Sign in to follow this  

OpenGL newbie question on glBufferData?

Recommended Posts

KaiserJohan    2317

I have a method like this for a first-try to draw triangles in OpenGL 3.3 (Vec3 is from glm library):

[CODE]void RenderOpenGL::DrawTriangle(const Vec3& pointA, const Vec3& pointB, const Vec3& pointC)
glBindBuffer(GL_ARRAY_BUFFER, mVertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(Vec3), glm::value_ptr(pointA), GL_STATIC_DRAW);
glBufferData(GL_ARRAY_BUFFER, sizeof(Vec3), glm::value_ptr(pointB), GL_STATIC_DRAW);
glBufferData(GL_ARRAY_BUFFER, sizeof(Vec3), glm::value_ptr(pointC), GL_STATIC_DRAW);

PrimitiveInfo primInfo;
primInfo.Mode = GL_TRIANGLES;
primInfo.Count = 3;

I would then go through my Vector<PrimitiveInfo> mPrimitiveInfo and draw as the elements indicated.

From what I read on [url=""]http://www.opengl.or...lBufferData.xml[/url] each consequent call to glBufferData will overwrite the old.
So how would you actually buffer this? Say I'm calling this method to draw three triangles, how would I actually buffer it without loosing the old data?

EDIT: and as a follow-up, how should you actually manage the VBOs? Right now I have a single buffer object to which I write data to but as you progress how would you distribute them? Edited by KaiserJohan

Share this post

Link to post
Share on other sites
clb    2147
glBufferData doesn't "queue up" the data you pass in, but specifies the full data contents. So, to specify a single triangle in a VBO, do
Vec3 points[3] = { pointA, pointB, pointC };
glBufferData(GL_ARRAY_BUFFER, sizeof(Vec3)*3, points, GL_STATIC_DRAW);

(Note however, that you never want to draw just a single triangle in a VBO, but you want to batch as many of them in a single array as possible for best batching)

Share this post

Link to post
Share on other sites
KaiserJohan    2317
Would it be wise to store all primitives I want to draw in one big vector and then in a "Render" method send them to the glBuffer and draw them?
Also, you would only have to rebuffer and redraw all of them if the content of that vector changed between frames right?

Share this post

Link to post
Share on other sites
Brother Bob    10344
The purpose of the buffer object is to store your vertex data so there's where you store it, not in some external vector only to copy it to the buffer when rendering. Load the buffer with data once and update it when it has to change.

Share this post

Link to post
Share on other sites
beans222    1248
Don't forget about glBufferSubData to alter a buffer without changing its size.
It's possible to pass a null pointer to glBufferData to just allocate the space.

If the buffer is holding just the single triangle, then clb's solution is best.

Share this post

Link to post
Share on other sites
clb    2147
Btw, as a note, on Android, I was in a habit of using glBufferData to initially create a VBO, then using glBufferSubData in all subsequent calls to update the full contents of that VBO, e.g. for per-frame particles. What I noticed was that it was slower than just directly using glBufferData each frame to update the particles, combined with manual double-buffering of the VBOs.

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 povilaslt2
      Hello. I'm Programmer who is in search of 2D game project who preferably uses OpenGL and C++. You can see my projects in GitHub. Project genre doesn't matter (except MMO's :D).
    • By ZeldaFan555
      Hello, My name is Matt. I am a programmer. I mostly use Java, but can use C++ and various other languages. I'm looking for someone to partner up with for random projects, preferably using OpenGL, though I'd be open to just about anything. If you're interested you can contact me on Skype or on here, thank you!
      Skype: Mangodoor408
    • By tyhender
      Hello, my name is Mark. I'm hobby programmer. 
      So recently,I thought that it's good idea to find people to create a full 3D engine. I'm looking for people experienced in scripting 3D shaders and implementing physics into engine(game)(we are going to use the React physics engine). 
      And,ye,no money =D I'm just looking for hobbyists that will be proud of their work. If engine(or game) will have financial succes,well,then maybe =D
      Sorry for late replies.
      I mostly give more information when people PM me,but this post is REALLY short,even for me =D
      So here's few more points:
      Engine will use openGL and SDL for graphics. It will use React3D physics library for physics simulation. Engine(most probably,atleast for the first part) won't have graphical fron-end,it will be a framework . I think final engine should be enough to set up an FPS in a couple of minutes. A bit about my self:
      I've been programming for 7 years total. I learned very slowly it as "secondary interesting thing" for like 3 years, but then began to script more seriously.  My primary language is C++,which we are going to use for the engine. Yes,I did 3D graphics with physics simulation before. No, my portfolio isn't very impressive. I'm working on that No,I wasn't employed officially. If anybody need to know more PM me. 
    • By Zaphyk
      I am developing my engine using the OpenGL 3.3 compatibility profile. It runs as expected on my NVIDIA card and on my Intel Card however when I tried it on an AMD setup it ran 3 times worse than on the other setups. Could this be a AMD driver thing or is this probably a problem with my OGL code? Could a different code standard create such bad performance?
    • By Kjell Andersson
      I'm trying to get some legacy OpenGL code to run with a shader pipeline,
      The legacy code uses glVertexPointer(), glColorPointer(), glNormalPointer() and glTexCoordPointer() to supply the vertex information.
      I know that it should be using setVertexAttribPointer() etc to clearly define the layout but that is not an option right now since the legacy code can't be modified to that extent.
      I've got a version 330 vertex shader to somewhat work:
      #version 330 uniform mat4 osg_ModelViewProjectionMatrix; uniform mat4 osg_ModelViewMatrix; layout(location = 0) in vec4 Vertex; layout(location = 2) in vec4 Normal; // Velocity layout(location = 3) in vec3 TexCoord; // TODO: is this the right layout location? out VertexData { vec4 color; vec3 velocity; float size; } VertexOut; void main(void) { vec4 p0 = Vertex; vec4 p1 = Vertex + vec4(Normal.x, Normal.y, Normal.z, 0.0f); vec3 velocity = (osg_ModelViewProjectionMatrix * p1 - osg_ModelViewProjectionMatrix * p0).xyz; VertexOut.velocity = velocity; VertexOut.size = TexCoord.y; gl_Position = osg_ModelViewMatrix * Vertex; } What works is the Vertex and Normal information that the legacy C++ OpenGL code seem to provide in layout location 0 and 2. This is fine.
      What I'm not getting to work is the TexCoord information that is supplied by a glTexCoordPointer() call in C++.
      What layout location is the old standard pipeline using for glTexCoordPointer()? Or is this undefined?
      Side note: I'm trying to get an OpenSceneGraph 3.4.0 particle system to use custom vertex, geometry and fragment shaders for rendering the particles.
  • Popular Now