Sign in to follow this  
choffstein

OpenGL What is batching?

Recommended Posts

choffstein    1090
I have heard the term batching several times before, and was wondering what it was (and how it can be applied in OpenGL -- though, I don't want this post to be OpenGL specific). From the very little I have found (at last, I found a topic that google let me down on!), I think it has something to do with organizing the rendering of your geometry to make as few changes as you possibly can. So, for an example, I thought a simple 2d tile engine would make sense. I personally would rendering it with a triangle list (maybe quad list?). Each quad would have its own texture (hence the tile engine). Does it make sense to sort the quads by their texture, and use GL_QUADS for each group? Please, help a confused nubblet. Thanks.

Share this post


Link to post
Share on other sites
mictian    138
I am not sure that what you describe is necessarily batching. However sorting geometry to minimize the number of opengl state changes is definitely a good thing to do for any moderately complex scene, I think.

I was under the impression that batching meant sending a bunch of polygons at the same time to the hardware (or at least the opengl layer) in order to maximize the throughput to the vertex pipelines.

Please correct me as I want to know this as well.

Rob

Share this post


Link to post
Share on other sites
SimmerD    1210
Good batching means to take advantage of as much coherence in your scene as possible, mainly in order to reduce CPU overhead.

You can look at each render state as a bunch of sort keys. The first question is which key do you sort your scene by?

object #1 :
Render target : A
blending mode : K
z enable : true

object #2 :
Render target : A
blending mode : K
z enable : false

object #3 :
Render target : B
blending mode : K
z enable : true

object #4 :
Render target : B
blending mode : K
z enable : true

etc.

Objects 3 & 4 have the same render states, so they can be drawn one after another without changing state in between, or they could even be drawn in one draw call in some cases ( like if they were both in world space ).

It is easy to worry too much about this, but I have found that thinking about it a bit when you start your engine can help in the long run.

Many games still draw small pieces of geometry in separate calls. It is preferable for things like clumps of grass or rocks, or maybe even buildings in an RTS, to put them all in a big dynamic vertex buffer and draw them all at once.

For my particle system, I group all particles that were created together ( like from an explosion ), as one object. I cull or draw all of them in one call. That is an example of good batching. Poor batching would be to draw each particle one at a time.

This is more of an issue in DirectX, but it still can help in OGL as well.

Share this post


Link to post
Share on other sites
turnpast    1011
In DirectX (you said you did not want this thread to be OpenGL specific) a batch is typically a single DrawPrimitives or DrawIndexedPrimitives (DrawXXX etc..) call. The more of these calls you make the more likely you are to be CPU bound. So batching in this context is not just reducing state changes, but rendering as much of your geometry as possible in a single call. A single call implies that the states must be the same for all the geometry in the call. There are a number of techinques and tricks for increasing batch size -- many of them the same as the techinques for decreasing state changes, save that even one cheep state change will split your batch.

Instancing, as an example technique, seems to be directly targeted at increasing batch size. Texture packing is another technique that applies to batching and perhaps to your 2d problem. To pack your textures you combine your textures into one and then adjust your texcoords so that each geometry uses only portion of the combined texture. This should allow you to draw a number of objects, with different textures, in one call.

The last paper I read from ATI said that a 1GHz box would be CPU bound with 25,000 batches a second (DirectX). I don't know how all this applies to opengl, but I have heard that it is not nearly as important.

Share this post


Link to post
Share on other sites
OrangyTang    1298
Typically you only really need to worry about expensive state changes - which boils down to texture binds and shader binds. Other state tends to be much more trivial to change, and you should instead worry about grouping individual polys into chunks o geometry to render at that point.

If you've got some really expensive shaders a rough front-to-back z sort can be good too.

Share this post


Link to post
Share on other sites
zedzeek    529
Quote:
The last paper I read from ATI said that a 1GHz box would be CPU bound with 25,000 batches a second (DirectX). I don't know how all this applies to opengl, but I have heard that it is not nearly as important.

here im getting ~10 million batches sec with opengl (athln64 2.0ghz gffx5900)

Share this post


Link to post
Share on other sites
SimmerD    1210
I'll give an example where grouping by render state was more helpful that just reducing draw calls.

In my engine, I created per-caster shadow maps, so each shadow caster had its own 64x64 shadow map. I would render to it, then draw the floor using this texture, then do the next character, etc.

It was way too slow - like 80 fps with only one light and a couple of characters.

I changed it so that all characters allocated a 64x64 chunk on a 256x256 shadow map instead, drew all characters into this map, changing some shader constants and the viewport in between, then drew the receiver geometry with each sub-texture in turn.

This brought me up to ~200 fps. So, I still made the same # of draw calls, but I was able to avoid switching render targets, so it was a huge win.

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. 
       
      Thanks
    • 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,
      http://www.informit.com/articles/article.aspx?p=2120983
      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.
      Thanks!
  • Popular Now