Sign in to follow this  

OpenGL Getting vertices / faces after depth buffer test?

Recommended Posts

I am looking for a way to retrieve all remaining and visible vertices or polygons (better both) after the depth test has been performed. What I require is the amount of vertices that are visible to the view with the current settings in form of vector data(and not as an image) Is this possible at all with opengl?

Share this post

Link to post
Share on other sites
No. You could use a hardware occlusion query, but that will only give you the number of pixels that got rendered. The depth test is performed during scan conversion, so there's no notion of vertices or faces.

What is it you're trying to achieve? I know there's a better way of doing it...

Share this post

Link to post
Share on other sites
Oh great, let's see :)

I am working with models which are loaded during runtime. I need to figure the amount of vertices and polygon out which are visible to the user with the current modelview settings. So I need to check if the normals are facing towards the screen AND additionally I need to know if these vertices and polygons are visible to the user, since they may face towards the user but be occluded by other geometry.

This is required for a project, where I am creating thumbnails of arbitrary models with the highest possible entropy. Which means picking a model and view to get the most significant perspective of the model

Share this post

Link to post
Share on other sites
As was said before, this can't be done in an easy way.

How is this:
1. Render the object normally with z-write enabled
2. enable GL_POLYGON_OFFSET_LINE with a small offset
(2 a) alternatively, glDepthFunc(GL_LEQUAL) should work too
3. start occlusion query
4. render the object again, with depth testing, and as wireframe
5. read occlusion query

This won't give you the number of faces or vertices, but it will give you a good figure of what you described. The z-test will cull away edges that are hidden by faces and leave only edges on the frontmost faces.

More visible faces mean more visible edges. Occlusion queries count pixels, but lines are made of pixels, so more visible lines will give more pixels.
Yes, some lines are longer than others, but on the average over a few thousand of them, it shouldn't matter.
A perspective with more visible faces (and thus edges) will have a greater number returned by the query.

If you really want to know the number of faces and execution time is secondary, you can of course do this:
1. Render with z-write enabled
2. enable z-test with GLLEQUAL
3. for each triangle:
- start occlusion query
- draw it
- read occlusion query
- if number of pixels > 0 increment counter
Note that this will be horrendously slow. You can remove some of the stalls by issuing a couple of queries at a time, but it will probably still be slow.

Share this post

Link to post
Share on other sites
Ok, thx so far. The first version you described can be also easily done by rendering the whole model white with a black background and then counting pixels :) This is already part of my testing suite but unfortunately the complex ones require some calculation and weightening. Maybe I have to create my owb z-buffer by processing this by hand ...

Thank you for your help

Share this post

Link to post
Share on other sites
The first version you described can be also easily done by rendering the whole model white with a black background and then counting pixels

No, there is a difference. Counting pixels will give you a figure of the object's cross-section. Querying lines that are z-culled actually gives a figure of "how much edge" is visible.

But... stupid me... why did I say "lines" in the first place?! You can render points!

If you render your object as points, what will you get? One point for every vertex. If you run an occlusion query over that, you get a count of 1 for every pixel (have to make sure the point size is 1 of course).

Now, if you do a "normal" render pass to fill the z-buffer first, all the vertices that aren't visible will be culled, so the number returned by the query will be... (drums) the number of visible vertices!

