gluUnProject() to select where the "magnet" is placed, but when the sides of the box are rendered, I can't get my magnets into the box.
Is there anyway to still have the sides of the wall to be rendered but ignore them for the sake of mouse clicks?
I've tried GL_CULL_FACE but at first glance that doesn't seem to be what I'm looking for.
So if I understand correctly, you have semi-transparent boxes and when the magnet is inside the boxes you want to see the magnet in according to the semi-transparency of the boxes.
My guess is that when you're drawing the boxes you have the depth writes turned on, this way if boxes happen to get drawn before the magnet, then when you draw the magnet it will fail the depth test and the part that's inside won't get drawn as a result.
The easiest way to do this is:
Draw all the solid objects first
Disable depth writes:
glDepthMask(GL_FALSE);
Use an order-independent blending function when drawing the semi-transparent objects, for example:
glBlendFunc(GL_ONE, GL_ONE)
Draw all your transparent objects
Enable depth writes again
glDepthMask(GL_TRUE);
Bear in mind this simple method will only work if you can get away with using an commutative blending equation, if not then consider using order-independent transparency, a good article is "Efficient Layered Fragment Buffer Techniques" By Pyarelal Knowles, Geoff Leach, and Fabio Zambetta
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I have a model with transparent quads for a beard. I can not tell what triangles belong to the beard because their color comes from the texture passed to the fragment shader. I have tried to presort the triangles back to front during export of the model, but this does not help. So I implemented MSAA and Alpha to Coverage, but this did not help either. My last attempt was to draw the model, with a depth mask off and skipping any transparent data, so the color buffer would have non-clear color values to blend with. Then I would draw the model a second time with depth testing on and drawing the alpha pieces.
Nothing I have tried so far has worked. What other techniques can I try to get the beard of the model to properly draw? I am looking for a way to handle this that doesn't use a bunch of extensions. I'd prefer techniques that can be handled with plain old OpenGL 4.
Here is an image of what I am dealing with.
This is what I got after I applied the selected answer.
What you're trying to do there is a still largely unsolved problem: Order independent transparency. MSAA is something entirely different, as is alpha coverage.
So far the best working solution is to separate the model into an opaque and a hairy part. Draw the opaque parts of your scene first, then draw everything (semi-)translucent, ordered far to near in a second pass.
The way your image looks like it seems like the beard is rendered as the first thing, which is quite the opposite of what you actually want.
Simple way:
Enable depth write (depth mask), disable alpha-blending, draw model without the beard.
Disable depth write, enable alpha-blending, draw the beard. Make sure face culling is enabled.
Hard way:
Because order-independent transparency in renderers that use z-buffer is an unsolved problem (as datenwolf said), you could try depth-peeling. I believe the paper is available within OpenGL SDK. Most likely it'll be slower than "simple way", and there'll be a limit on number of maximum overlapping transparent polygons. Also check wikipedia article on order-independent transparency.
I'm working on an OpenGL 3 renderer for a GUI toolkit called Gwen. I nearly have everything working, but I'm having some issues getting everything to blend correctly. I've sorted the triangles by which texture they use and packed them into a VBO, so with the Unit Test, it basically boils down into 3 layers: Filled Rects with no texture, Text, and the windows, buttons, etc that use a skin texture.
The Filled Rects are usually drawn on top of everything else and blended in, but the background behind everything is also a Filled Rect, so I can't count on that. There is a Z-value conflict if you draw them last (ex: the windows have a textured shadow around the edges that turns black because the background fails the depth test) and a blending/z-value conflict if you draw them first (ex: some of the selection highlights get drawn on top of instead of blending like they're supposed to).
I can't count on being able to identify any specific layer except the Filled Rects. The different layers have a mix of z-values, so I can't just draw them in a certain order to make things work. While writing this, I thought of a simple method of drawing the triangles sorted back to front, but it could mean lots of little draw calls, which I'm hoping to avoid. Is there some method that involves some voodoo magic blending that would let me keep my big batches of triangles?
