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I am just starting to learn OpenGL today from this tutorial: http://openglbook.com/the-book/
I got to chapter 2, where I draw a triangle, and I understand everything except VAOs (is this acronym OK?). The tutorial has this code:
glGenVertexArrays(1, &VaoId);
glBindVertexArray(VaoId);
While I understand that the code is necessary, I have no clue what it does. Although I never use VaoId past this point (except to destroy it), the code does not function without it. I am assuming this is because it is required to be bound, but I don't know why. Does this exact code just need to be part of every OpenGL program? The tutorial explains VAOs as:
A Vertex Array Object (or VAO) is an object that describes how the vertex attributes are stored in a Vertex Buffer Object (or VBO). This means that the VAO is not the actual object storing the vertex data, but the descriptor of the vertex data. Vertex attributes can be described by the glVertexAttribPointer function and its two sister functions glVertexAttribIPointer and glVertexAttribLPointer, the first of which we’ll explore below.
I don't understand how the VAO describes the vertex attributes. I have not described them in any way. Does it get the information from the glVertexAttribPointer? I guess this must be it. Is the VAO simply a destination for the information from glVertexAttribPointer?
On a side note, is the tutorial I am following acceptable? Is there anything I should watch out for or a better tutorial to follow?
"Vertex Array Object" is brought to you by the OpenGL ARB Subcommittee for Silly Names.
Think of it as a geometry object. (As an old time SGI Performer programmer, I call them geosets.) The instance variables/members of the object are your vertex pointer, normal pointer, color pointer, attrib N pointer, ...
When a VAO is first bound, you assign these members by calling
glEnableClientState(GL_VERTEX_ARRAY); glVertexPointer...;
glEnableClientState(GL_NORMAL_ARRAY); glNormalPointer...;
and so on. Which attributes are enabled and the pointers you supply are stored in the VAO.
After that when you bind the VAO again, all the those attributes and pointers also become current. So one glBindVertexArray call is equivalent to all the code previously needed to set up all the attributes. It's handy for passing geometry around between functions or methods without having to create your own structs or objects.
(One time setup, multiple use is the easiest way to use VAOs, but you can also change attributes just by binding it and doing more enable/pointer calls. VAOs are not constants.)
More info in response to Patrick's questions:
The default for a newly created VAO is that it's empty (AFAIK). No geometry at all, not even vertexes, so if you try to draw it, you'll get an OpenGL error. This is reasonably sane, as in "initialize everything to False/NULL/zero".
You only need to glEnableClientState when you set things up. The VAO remembers the enable/disable state for each pointer.
Yes the VAO will store glEnableVertexAttribArray and glVertexAttrib. The old vertex, normal, color, ... arrays are the same as attribute arrays, vertex == #0 and so on.
I always think about VAO as an array of data buffers used by OpenGL. Using modern OpenGL you will create a VAO and Vertex Buffer Objects.
//vaoB is a buffer
glGenVertexArrays(1, vaoB); //creates one VAO
glBindVertexArray(vao.get(0));
glGenBuffers(vbo.length, vbo, 0); //vbo is a buffer
glBindVertexArray(vao.get(1));
glGenBuffers(vbo1.length, vbo1, 0); //vbo1 is a buffer
glBindVertexArray(vao.get(2));
glGenBuffers(vbo2.length, vbo2, 0); //vbo2 is a buffer
The next step is to bind data to a buffer:
glBindBuffer(GL_ARRAY_BUFFER, vbo[0]);
glBufferData(GL_ARRAY_BUFFER,vertBuf.limit()*4, vertBuf, GL_STATIC_DRAW); //vertf buf is a floatbuffer of vertices
At this point OpenGL Sees:
Now we can use glVertexAttribPointer to tell OpenGL what the data in the buffer represents:
glBindBuffer(GL_ARRAY_BUFFER, 0); //bind VBO at 0
glVertexAttribPointer(0, 3, GL_FLOAT, false, 0, 0); //each vertex has 3 components of size GL_FLOAT with 0 stride (space) between them and the first component starts at 0 (start of data)
OpenGL now has the data in the buffer and knows how the data is organized into vertices. The same process can be applied to texture coordinates etc but for texture coordinates there would be two values.
glBindBuffer(GL_ARRAY_BUFFER, vbo[1]);
glBufferData(GL_ARRAY_BUFFER,coordBuf.limit()*4, coordBuf, GL_STATIC_DRAW);
glVertexAttribPointer(0, 2, GL_FLOAT, false, 0, 0);
Next you can bind texture and draw arrays, you will want to create a Vert and Frag shader, compile and attach it to a program (not included here).
glActiveTexture(textureID); //bind our texture
glBindTexture(GL_TEXTURE_2D, textureID);
glDrawArrays(GL_TRIANGLES,0,6); //in this case 6 indices are used for two triangles forming a square
Vertex Array Objects are like macros in word processing programs and the like. A good description is found here.
