I came across this issue when I was trying to triangulate Text entities using GLUtesselator. However, it can occur during triangulation of any polygon using GLUtesselator. The problem is that sometimes GLUtesselator generates zero-area triangles. Most of the times you can ignore them but there are cases where they can’t be ignored. I am trying to find a solution so that final triangulation of a given polygon do not have any zero-area triangles or T-intersections. As far as I know, GLUtesselator is one of the most robust and stable tesselator available so I would like to stick to it and won’t mind doing some post-processing to fix the triangulation rather than writing a new tesselator myself.
I will try to demonstrate the problem with tessellation of character ‘H’. Please note that I am a new user of stackflow so I can't post images yet and it is impossible to explain this problem without images so I am just posting the URL's to the blog where I have described the problem. You can also check it at GLUtesselator : Issues with zero-area triangles and T-intersections http://www.dixittech.com/blog
I think its a very fundamental problem and a solution to it will benefit lot of developers working in a similar area. I am sure I am not the first one to stumble upon it. Any suggestions how it can be done? Any alternate approach is very welcome as well.
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I am going to split this question in 3 parts
First, I've been given this problem, and I don't know where to start, if you have been solving related problem, would you give me some hints and keywords to help me do some more research?
I have done some research on my own
So here is some 2D chest CT scans (sorry due to reputation rule i can't implement images directly)
All photos are in the same angle. So I think I can simply read each photo to a vector of pixels, do some thresh holding to make all black and black-ish pixels going to be a non-colored pixel. Next, I'll create a vector called vector_of_photo of those vectors. Then the index of each vector in vector_of_photo are now the Z-index.
Now I can render a 3d photo from those vectors of pixels right?
In the second place, I got trouble understand raycasting algorithm,
I think the idea here is, when I already got a box of pixel then everytime I rotate the box, it cast straight-lines from that angle of the camera to the box, each line found a has-colored pixel going to stop casting and render that pixel (or more specific, copy the pixel to the exactly location on the plane).
Did I understand it correctly?
At last, the OPENGL/c++ part is just the option I think I'm going to use to solve this problem. And I'm not pretty sure it is a good idea or not, so give me some more hint about the programming language, library or module I should take a look at.
I happen to be working on the same problem in my spare time. Haha :)
Here is one approach to your problem:
Load the images into your application, such that you get the 3D volumetric dataset that you describe
Remove all points that don't fit within some range of values (e.g. 0.4/1.0 to 0.6/1.0 brightness). You may need to apply preprocessing and filtering.
Fit a mesh to the resulting point cloud with open-source software. Here is a good blog post about that
https://towardsdatascience.com/5-step-guide-to-generate-3d-meshes-from-point-clouds-with-python-36bad397d8ba
Take the resulting mesh (probably, an STL file) and visualize it in any software your want (Blender 3D, Unity 3D, Cinema 4D, a custom OpenGL application), anything really.
My own approach to this problem is very similar to the one you suggest in your question, and I have already made some headway. Therefore, I thought it would be good to suggest another route.
NOTE Please be aware that what you are working on is not a trivial problem. It's a large project, and there are many Commerical companies that put years into doing just this. This is a great project for learning OpenGL, rendering, and other concepts. It's perfectly doable, but you may be looking at several months of work, and lots of trial and error. Good luck!
Its not often that two people would happen to work on the same problem, so if you want to discuss further, feel free to contact me over linkedin and/or post a comment below. www.linkedin.com/in/michael-sohnen-a2454b1b2
I'm newbee in opengl, and I'm not so familiar with the billboard techniques. Recently I have a project to make realistic tree models, I have rendered tree branches using cylinders, like below:
I'm satisfied with this look, but when I add leaves to the branches, I encountered problems. I'm not sure how to make it natural and realistic, I tried to use small leaf billboard and attach the billboard along the branches, the result is shown below:
As you can see, it's really ugly and unnatural, so please give me some ideas on that, how can I attach the leaves to my branches in a correct way? Please make it more detailed because I'm really new to opengl.
EDIT:
I've implemented what I want. I've done that by attaching cross-billboards to the end nodes of tree. Each cross-billboard contains 3 billboards, the angle between any two billboard is 60 degress. And below is the result:
I guess, general approach in graphic techniques(this question is not about OpenGL, in fact) is to look around, understand the maths, implement them, optimize them. To get perspective at common and proved techniques used, read articles about state-of-art graphics stuff. NVidia has good series, called GPU Gems. More over, there is even an article about rendering realistic trees - watch there
When it comes to such particular question, how to place the leaves, it depends on the tree, you actually want to render. Okay, let's watch some marginal tree, like this one:
http://i.stack.imgur.com/V49jl.jpg
I suggest, that you build tree structure with some sort of fractal function. Try to tweak it in the way, that end-branches are more likely to grow up away from ground part. The leaves are also more likely to grow on the outer part of tree's volume and on the smaller branches. Play with values and watch the results. It's all about you. There is no recipe to succeed
I am using a nice little line rendering library that utilizes GL_TRIANGLE_STRIPS to make smooth lines (described here). The lines look great EXCEPT that there are random missing triangles. See image at below link.
