Sign in to follow this  
X Abstract X

OpenGL Model Loading Performance

Recommended Posts

X Abstract X    109
I'm wondering if I'm loading a model in OpenGL to be used in a VBO, is it better to: - read vertex normal data from the model file OR - calculate the normals at runtime If I do go about loading the vertex normals from file, I believe there might be some situations where I need to use a surface normal, like backface culling maybe? Is there any need for surface normals if I'm using vertex normals? Thanks.

Share this post

Link to post
Share on other sites
karwosts    840
I can't imagine wanting to have to recalculate all of the normals at runtime. If you load from a binary file you can probably read hundreds of thousands of normals in a totally trivial amount of time.

I don't believe you need surface normals for anything, but if I'm wrong someone can correct me. Backface culling is performed in the primitive assembly and I don't think it needs any input from the user to function properly.

Share this post

Link to post
Share on other sites
Jason Z    6436
I think in some cases you want to be able to use the model normals intentionally - i.e. the model normals have been manipulated somehow.

Share this post

Link to post
Share on other sites
Ohforf sake    2052
Backface culling is performed in the primitive assembly and I don't think it needs any input from the user to function properly.

This is correct, backface culling works by projecting the vertices on the screen and checking, if they are in clockwise or counter clockwise order. The normal(s) are not taken into consideration.

What about tangent-space-calculations? There you'd need normals.

True, but you'd need the vertex normals because a) that is how you pass normals to the API and b) there are cases where you want the faces top be "smooth shaded".

You will need surface normals if you are performing collision detection yourself, or you need to place decals etc. For the graphics side alone, I agree with karwosts, I can't think of anything usefull.

About whether or not to store the normals in the file. As was already pointed out, it's not really a matter of performance, although in my experience recalculating them after loading takes a little bit longer.
If the mesh is animated, you'll need to recalculate the normals somehow either way, so I'll assume you are talking about static geometry.

In that case I would either write a dedicated file format for your engine, which your model editor exports and which has all normals stored within. The format should be in a way that loading would just be a bunch of block reads.
If however you wrote the model editor yourself (or wrote an intermediate tool to apply the final touches and settings) then I would reuse the file format of that tool (which should feature things like smoothing groups and face orientations, rather than normals). In that case you can also reuse the loading and normal calculation code from the model editor in your engine and you are sure, that normals look exactly the way they look in your editor.

Share this post

Link to post
Share on other sites
TyrianFin    122
Good artist can make manual tweeks to normals of model,
and create better looking models.

And better file size savings come from compressing complite model file
vs. dropping data from file. (pkzip or some custom packing algo.)

Geometry normals (face normals) are not normaly neaded if vertex normals
are given. (normals are extra info descriping surfaces accurate form,
ie. in ultra-hi-poly models (10.000.000+ poly) there is no nead to save
normals, becouse polygons can be used to describe accurate surface.)


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  

  • Similar Content

    • By pseudomarvin
      I assumed that if a shader is computationally expensive then the execution is just slower. But running the following GLSL FS instead just crashes
      void main() { float x = 0; float y = 0; int sum = 0; for (float x = 0; x < 10; x += 0.00005) { for (float y = 0; y < 10; y += 0.00005) { sum++; } } fragColor = vec4(1, 1, 1 , 1.0); } with unhandled exception in nvoglv32.dll. Are there any hard limits on the number of steps/time that a shader can take before it is shut down? I was thinking about implementing some time intensive computation in shaders where it would take on the order of seconds to compute a frame, is that possible? Thanks.
    • By Arulbabu Donbosco
      There are studios selling applications which is just copying any 3Dgraphic content and regenerating into another new window. especially for CAVE Virtual reality experience. so that the user opens REvite or CAD or any other 3D applications and opens a model. then when the user selects the rendered window the VR application copies the 3D model information from the OpenGL window. 
      I got the clue that the VR application replaces the windows opengl32.dll file. how this is possible ... how can we copy the 3d content from the current OpenGL window.
      anyone, please help me .. how to go further... to create an application like VR CAVE. 
    • By cebugdev
      hi all,

      i am trying to build an OpenGL 2D GUI system, (yeah yeah, i know i should not be re inventing the wheel, but this is for educational and some other purpose only),
      i have built GUI system before using 2D systems such as that of HTML/JS canvas, but in 2D system, i can directly match a mouse coordinates to the actual graphic coordinates with additional computation for screen size/ratio/scale ofcourse.
      now i want to port it to OpenGL, i know that to render a 2D object in OpenGL we specify coordiantes in Clip space or use the orthographic projection, now heres what i need help about.
      1. what is the right way of rendering the GUI? is it thru drawing in clip space or switching to ortho projection?
      2. from screen coordinates (top left is 0,0 nd bottom right is width height), how can i map the mouse coordinates to OpenGL 2D so that mouse events such as button click works? In consideration ofcourse to the current screen/size dimension.
      3. when let say if the screen size/dimension is different, how to handle this? in my previous javascript 2D engine using canvas, i just have my working coordinates and then just perform the bitblk or copying my working canvas to screen canvas and scale the mouse coordinates from there, in OpenGL how to work on a multiple screen sizes (more like an OpenGL ES question).
      lastly, if you guys know any books, resources, links or tutorials that handle or discuss this, i found one with marekknows opengl game engine website but its not free,
      Just let me know. Did not have any luck finding resource in google for writing our own OpenGL GUI framework.
      IF there are no any available online, just let me know, what things do i need to look into for OpenGL and i will study them one by one to make it work.
      thank you, and looking forward to positive replies.
    • By fllwr0491
      I have a few beginner questions about tesselation that I really have no clue.
      The opengl wiki doesn't seem to talk anything about the details.
      What is the relationship between TCS layout out and TES layout in?
      How does the tesselator know how control points are organized?
          e.g. If TES input requests triangles, but TCS can output N vertices.
             What happens in this case?
      In this article,
      the isoline example TCS out=4, but TES in=isoline.
      And gl_TessCoord is only a single one.
      So which ones are the control points?
      How are tesselator building primitives?
    • By Orella
      I've been developing a 2D Engine using SFML + ImGui.
      Here you can see an image
      The editor is rendered using ImGui and the scene window is a sf::RenderTexture where I draw the GameObjects and then is converted to ImGui::Image to render it in the editor.
      Now I need to create a 3D Engine during this year in my Bachelor Degree but using SDL2 + ImGui and I want to recreate what I did with the 2D Engine. 
      I've managed to render the editor like I did in the 2D Engine using this example that comes with ImGui. 
      3D Editor preview
      But I don't know how to create an equivalent of sf::RenderTexture in SDL2, so I can draw the 3D scene there and convert it to ImGui::Image to show it in the editor.
      If you can provide code will be better. And if you want me to provide any specific code tell me.
  • Popular Now