You're drawing a GUI; batching shouldn't be your first priority for the simple fact that a GUI just doesn't do much. A GUI will almost never be your performance bottleneck. This smells of premature optimization; first, get it to work. Then, if it's too slow, make it work faster.
There is no simple mechanism for order-independent transparency. Your best bet is to just render things in the proper Z order.
I am going through the NeHe tutorials for OpenGL... I am at lesson 8 (drawing a cube with blending). http://nehe.gamedev.net/data/lessons/lesson.asp?lesson=08
I wanted to experiment and change half of the faces to be opaque so that there is always a semi-transparent face opposite to an opaque one and be able to rotate the cube...
I changed the code a little bit, the entire source is there : http://pastebin.com/uzfSk2wB
I changed a few things :
enable blending by default and set the blending function to glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
changed the order of drawing the faces and colors for each face.
I set the depth testing
I draw all the opaque faces
I disable depth test
I draw all the transparent faces
Now, it's hard to tell exactly what is going wrong, but it definitely does not look right, I cannot recognize what face is opaque compared to the transparent ones, sometimes some faces do not seem to get drawn when they should...etc...
Seems like calculating what face is in front compared to back would not be trivial (although I am sure possible), I hope there is a way of doing that would not need to do that.
Looking either for what is wrong in my code or whether this is not the right way of doing it in the first place.
If you disable depth testing before drawing the transparent faces, then they will be drawn with no regard for their correct z-ordering. It probably looks like the transparent faces are being drawn atop all the other faces. Leave depth testing on.
I'm learning about how to use JOGL and OpenGL to render texture-mapped quads. I have a test program and a test quad, and I figured out how to enable GL_BLEND so that I can specify the alpha value of a vertex to make a quad with a sort of gradient... but now I want this to show through to another textured quad at the same position.
Drawing two quads with the same vertex locations didn't work, it only renders the first quad. Is this possible then, or will I need to basically construct a custom texture on-the-fly based on what I want and then draw one quad with this texture? I was really hoping to take advantage of blending in this case...
Have a look at which glDepthFunc you're using, perhaps you're using GL_LESS/GL_GREATER and it could work if you're using GL_LEQUAL/GL_GEQUAL.
Its difficult to make out of the question what exactly you're trying to achieve but here's a try
For transparency to work correctly in OpenGL you need to draw the polygons from the furthest to the nearest to the camera. If you're scene is static this is definitely something you can do. But if it's rotating and moving then this is usually not feasible since you'll have to sort the polygons for each and every frame.
More on this can be found in this FAQ page:
http://www.opengl.org/resources/faq/technical/transparency.htm
For alpha blending, the renderer blends all colors behind the current transparent object (from the camera's point of view) at the time the transparent object is rendered. If the transparent object is rendered first, there is nothing behind it to blend with. If it's rendered second, it will have something to blend it with.
Try rendering your opaque quad first, then rendering your transparent quad second. Plus, make sure your opaque quad is slightly behind your transparent quad (relative to the camera) so you don't get z-buffer striping.
I am rendering an OpenGL scene that include some bitmap text. It is my understanding the order I draw things in will determine which items are on top.
However, my bitmap text, even though I draw it last, is not on top!
For instance, I am drawing:
1) Background
2) Buttons
3) Text
All at the same z depth. Buttons are above the background, but text is invisible. It I change the z depth of the text, I can see it, but I then have other problems.
I am using the bitmap text method from Nehe's Tutorials.
How can I make the text visible without changing the z depth?
You can simply disable the z-test via
glDisable (GL_DEPTH_TEST); // or something related..
If you do so the Z of your text-primitives will be ignored. Primitives are drawn in the same order as your call the gl-functions.
Another way would be to set some constant z-offset via glPolygonOffset (not recommended) or set the depth-compare mode to something like GL_LESS_EQUAL (the EQUAL is the important one). That makes sure that primitives drawn with the same depth are rendered ontop of each other.
Hope that helps.
You can also use glDepthFunc (GL_ALWAYS).