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Announcements

  • Forum Statistics

    • Total Topics
    • Total Posts
  • Similar Content

    • By test opty
      Hi all,
      I'm starting OpenGL using a tut on the Web. But at this point I would like to know the primitives needed for creating a window using OpenGL. So on Windows and using MS VS 2017, what is the simplest code required to render a window with the title of "First Rectangle", please?
    • By DejayHextrix
      Hi, New here. 
      I need some help. My fiance and I like to play this mobile game online that goes by real time. Her and I are always working but when we have free time we like to play this game. We don't always got time throughout the day to Queue Buildings, troops, Upgrades....etc.... 
      I was told to look into DLL Injection and OpenGL/DirectX Hooking. Is this true? Is this what I need to learn? 
      How do I read the Android files, or modify the files, or get the in-game tags/variables for the game I want? 
      Any assistance on this would be most appreciated. I been everywhere and seems no one knows or is to lazy to help me out. It would be nice to have assistance for once. I don't know what I need to learn. 
      So links of topics I need to learn within the comment section would be SOOOOO.....Helpful. Anything to just get me started. 
      Dejay Hextrix 
    • By mellinoe
      Hi all,
      First time poster here, although I've been reading posts here for quite a while. This place has been invaluable for learning graphics programming -- thanks for a great resource!
      Right now, I'm working on a graphics abstraction layer for .NET which supports D3D11, Vulkan, and OpenGL at the moment. I have implemented most of my planned features already, and things are working well. Some remaining features that I am planning are Compute Shaders, and some flavor of read-write shader resources. At the moment, my shaders can just get simple read-only access to a uniform (or constant) buffer, a texture, or a sampler. Unfortunately, I'm having a tough time grasping the distinctions between all of the different kinds of read-write resources that are available. In D3D alone, there seem to be 5 or 6 different kinds of resources with similar but different characteristics. On top of that, I get the impression that some of them are more or less "obsoleted" by the newer kinds, and don't have much of a place in modern code. There seem to be a few pivots:
      The data source/destination (buffer or texture) Read-write or read-only Structured or unstructured (?) Ordered vs unordered (?) These are just my observations based on a lot of MSDN and OpenGL doc reading. For my library, I'm not interested in exposing every possibility to the user -- just trying to find a good "middle-ground" that can be represented cleanly across API's which is good enough for common scenarios.
      Can anyone give a sort of "overview" of the different options, and perhaps compare/contrast the concepts between Direct3D, OpenGL, and Vulkan? I'd also be very interested in hearing how other folks have abstracted these concepts in their libraries.
    • By aejt
      I recently started getting into graphics programming (2nd try, first try was many years ago) and I'm working on a 3d rendering engine which I hope to be able to make a 3D game with sooner or later. I have plenty of C++ experience, but not a lot when it comes to graphics, and while it's definitely going much better this time, I'm having trouble figuring out how assets are usually handled by engines.
      I'm not having trouble with handling the GPU resources, but more so with how the resources should be defined and used in the system (materials, models, etc).
      This is my plan now, I've implemented most of it except for the XML parts and factories and those are the ones I'm not sure of at all:
      I have these classes:
      For GPU resources:
      Geometry: holds and manages everything needed to render a geometry: VAO, VBO, EBO. Texture: holds and manages a texture which is loaded into the GPU. Shader: holds and manages a shader which is loaded into the GPU. For assets relying on GPU resources:
      Material: holds a shader resource, multiple texture resources, as well as uniform settings. Mesh: holds a geometry and a material. Model: holds multiple meshes, possibly in a tree structure to more easily support skinning later on? For handling GPU resources:
      ResourceCache<T>: T can be any resource loaded into the GPU. It owns these resources and only hands out handles to them on request (currently string identifiers are used when requesting handles, but all resources are stored in a vector and each handle only contains resource's index in that vector) Resource<T>: The handles given out from ResourceCache. The handles are reference counted and to get the underlying resource you simply deference like with pointers (*handle).  
      And my plan is to define everything into these XML documents to abstract away files:
      Resources.xml for ref-counted GPU resources (geometry, shaders, textures) Resources are assigned names/ids and resource files, and possibly some attributes (what vertex attributes does this geometry have? what vertex attributes does this shader expect? what uniforms does this shader use? and so on) Are reference counted using ResourceCache<T> Assets.xml for assets using the GPU resources (materials, meshes, models) Assets are not reference counted, but they hold handles to ref-counted resources. References the resources defined in Resources.xml by names/ids. The XMLs are loaded into some structure in memory which is then used for loading the resources/assets using factory classes:
      Factory classes for resources:
      For example, a texture factory could contain the texture definitions from the XML containing data about textures in the game, as well as a cache containing all loaded textures. This means it has mappings from each name/id to a file and when asked to load a texture with a name/id, it can look up its path and use a "BinaryLoader" to either load the file and create the resource directly, or asynchronously load the file's data into a queue which then can be read from later to create the resources synchronously in the GL context. These factories only return handles.
      Factory classes for assets:
      Much like for resources, these classes contain the definitions for the assets they can load. For example, with the definition the MaterialFactory will know which shader, textures and possibly uniform a certain material has, and with the help of TextureFactory and ShaderFactory, it can retrieve handles to the resources it needs (Shader + Textures), setup itself from XML data (uniform values), and return a created instance of requested material. These factories return actual instances, not handles (but the instances contain handles).
      Is this a good or commonly used approach? Is this going to bite me in the ass later on? Are there other more preferable approaches? Is this outside of the scope of a 3d renderer and should be on the engine side? I'd love to receive and kind of advice or suggestions!
    • By nedondev
      I 'm learning how to create game by using opengl with c/c++ coding, so here is my fist game. In video description also have game contain in Dropbox. May be I will make it better in future.
  • Popular Now