• Advertisement
Sign in to follow this  

OpenGL Vertex Arrays vs. VBOs vs. VAOs

This topic is 2112 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts


I was wondering if someone would help me check my understanding of these three concepts, in order to make sure that I have them straight.

[indent=1]1. With all three of these methods, DrawArrays is used to make the actual draw call (or DrawElements, if there are indices). There are other draw functions as well (e.g., for instanced geometry), but these are the simplest ones.

[indent=1]2. The "Arrays" in question refer to individual vertex attribute data arrays.

[indent=1]3. For fixed-function pipeline stuff, the available arrays include GL_VERTEX_ARRAY, GL_COLOR_ARRAY, GL_TEXTURE_COORD_ARRAY, and so on. These arrays can be enabled and disabled individually, and are disabled by default. The data for these arrays can be specified using glVertexPointer, glColorPointer, glTexCoordPointer, etc. If a VBO is bound to GL_ARRAY_BUFFER at the time that gl*Pointer is called, then the pointer parameter specified is assumed to be an offset into that VBO. Otherwise, the pointer specified is assumed to be a direct pointer to the data on the CPU side. Although I've described this as "fixed-function pipeline stuff", the truth is that there are some deprecated built-in vertex attributes that you can use to access this data from GLSL shader code.

[indent=1]4. Under the new-ish shader-only approach, the "Arrays" in question are "vertex attribute arrays", which are analogous to GL_VERTEX_ARRAY, GL_COLOR_ARRAY, etc., except they are generic and can be used for anything. So, they are no longer referred-to by such symbolic constants, but rather are referred to by a GLuint index. The data for these arrays is set using glVertexAttribPointer, which is analogous to glVertexPointer, glColorPointer, etc., except that this one generic function is used for vertex attributes of all types. Because there is just one generic function for all vertex attribute arrays, you must provide information about the data type, size, stride, etc. so that it knows how to interpret the data.

[indent=1]5. My understanding is that, with #4, a VBO must be bound to GL_ARRAY_BUFFER and that supplying a direct pointer to the data (as with "Vertex Arrays") will not work.

[indent=1]6. While performing the steps alluded to in #4, a VAO object must be bound. Subsequent calls to glVertexAttribPointer and glEnableVertexAttribArray will affect that VAO. As such, the VBOs that are bound during calls to glVertexAttribPointer are indirectly associated with the VAO. When it is time to draw, one need only bind the VAO beforehand and there is no need to bind any VBOs or enable/disable specific arrays.

The above is basically what I've been able to come up with after reading the man pages, the OpenGL wiki, and some example source code here and there. I was a bit confused about the reuse of functions in each method, and the reuse of terms such as "vertex arrays" to describe (apparently) different things. There was also some ambiguity in the code samples that I examined. For instance, they would often call glEnableVertexAttribArray only once, when the VAO was first being set up. However, I couldn't tell if they were doing this because the VAO "remembers" which indices were enabled, or if they were just being efficient because the example never wanted those arrays to be disabled. Things like that. I'm hoping I have it straight now, or at least close.

Thank you.

Share this post

Link to post
Share on other sites
glVertexAttribPointer and friends actually predate VAOs and even go back to a time before usage of VBOs was widespread. You don't need either for to use generic attribs, although you do need shaders of some description (with one exception - attrib 0 always maps to position and can even be used in the fixed pipeline). Doom 3 for example used generic attrib arrays and didn't have VAOs, with use of VBOs being optional.

Otherwise you have it broadly correct - a VAO encapsulates all of the various vertex attrib states and when bound the states that were set for that VAO are automatically applied.

Share this post

Link to post
Share on other sites
Thanks very much!

glVertexAttribPointer and friends actually predate VAOs and even go back to a time before usage of VBOs was widespread. You don't need either for to use generic attribs, although you do need shaders of some description (with one exception - attrib 0 always maps to position and can even be used in the fixed pipeline).

That is interesting. So, in cases where there is no VBO and no VAO bound, is the pointer just assumed to be a direct pointer to data in system memory? Like with the gl*Pointer functions?

Share this post

Link to post
Share on other sites
Yup, that should work.

There is also the - I guess - little-known glVertexAttrib function which can even be used with immediate mode (and which may come in handy if you need to quickly prototype something new).

