Sign in to follow this  
FluxCapacitor

OpenGL Order of pixel operations - pixel shader/depth test

Recommended Posts

FluxCapacitor    200
I've heard that performing a "Pre-Z" depth only rendering pass can improve performance when you're executing complex pixel shaders for potentially occluded objects, the idea being (I think) that occluded pixels will be rejected by the depth test before having to execute their pixel shaders. This makes sense, and judging by my searches of gamedev and google, this technique seems to be fairly well known, even having been recommended in a couple of presentations I saw from ATI. However, according to the OpenGL and Direct3D documentation, the depth test in both API's is not performed until after the pixel shader is executed (this makes sense too, since the pixel shader could potentially modify a fragment's depth). So does a "Pre-Z" pass actually do anything? If so, what am I missing? Direct3D pipeline diagram, from Gamasutra: Direct3D9 pipeline

Share this post


Link to post
Share on other sites
nts    968
If the pixel shader doesn't modify the depth then the pixel/fragment can be culled before the pixel shader is even run (early-z). If the pixel shader does modify the depth then the early depth test can not be performed and the pixel shader must be ran before the depth test can be done.

Share this post


Link to post
Share on other sites
mattnewport    1038
The z-test is conceptually performed after the pixel shader is modified. The APIs require the hardware to behave 'as if' the test is performed at that point. In actual hardware though there is a fair bit of silicon dedicated to optimizing things so that in the common case of the pixel shader not modifying depth the hardware can do an early reject of the fragments and never run the pixel shader. Most hardware uses some kind of hierarchical z buffer to optimize this early z rejection and this often allows large numbers of fragments to be discarded early in the pipeline.

Certain things tend to disable this fast path though - modifying z in the pixel shader is an obvious one. Alpha testing and texkill can also disable this optimization on some hardware. This is the main reason you'll generally see hardware vendors recommend against changing depth in the pixel shader as it can have a large performance penalty by disabling all this valuable optimization.

Share this post


Link to post
Share on other sites
You're missing the point of the Z prepass though.

Lets say your Z prepass shader is simple, which is what you want. The vertex shader just transforms position, and the pixel shader does nothing. Lets say this takes 1 cycle/pixel to run.

Now lets assume you have heavy shaders, using multiple textures, complex per pixel lighting, taking 100 cycles/pixel to run.

Lets assume the average overdraw in your scene is 2. Overdraw is inevitable unless you have perfect Z sorting, which never happens. Some pixel tend to be shaded 3 times, some twice, some once, but on average, every pixel is drawn to twice.

Lets further assume that when a Z test fails, it takes a cycle/pixel to process.

Regular render: Cost = 200 cycles/pixel on average.
Z prepass render: 103 cycles/pixel on average.

The Z prepass is 2 draws/pixel of the quick Z only render, followed by a full render which will shade this pixel once, for the nearest depth, and Z fail once.

Lets assume random Z ordering of those heavy passes during regular rendering, Half the pixels will be drawn twice (200 cycles/pixel), and half will be drawn once and Z failed once (101 cycles/pixel), for an average of 150.5 cycles/pixel.

More realistic regular render = 150.5 cycles/pixel
Z prepass render: 103 cycles/pixel on average.


There are so many things which can change the balance of this that you need to test your app to see if it's a win for you, but the theory behind doing a Z pre-pass is quite sound.

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 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!
    • By Picpenguin
      Hi
      I'm new to learning OpenGL and still learning C. I'm using SDL2, glew, OpenGL 3.3, linmath and stb_image.
      I started following through learnopengl.com and got through it until I had to load models. The problem is, it uses Assimp for loading models. Assimp is C++ and uses things I don't want in my program (boost for example) and C support doesn't seem that good.
      Things like glVertexAttribPointer and shaders are still confusing to me, but I have to start somewhere right?
      I can't seem to find any good loading/rendering tutorials or source code that is simple to use and easy to understand.
      I have tried this for over a week by myself, searching for solutions but so far no luck. With tinyobjloader-c and project that uses it, FantasyGolfSimulator, I was able to actually load the model with plain color (always the same color no matter what I do) on screen and move it around, but cannot figure out how to use textures or use its multiple textures with it.
      I don't ask much: I just want to load models with textures in them, maybe have lights affect them (directional spotlight etc). Also, some models have multiple parts and multiple textures in them, how can I handle those?
      Are there solutions anywhere?
      Thank you for your time. Sorry if this is a bit confusing, English isn't my native language
    • By dpadam450
      FINALLY, upgrading my engine to openGL 4. I was having some trouble so I started with a stripped down application and was wondering if VAO's are required, because I have a sample working, but if I remove the VAO then it doesn't seem to like drawing my triangle.
  • Popular Now