Sign in to follow this  

OpenGL Weird Wave Behaviour in Fluid Dynamics

Recommended Posts

So I have a 2D grid of vertices, each with a Z value representing the displacement at that point. Collectively, these form waves on the surface. I initialize this Z-map to all zeros (state of rest), with a disturbance in the middle (-0.5, and -0.3 to surrounding 8 vertices). A 9x9 grid:
0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	
0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	
0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	
0.00	0.00	0.00	-0.30	-0.30	-0.30	0.00	0.00	0.00	
0.00	0.00	0.00	-0.30	-0.50	-0.30	0.00	0.00	0.00	
0.00	0.00	0.00	-0.30	-0.30	-0.30	0.00	0.00	0.00	
0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	
0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	
0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
I call a method EvaluateWaves() at regular intervals to update the Z values. It takes into account preset constants, such as desired wave speed, fluid viscosity, and so on. Since a disturbance in the middle should result in the same thing happening outward radially in all directions, I was surprised to see the result after an EvaluateWaves() call resulting in some non-symmetrical results:
0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	
0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	
0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	
0.00	0.00	0.00	0.00	-0.11	-0.11	-0.11	0.00	0.00	
0.00	0.00	0.00	-0.11	-0.08	-0.26	-0.08	-0.11	0.00	
0.00	0.00	0.00	-0.11	-0.26	-0.21	-0.26	-0.11	0.00	
0.00	0.00	0.00	-0.11	-0.08	-0.26	-0.08	-0.11	0.00	
0.00	0.00	0.00	0.00	-0.11	-0.11	-0.11	0.00	0.00	
0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
I am sending the grid to my fragment shader which performs the calculations and writes it out to a texture bound to the framebuffer object. The texture is ping-ponged back and forth at each evaluation. The EvaluateWaves() call:
// Re-evaluate the waves, but must be called at constant interval...
void FluidSurface::EvaluateWaves()
    // Variables...
    GLint   Viewport[4] = { 0, 0, 0, 0 };

    // Setup viewport for one to one pixel = texel = geometry mapping...

        // Projection should not transform...
        gluOrtho2D(0.0, m_Width, 0.0, m_Height);

        // There should be no modelview transformations either...

        // Lastly, backup viewport size, and resize to the size of grid maps...
        glGetIntegerv(GL_VIEWPORT, Viewport);
        glViewport(0, 0, m_Width, m_Height);

    // Redirect rendering through our framebuffer object...
    glBindFramebuffer(GL_FRAMEBUFFER, m_FrameBufferObject);

        // Check for OpenGL errors...

    // Prepare shader state and dispatch grid location data to program...

        // Install program...

        // Prepare texture units...

            // First texture unit always contains first z-map...
            glActiveTexture(GL_TEXTURE0 + m_ZMaps_TextureUnit[0]);
            glBindTexture(GL_TEXTURE_2D, m_ZMaps_TextureID[0]);

            // Second texture unit always contains second z-map...
            glActiveTexture(GL_TEXTURE0 + m_ZMaps_TextureUnit[1]);
            glBindTexture(GL_TEXTURE_2D, m_ZMaps_TextureID[1]);

            // Check for OpenGL errors...

        // Update uniforms...

            // Z-maps...
                // Previous...
                glUniform1i(GetUniformIndex(m_PO_EvaluateWaves, "ZMapPrevious"), 

                // Current...
                glUniform1i(GetUniformIndex(m_PO_EvaluateWaves, "ZMapCurrent"), 

        // Bind fragment shader outputs to respective data buffers...

            // Create list of attachment points for each fragment output ID...
            GLuint const DrawBuffers[] = {

            // Load the draw buffers...
            glDrawBuffers(sizeof(DrawBuffers) / sizeof(DrawBuffers[0]), DrawBuffers);

        // Enable required vertex arrays...

        // Select the grid indices...
        glBindBuffer(GL_ARRAY_BUFFER, m_VBO_GridLocations);

        // Commit data to shader...
        glDrawArrays(GL_POINTS, 0, m_Width * m_Height);

        // Disable required vertex arrays...

        // Swap the source buffer for next evaluation...
        m_SourceBufferSwitch = !m_SourceBufferSwitch;

        // Check for OpenGL errors...

    // Restore state...

        // Default framebuffer...
        glBindFramebuffer(GL_FRAMEBUFFER, 0);

        // Viewport back to original dimensions...
        glViewport(Viewport[0], Viewport[1], Viewport[2], Viewport[3]);

        // Check for OpenGL errors...
Vertex shader:
// Input variables...

    // Grid location...
    in vec2         GridLocation;

// Uniform variables...