Share this post

Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
  • Popular Tags

  • Advertisement
  • Popular Now

  • Similar Content

    • By QQemka
      Hello. I am coding a small thingy in my spare time. All i want to achieve is to load a heightmap (as the lowest possible walking terrain), some static meshes (elements of the environment) and a dynamic character (meaning i can move, collide with heightmap/static meshes and hold a varying item in a hand ). Got a bunch of questions, or rather problems i can't find solution to myself. Nearly all are deal with graphics/gpu, not the coding part. My c++ is on high enough level.
      Let's go:
      Heightmap - i obviously want it to be textured, size is hardcoded to 256x256 squares. I can't have one huge texture stretched over entire terrain cause every pixel would be enormous. Thats why i decided to use 2 specified textures. First will be a tileset consisting of 16 square tiles (u v range from 0 to 0.25 for first tile and so on) and second a 256x256 buffer with 0-15 value representing index of the tile from tileset for every heigtmap square. Problem is, how do i blend the edges nicely and make some computationally cheap changes so its not obvious there are only 16 tiles? Is it possible to generate such terrain with some existing program?
      Collisions - i want to use bounding sphere and aabb. But should i store them for a model or entity instance? Meaning i have 20 same trees spawned using the same tree model, but every entity got its own transformation (position, scale etc). Storing collision component per instance grats faster access + is precalculated and transformed (takes additional memory, but who cares?), so i stick with this, right? What should i do if object is dynamically rotated? The aabb is no longer aligned and calculating per vertex min/max everytime object rotates/scales is pretty expensive, right?
      Drawing aabb - problem similar to above (storing aabb data per instance or model). This time in my opinion per model is enough since every instance also does not have own vertex buffer but uses the shared one (so 20 trees share reference to one tree model). So rendering aabb is about taking the model's aabb, transforming with instance matrix and voila. What about aabb vertex buffer (this is more of a cosmetic question, just curious, bumped onto it in time of writing this). Is it better to make it as 8 points and index buffer (12 lines), or only 2 vertices with min/max x/y/z and having the shaders dynamically generate 6 other vertices and draw the box? Or maybe there should be just ONE 1x1x1 cube box template moved/scaled per entity?
      What if one model got a diffuse texture and a normal map, and other has only diffuse? Should i pass some bool flag to shader with that info, or just assume that my game supports only diffuse maps without fancy stuff?
      There were several more but i forgot/solved them at time of writing
      Thanks in advance
    • By RenanRR
      Hi All,
      I'm reading the tutorials from learnOpengl site (nice site) and I'm having a question on the camera (https://learnopengl.com/Getting-started/Camera).
      I always saw the camera being manipulated with the lookat, but in tutorial I saw the camera being changed through the MVP arrays, which do not seem to be camera, but rather the scene that changes:
      Vertex Shader:
      #version 330 core layout (location = 0) in vec3 aPos; layout (location = 1) in vec2 aTexCoord; out vec2 TexCoord; uniform mat4 model; uniform mat4 view; uniform mat4 projection; void main() { gl_Position = projection * view * model * vec4(aPos, 1.0f); TexCoord = vec2(aTexCoord.x, aTexCoord.y); } then, the matrix manipulated:
      ..... glm::mat4 projection = glm::perspective(glm::radians(fov), (float)SCR_WIDTH / (float)SCR_HEIGHT, 0.1f, 100.0f); ourShader.setMat4("projection", projection); .... glm::mat4 view = glm::lookAt(cameraPos, cameraPos + cameraFront, cameraUp); ourShader.setMat4("view", view); .... model = glm::rotate(model, glm::radians(angle), glm::vec3(1.0f, 0.3f, 0.5f)); ourShader.setMat4("model", model);  
      So, some doubts:
      - Why use it like that?
      - Is it okay to manipulate the camera that way?
      -in this way, are not the vertex's positions that changes instead of the camera?
      - I need to pass MVP to all shaders of object in my scenes ?
      What it seems, is that the camera stands still and the scenery that changes...
      it's right?
      Thank you
    • By dpadam450
      Sampling a floating point texture where the alpha channel holds 4-bytes of packed data into the float. I don't know how to cast the raw memory to treat it as an integer so I can perform bit-shifting operations.