Macros just remember the actions you did, such as activate this attribute, bind that buffer, etc. When you call glBindVertexArray( yourVAOId ), it simply replays those attribute pointer bindings and buffer bindings.
So your next call to draw uses whatever was bound by the VAO.
VAO's don't store vertex data. No. The vertex data is stored in a vertex buffer or in an array of client memory.
VAO is an object that represents the vertex fetch stage of the OpenGL pipeline and is used to supply input to the vertex shader.
You can create vertex array object like this
GLuint vao;
glCreateVertexArrays(1, &vao);
glBindVertexArray(vao);
First let' do a simple example. Consider such an input parameter in a shader code
layout (location = 0) in vec4 offset; // input vertex attribute
To fill in this attribute we can use
glVertexAttrib4fv(0, attrib); // updates the value of input attribute 0
Although the vertex array object stores these static attribute values for
you, it can do a lot more.
After creating vertex array object we can start filling in its state. We will ask OpenGL to fill it automatically using the data stored in a buffer object that we supply. Each vertex attribute gets to fetch data from a buffer bound to one of several vertex buffer bindings. For this end we use glVertexArrayAttribBinding(GLuint vao, GLuint attribindex, GLuint bindingindex). Also we use the glVertexArrayVertexBuffer() function to bind a buffer to one of the vertex buffer bindings. We use the glVertexArrayAttribFormat() function to describe the layout and format of the data, and finally we enable automatic filling of the attribute by calling glEnableVertexAttribArray().
When a vertex attribute is enabled, OpenGL will feed data to the vertex shader based on the format and location information you’ve provided with
glVertexArrayVertexBuffer() and glVertexArrayAttribFormat(). When
the attribute is disabled, the vertex shader will be provided with the static information you provide with a call to glVertexAttrib*().
// First, bind a vertex buffer to the VAO
glVertexArrayVertexBuffer(vao, 0, buffer, 0, sizeof(vmath::vec4));
// Now, describe the data to OpenGL, tell it where it is, and turn on automatic
// vertex fetching for the specified attribute
glVertexArrayAttribFormat(vao, 0, 4, GL_FLOAT, GL_FALSE, 0);
glEnableVertexArrayAttrib(vao, 0);
And code in a shader
layout (location = 0) in vec4 position;
After all you need to call to glDeleteVertexArrays(1, &vao).
You can read OpenGL SuperBible to understand it better.
I was trying to understand this as well and now that I think I do, it would be prudent to post a code example aimed at
people less familiar with OpenGL architecture, as I found the previous examples not very illuminating and most tutorials
just tell you to copy paste the code without explaining it.
(This is in C++ but the code can be easily translated to C)
In this example, we'll be rendering a rectangle, which has 4 vertices. Each vertex has a position (vec3, xyz), texture coordinate (vec2, uv) and color attribute (vec4, rgba).
I think it's cleanest to separate each attribute into their own array:
float positions[] = {
+0.5, +0.5, 0,
+0.5, -0.5, 0,
-0.5, -0.5, 0,
-0.5, +0.5, 0
};
float colors[] = {
1, 1, 1, 1,
1, 1, 1, 1,
1, 1, 1, 1,
1, 1, 1, 1
};
float tex_coords[] = {
0, 0,
0, 1,
1, 1,
1, 0
};
Our vertex array object will describe the four vertices with these properties.
First, we need to create the vertex array:
GLuint vertex_array;
glGenVertexArrays(1, &vertex_array);
Each vertex array has a number of buffers, these can be thought of as properties of the array. Each vertex array has an
arbitrary number of "slots" for the buffers. Along with which buffer is in which slot, it saves the CPU-side pointer to
the data for the buffer, and the CPU-side datas format. We need to make OpenGL aware of both which slot to use, where the
data is, and how it is formatted.
The buffers slots are indexed, so the first buffer is index 0, the second is 1, etc.