Since I have not seen any other complaints on the blog for this software, I am thinking there might be something wrong with my OpenGL state that is causing this, but I don't know what. Disabling GL_CULL_FACE had no effect (though my understanding is that all triangles in a strip must face the same way anyways) and I am low on ideas.
I'm wondering if anyone could point me to any resources that would deal in rotating one or several 2D textured planes in 3D space. Something like this but with openGL (preferably C++):
I'm attempting to do pretty much the exact same thing but no matter how I order my operations I'm getting right-screwy results. So I figure asking for some resources on the subject is better than posting tons of code and asking for people to fix my problems for me. :)
If you havent already, do a search for 'NeHe tutorials'. An excellent set of OpenGL tutorials.
Here is a link to the rotation tutorial, includes all the source code in downloadable format and the tutorial walks you through each relevant line.
http://nehe.gamedev.net/tutorial/rotation/14001/
I believe this is working in a 2D space, the step up to 3D probably involves a bit more matrix math but...doable
The NeHe tutorials are a very popular place to learn the basics of OpenGL. In particular, the tutorial about texture mapping should help you:
http://nehe.gamedev.net/tutorial/texture_mapping/12038/
Mind you though that these tutorials are written for older OpenGL versions which are more beginner friendly IMHO.
You should look into scene graphs. Basically it is a way to define a bunch of objects (2D textured planes) and their transforms in 3D space. This allows you to define transforms that work on multiple nodes (objects) as well as single nodes. You can make a pretty simple one in C++ with little effort, or use one such as OpenSG or OSG (slight learning curve needed).
Wikipedia - http://en.wikipedia.org/wiki/Scene_graph
I'm trying to, in JOGL, pick from a large set of rendered quads (several thousands). Does anyone have any recommendations?
To give you more detail, I'm plotting a large set of data as billboards with procedurally created textures.
I've seen this post OpenGL GL_SELECT or manual collision detection? and have found it helpful. However it can take my program up to several minutes to complete a rendering of the full set, so I don't think drawing 2x (for color picking) is an option.
I'm currently drawing with calls to glBegin/glVertex.../glEnd. Given that I made the switch to batch rendering on the GPU with vao's and vbo's, do you think I would receive a speedup large enough to facilitate color picking?
If not, given all of the recommendations against using GL_SELECT, do you think it would be worth me using it?
I've investigated multithreaded CPU approaches to picking these quads that completely sidestep OpenGL all together. Do you think a OpenGL-less CPU solution is the way to go?
Sorry for all the questions. My main question remains to be, whats a good way that one can pick from a large set of quads using OpenGL (JOGL)?
The best way to pick from a large number of quad cannot be easily defined. I don't like color picking or similar techniques very much, because they seem to be to impractical for most situations. I never understood why there are so many tutorials that focus on people that are new to OpenGl or even programming focus on picking that is just useless for nearly everything. For exmaple: Try to get a pixel you clicked on in a heightmap: Not possible. Try to locate the exact mesh in a model you clicked on: Impractical.
If you have a large number of quads you will probably need a good spatial partitioning or at least (better also) a scene graph. Ok, you don't need this, but it helps A LOT. Look at some tutorials for scene graphs for further information's, it's a good thing to know if you start with 3D programming, because you get to know a lot of concepts and not only OpenGl code.
So what to do now to start with some picking? Take the inverse of your modelview matrix (iirc with glUnproject(...)) on the position where your mouse cursor is. With the orientation of your camera you can now cast a ray into your spatial structure (or your scene graph that holds a spatial structure). Now check for collisions with your quads. I currently have no link, but if you search for inverse modelview matrix you should find some pages that explain this better and in more detail than it would be practical to do here.
With this raycasting based technique you will be able to find your quad in O(log n), where n is the number of quads you have. With some heuristics based on the exact layout of your application (your question is too generic to be more specific) you can improve this a lot for most cases.
An easy spatial structure for this is for example a quadtree. However you should start with they raycasting first to fully understand this technique.
Never faced such problem, but in my opinion, I think the CPU based picking is the best way to try.
If you have a large set of quads, maybe you can group quads by space to avoid testing all quads. For example, you can group the quads in two boxes and firtly test which box you
I just implemented color picking but glReadPixels is slow here (I've read somehere that it might be bad for asynchron behaviour between GL and CPU).
Another possibility seems to me using transform feedback and a geometry shader that does the scissor test. The GS can then discard all faces that do not contain the mouse position. The transform feedback buffer contains then exactly the information about hovered meshes.
You probably want to write the depth to the transform feedback buffer too, so that you can find the topmost hovered mesh.
This approach works also nice with instancing (additionally write the instance id to the buffer)
I haven't tried it yet but I guess it will be a lot faster then using glReadPixels.
I only found this reference for this approach.
I'm using the solution that I've borrowed from DirectX SDK, there's a nice example how to detect the selected polygon in a vertext buffer object.
The same algorithm works nice with OpenGL.