      int rgbValue = int(textureSample.w);//4 bytes of data packed as color
      // algorithm might not be correct and endianness might need switching.
      vec3 extractedData = vec3(  rgbValue & 0xFF000000,  (rgbValue << 8) & 0xFF000000, (rgbValue << 16) & 0xFF000000);
      extractedData /= 255.0f;
    • By Devashish Khandelwal
      While writing a simple renderer using OpenGL, I faced an issue with the glGetUniformLocation function. For some reason, the location is coming to be -1.
      Anyone has any idea .. what should I do?
    • By Andrey OGL_D3D
      Hi all!
      I try to use the Sun shafts effects via post process in my 3DEngine, but i have some artefacts on final image(Please see attached images).
      The effect contains the following passes:
      1) Depth scene pass;
      2) "Shafts pass" Using DepthPass Texture + RGBA BackBuffer texture.
      3) Shafts pass texture +  RGBA BackBuffer texture.
      Shafts shader for 2 pass:
      // uniform sampler2D FullSampler; // RGBA Back Buffer uniform sampler2D DepthSampler; varying vec2 tex; #ifndef saturate float saturate(float val) {     return clamp(val, 0.0, 1.0); } #endif void main(void) {     vec2 uv = tex;     float sceneDepth = texture2D(DepthSampler, uv.xy).r;     vec4  scene        = texture2D(FullSampler, tex);     float fShaftsMask     = (1.0 - sceneDepth);     gl_FragColor = vec4( scene.xyz * saturate(sceneDepth), fShaftsMask ); } final shader:
      // uniform sampler2D FullSampler; // RGBA Back Buffer uniform sampler2D BlurSampler; // shafts sampler varying vec4 Sun_pos; const vec4    ShaftParams = vec4(0.1,2.0,0.1,2.0); varying vec2 Tex_UV; #ifndef saturate  float saturate(float val) {     return clamp(val, 0.0, 1.0); } #endif vec4 blendSoftLight(vec4 a, vec4 b) {   vec4 c = 2.0 * a * b + a * a * (1.0 - 2.0 * b);   vec4 d = sqrt(a) * (2.0 * b - 1.0) + 2.0 * a * (1.0 - b);       // TODO: To look in Crysis what it the shit???   //return ( b < 0.5 )? c : d;   return any(lessThan(b, vec4(0.5,0.5,0.5,0.5)))? c : d; } void main(void) {     vec4 sun_pos = Sun_pos;     vec2    sunPosProj = sun_pos.xy;     //float    sign = sun_pos.w;     float    sign = 1.0;     vec2    sunVec = sunPosProj.xy - (Tex_UV.xy - vec2(0.5, 0.5));     float    sunDist = saturate(sign) * saturate( 1.0 - saturate(length(sunVec) * ShaftParams.y ));     sunVec *= ShaftParams.x * sign;     vec4 accum;     vec2 tc = Tex_UV.xy;     tc += sunVec;     accum = texture2D(BlurSampler, tc);     tc += sunVec;     accum += texture2D(BlurSampler, tc) * 0.875;     tc += sunVec;     accum += texture2D(BlurSampler, tc) * 0.75;     tc += sunVec;     accum += texture2D(BlurSampler, tc) * 0.625;     tc += sunVec;     accum += texture2D(BlurSampler, tc) * 0.5;     tc += sunVec;     accum += texture2D(BlurSampler, tc) * 0.375;     tc += sunVec;     accum += texture2D(BlurSampler, tc) * 0.25;     tc += sunVec;     accum += texture2D(BlurSampler, tc) * 0.125;     accum  *= 0.25 * vec4(sunDist, sunDist, sunDist, 1.0);           accum.w += 1.0 - saturate(saturate(sign * 0.1 + 0.9));     vec4    cScreen = texture2D(FullSampler, Tex_UV.xy);           vec4    cSunShafts = accum;     float fShaftsMask = saturate(1.00001 - cSunShafts.w) * ShaftParams.z * 2.0;              float fBlend = cSunShafts.w;     vec4 sunColor = vec4(0.9, 0.8, 0.6, 1.0);     accum =  cScreen + cSunShafts.xyzz * ShaftParams.w * sunColor * (1.0 - cScreen);     accum = blendSoftLight(accum, sunColor * fShaftsMask * 0.5 + 0.5);     gl_FragColor = accum; } Demo project:
      Demo Project
      Shaders for postprocess Shaders/SunShaft/
      What i do wrong ?

  • Advertisement