    // Dimensions...
    uniform uint    Width;
    uniform uint    Height;

// Variables for the fragment shader, none of which need be interpolated...

    // Normalized location cooresponding to grid location...
    out vec2        NormalizedLocation;
    // Texture location cooresponding to grid location...
    out vec2        TextureLocation;

// Entry point...
void main()
    // Calculate texture location which has components in [0, 1] range...
    TextureLocation = GridLocation / vec2(Width, Height); 

    // Calculate normalized location which has components in [-1, 1] range...
    NormalizedLocation = TextureLocation * vec2(2.0, 2.0) - vec2(1.0, 1.0);

    // Generate a fragment...
    gl_Position = vec4(NormalizedLocation, 0.0, 1.0);
Fragment shader:
// Input variables...

    // Normalized location cooresponding to grid location...
    in vec2             NormalizedLocation;
    // Texture location cooresponding to grid location...
    in vec2             TextureLocation;

// Uniform variables...

    // Dimensions...
    uniform uint        Width;
    uniform uint        Height;

    // Grid spacing between vertices...
    uniform float       DistanceBetweenVertices;
    // Pre-computed equation constants...
    uniform float       CachedConstant1;
    uniform float       CachedConstant2;
    uniform float       CachedConstant3;
    // Z-displacement maps...
    uniform sampler2D   ZMapPrevious;
    uniform sampler2D   ZMapCurrent;

// Output variables...

    // Z-displacement output buffer...
    out float           ZPreviousOut;
    // Normal...
    out vec3            NormalOut;
    // Tangent...
    out vec3            TangentOut;

// Take a grid location and transform to a normalized texture coordinate...
vec2 GridToTexture(const uvec2 Location)
    // Transform to [0..1] and return it...
    return (Location / vec2(Width, Height));

// Take coordinate normalized in [-1..1] and transform to [0..Width or Height]...
uvec2 NormalizedPointToGrid(const vec2 Normalized)
    // Transform and return it... p = (d/2)(p'+1)
    return uvec2((vec2(Width, Height) * vec2(0.5, 0.5)) * (Normalized + vec2(1.0, 1.0)));

// Transform coordinate in [-1..1] to [0..1]
vec2 NormalizedPointToTexture(const vec2 Normalized)
    // Transform normalized form to texture coordinate... p_t = (p' + <1,1>) / 2
    return (Normalized + vec2(1.0, 1.0)) * vec2(0.5, 0.5);

// Entry point...
void main()
    // Get location on grid...
    uvec2 GridLocation = NormalizedPointToGrid(NormalizedLocation);
        // We assume the outer edge of fluid surface is fixed. This also makes
        //  querying neighbours easier since we can assume there always will be
        //  exactly eight all around it after this, though we don't affect the
        //  diagonal neighbours...

        if(GridLocation.x == 0u || GridLocation.x == (Width - 1u) ||
           GridLocation.y == 0u || GridLocation.y == (Height - 1u))

    // Lookup displacement of current location...
    float CurrentZ      = texture(ZMapCurrent, TextureLocation).r;
    // Lookup displacements of current location's neighbours...
    float AboveCurrentZ = texture(ZMapCurrent, GridToTexture(GridLocation + uvec2( 0,  1))).r;
    float BelowCurrentZ = texture(ZMapCurrent, GridToTexture(GridLocation + uvec2( 0, -1))).r;
    float RightCurrentZ = texture(ZMapCurrent, GridToTexture(GridLocation + uvec2( 1,  0))).r;
    float LeftCurrentZ  = texture(ZMapCurrent, GridToTexture(GridLocation + uvec2(-1,  0))).r;
    // Lookup displacement of last passes displacement at current location...
    float PreviousZ     = texture(ZMapPrevious, TextureLocation).r;

    // Update previous buffer's displacement here using equation 12.25, p.335, 
    //  of Mathematics for 3D Game Programming and Computer Graphics. */
    ZPreviousOut    = CachedConstant1 * CurrentZ +                        /* first term */
                      CachedConstant2 * PreviousZ +                       /* second term */
                      CachedConstant3 * (RightCurrentZ + LeftCurrentZ +   /* third term */
                                         AboveCurrentZ + BelowCurrentZ);

    // Calculate normal...
    NormalOut       = vec3(LeftCurrentZ  - RightCurrentZ, 
                           BelowCurrentZ - AboveCurrentZ,
                           2.0 * DistanceBetweenVertices);

    // Calculate tangent...
    TangentOut      = vec3(2.0 * DistanceBetweenVertices, 
                           RightCurrentZ - LeftCurrentZ);
I know this isn't a trivial algorithm, but any light one can shed on this is appreciated. Kip

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