These locations correspond to the layout defined in the vertex shader:
// vertex shader
std::string _noop_vertex_shader_source = R"(
#version 420
layout (location = 0) in vec3 _position_3d; // slot 0: xyz
layout (location = 1) in vec4 _color_rgba; // slot 1: rgba
layout (location = 2) in vec2 _tex_coord; // slot 2: uv
out vec2 _vertex_tex_coord;
out vec4 _vertex_color_rgba;
void main()
{
gl_Position = vec4(_position_3d.xy, 1, 1); // forward position to fragment shader
_vertex_color_rgba = _color_rgba; // forward color to fragment shader
_vertex_tex_coord = _tex_coord; // forward tex coord to fragment shader
}
)";
We see that the position property is at location 0, the color property at 1 and the tex coords at 2. We'll store these
for clarity:
// property locations from our shader
const auto vertex_pos_location = 0;
const auto vertex_color_location = 1;
const auto vertex_tex_coord_location = 2;
We now need to tell OpenGL the information about each buffer outlined above:
// bind the array, this makes OpenGL aware that we are modifying it with future calls
glBindVertexArray(vertex_array);
// create the position buffer
glGenBuffers(1, &position_buffer);
// bind the buffer so opengl knows we are currently operating on it
glBindBuffer(GL_ARRAY_BUFFER, position_buffer);
// tell opengl where the data pointer is
glBufferData(GL_ARRAY_BUFFER, sizeof(positions), positions, GL_STATIC_DRAW);
// tell opengl how the data is formatted
glVertexAttribPointer(vertex_pos_location, 3, GL_FLOAT, GL_FALSE, 0, (void*) 0);
// tell opengl that this slot should be used
glEnableVertexAttribArray(vertex_pos_location);
Here, we generate a buffer that will hold our position data. For glVertexAttribPointer, we choose the
correct location, 3 elements (as the positions are xyz coordinates), and no offset or stride.
Because we have a separate array for all our properties, we can leave both as 0.
Similar to the position, we generate and fill the buffers for the color and tex coord property:
// color
glGenBuffers(1, &color_buffer); // generate
glBindBuffer(GL_ARRAY_BUFFER, color_buffer); // bind
glBufferData(GL_ARRAY_BUFFER, sizeof(colors), colors, GL_STATIC_DRAW); // set pointer
glVertexAttribPointer(vertex_color_location, 4, GL_FLOAT, GL_FALSE, 0, (void*) 0); // set data format
glEnableVertexAttribArray(vertex_color_location); // enable slot
// tex coords
glGenBuffers(1, &tex_coord_buffer);
glBindBuffer(GL_ARRAY_BUFFER, tex_coord_buffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(tex_coords), tex_coords, GL_STATIC_DRAW);
glVertexAttribPointer(vertex_tex_coord_location, 2, GL_FLOAT, GL_FALSE, 0, (void*) 0);
glEnableVertexAttribArray(vertex_tex_coord_location);
Where we chose 4 elements for the colors because they are in RGBA format and 2 for the tex coords for obvious reasons.
The last thing we need to render a vertex array is an element buffer. These can be thought of as a list of
indices that define which order the vertices will be rendered in. For us, we want to render the
rectangle as two tris in a triangle fan, so we choose the following element buffer:
// vertex order
static uint32_t indices[] = {
0, 1, 2, 1, 2, 3
};
glGenBuffers(1, &element_buffer); // generate
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, element_buffer); // bind
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW) // set pointer
We do not need to enable the element buffers slot, it is separate from the vertex array. We don't have to specify the format of the elements buffer here, that will be done during glDrawElements in the render step.
So why all this? All these functions tell OpenGL where to look for the data for the vertices. Specifying the pointers to
the correct buffer data and their layout, if we now bind the vertex array during a render step:
glUseProgram(shader.get_program_id()); // shader program with our vertex shader
glBindVertexArray(vertex_array);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, element_buffer);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
Where 6 is the number of elements in the element buffer.
This is all that's needed to correctly update the in values in the vertex shader. OpenGL will move the data from
our CPU-side positions, colors and tex_coords into the correct locations 0, 1 and 2 of the vertex shader respectively.
We don't need to bind anything else, the vertex array remembers what we gave it and does it for us, which is why it's convenient and should be preferred in modern OpenGL.
In summary:
Each vertex array has n buffers for arbitrary properties and 1 element buffer. For each property / buffer, we need to
a) generate it (glGenBuffers)
b) bind it (glBindBuffer(GL_ARRAY_BUFFER)
c) tell OpenGL where the data is located in RAM (glBufferData)
d) tell OpenGL how the data is formatted (glVertexAttribPointer)
e) tell OpenGL to use that slot (glEnableVertexAttribArray)
for the element buffer, we only need to generate it, bind it to GL_ELEMENT_ARRAY_BUFFER, then tell opengl
where the data is.
Hopefully that helped shed some light on things. I'm almost positive there will be factual errors in this post as
I'm also mostly new to OpenGL but this was the way I conceptualized it to get my code working.
I have been working to get OpenGL to render multiple different entities on the scene.
According to http://www.opengl.org/wiki/Vertex_Specification,
Vertex Array Object should remember what Vertex Buffer Object was bound to GL_ARRAY_BUFFER, GL_ELEMENT_ARRAY_BUFFER. (Or at least that is how I understood what it was saying)
Yet, even though I call draw after binding any vao, the application will use only the last one bound to GL_ARRAY_BUFFER.
Question - Am I understanding it right? Considering the code below, is all sequence of calling to gl functions is correct?
void OglLayer::InitBuffer()
{
std::vector<float> out;
std::vector<unsigned> ibOut;
glGenVertexArrays(V_COUNT, vaos);
glGenBuffers(B_COUNT, buffers);
// -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= //
glBindVertexArray(vaos[V_PLANE]);
PlaneBuffer(out, ibOut, 0.5f, 0.5f, divCount, divCount);
OGL_CALL(glBindBuffer(GL_ARRAY_BUFFER, buffers[B_PLANE_VERTEX]));
OGL_CALL(glBufferData(GL_ARRAY_BUFFER, out.size()*sizeof(float), out.data(), GL_DYNAMIC_DRAW));
//GLuint vPosition = glGetAttribLocation( programs[P_PLANE], "vPosition" );
OGL_CALL(glEnableVertexAttribArray(0));
//OGL_CALL(glVertexAttribPointer( vPosition, 3, GL_FLOAT, GL_FALSE, sizeof(float)*3, BUFFER_OFFSET(0) ));
OGL_CALL(glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, sizeof(float)*3, BUFFER_OFFSET(0) ));
bufferData[B_PLANE_VERTEX].cbSize = sizeof(float) * out.size();
bufferData[B_PLANE_VERTEX].elementCount = out.size()/3;
OGL_CALL(glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, buffers[B_PLANE_INDEX]));
OGL_CALL(glBufferData(GL_ELEMENT_ARRAY_BUFFER, ibOut.size()*sizeof(unsigned), ibOut.data(), GL_STATIC_DRAW));
bufferData[B_PLANE_INDEX].cbSize = sizeof(float) * ibOut.size();
bufferData[B_PLANE_INDEX].elementCount = ibOut.size();
// -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= //
glBindVertexArray(vaos[V_CUBE]);
out.clear();
ibOut.clear();
GenCubeMesh(out, ibOut);
glBindBuffer(GL_ARRAY_BUFFER, buffers[B_CUBE_VERTEX]);
glBufferData(GL_ARRAY_BUFFER, out.size()*sizeof(float), out.data(), GL_STATIC_DRAW);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3*sizeof(float), BUFFER_OFFSET(0));
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, buffers[B_CUBE_INDEX]);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(unsigned), ibOut.data(), GL_STATIC_DRAW);
}
void RenderPlane::Render( float dt )
{
// Putting any vao here always results in using the lastly buffer bound.
OGL_CALL(glBindVertexArray(g_ogl.vaos[m_pRender->vao]));
{
OGL_CALL(glUseProgram(g_ogl.programs[m_pRender->program]));
// uniform location
GLint loc = 0;
// Send Transform Matrix ( Rotate Cube over time )
loc = glGetUniformLocation(g_ogl.programs[m_pRender->program], "transfMat");
auto transf = m_pRender->pParent->m_transform->CreateTransformMatrix();
glUniformMatrix4fv(loc, 1, GL_TRUE, &transf.matrix()(0,0));
// Send View Matrix
loc = glGetUniformLocation(g_ogl.programs[m_pRender->program], "viewMat");
mat4 view = g_ogl.camera.transf().inverse();
glUniformMatrix4fv(loc, 1, GL_TRUE, &view(0,0));
// Send Projection Matrix
loc = glGetUniformLocation(g_ogl.programs[m_pRender->program], "projMat");
mat4 proj = g_ogl.camera.proj();
glUniformMatrix4fv(loc, 1, GL_TRUE, &proj(0,0));
}
OGL_CALL(glDrawElements(GL_TRIANGLES, g_ogl.bufferData[m_pRender->ib].elementCount, GL_UNSIGNED_INT, 0));
}
The GL_ARRAY_BUFFER binding is not part of the VAO state. The wiki page you link explains that correctly:
Note: The GL_ARRAY_BUFFER binding is NOT part of the VAO's state! I know that's confusing, but that's the way it is.
The GL_ELEMENT_ARRAY_BUFFER binding on the other hand is part of the VAO state.
While I don't completely disagree with calling this confusing, it actually does make sense if you start thinking about it. The goal of a VAO is to capture all vertex state that you would typically set up before making a draw call. When using indexed rendering, this includes binding the proper index buffer used for the draw call. Therefore, including the GL_ELEMENT_ARRAY_BUFFER binding in the VAO state makes complete sense.
On the other hand, the current GL_ARRAY_BUFFER binding does not influence a draw call at all. It only matters that the correct binding is established before calling glVertexAttribPointer(). And all the state set by glVertexAttribPointer() is part of the VAO state. So the VAO state contains the vertex buffer reference used for each attribute, which is established by the glVertexAttribPointer() call. The current GL_ARRAY_BUFFER on the other hand is not part of the VAO state because the current binding at the time of the draw call does not have any effect on the draw call.
Another way of looking at this: Since attributes used for a draw call can be pulled from different vertex buffers, having a single vertex buffer binding tracked in the VAO state would not be useful. On the other hand, since OpenGL only ever uses a single index buffer for a draw call, and uses the current index buffer binding for the draw call, tracking the index buffer binding in the VAO makes sense.
I have generally learned OpenGL Interoperability with CUDA, but my problem is like this:
I have a lot of arrays, some for vertex, some for norm and some for alpha value alone, and some pointers to these arrays on device memory (something like dev_ver, dev_norm) which are used in kernel. I have already mapped the resource and now I want to use these data in shaders to make some effects. My rendering code is like this:
glUseProgram (programID);
glBindBuffer (GL_ARRAY_BUFFER, vertexBuffer_0);
glBufferData(GL_ARRAY_BUFFER, size, _data_on_cpu_0, GL_DYNAMIC_DRAW);
glVertexAttribPointer (0, 3, GL_FLOAT, GL_FALSE, 0, (void*)0);
glBindBuffer (GL_ARRAY_BUFFER, vertexBuffer_1);
glBufferData(GL_ARRAY_BUFFER, size, _data_on_cpu_1, GL_DYNAMIC_DRAW);
glVertexAttribPointer (1, 3, GL_FLOAT, GL_FALSE, 0, (void*)0);
glBindBuffer (GL_ARRAY_BUFFER, vertexBuffer_2);
glBufferData(GL_ARRAY_BUFFER, size, _data_on_cpu_2, GL_DYNAMIC_DRAW);
glVertexAttribPointer (2, 3, GL_FLOAT, GL_FALSE, 0, (void*)0);
glEnableVertexAttribArray (0);
glEnableVertexAttribArray (1);
glEnableVertexAttribArray (2);
glDrawArrays (GL_TRIANGLES, 0, _max_);
glDisableVertexAttribArray (0);
glDisableVertexAttribArray (1);
glDisableVertexAttribArray (2);
However, now I have no _data_on_cpu_, is it still possible to do the same thing ? The sample in cuda 6.0 is something like this:
glBindBuffer(GL_ARRAY_BUFFER, posVbo);
glVertexPointer(4, GL_FLOAT, 0, 0);
glEnableClientState(GL_VERTEX_ARRAY);
glBindBufferARB(GL_ARRAY_BUFFER_ARB, normalVbo);
glNormalPointer(GL_FLOAT, sizeof(float)*4, 0);
glEnableClientState(GL_NORMAL_ARRAY);
glColor3f(1.0, 0.0, 0.0);
glDrawArrays(GL_TRIANGLES, 0, totalVerts);
glDisableClientState(GL_VERTEX_ARRAY);
glDisableClientState(GL_NORMAL_ARRAY);
I don't exactly understand how this could work and what to do in my case.
By the way, the method I have used is to cudaMemcpy the dev_ to host and do the render like usual, but this is obviously not efficient, because when I do rendering I again send the data back to GPU by OpenGL (if I'm right).
It's not really clear what your asking for, you mention CUDA yet none of the code you have posted is CUDA specific. I'm guessing vertexbuffer_2 contains additional per vertex information you want to access in the shader?
OpenGL calls are as efficient as you will get it, they aren't actually copying any data back from device to host. They are simply sending the addresses to the device, telling it where to get the data from and how much data to use to render.
You only need to fill the vertex and normal information at the start of your program, there isn't much reason to be changing this information during execution. You can then change data stored in texture buffers to pass additional per entity data to shaders to change model position, rotation, colour etc.
When you write your shader you must include in it;
attribute in vec3 v_data; (or similar)
When you init your shader you must then;
GLuint vs_v_data = glGetAttribLocation(p_shaderProgram, "v_data");
Then instead of your;
glBindBuffer (GL_ARRAY_BUFFER, vertexBuffer_2);
glBufferData(GL_ARRAY_BUFFER, size, _data_on_cpu_2, GL_DYNAMIC_DRAW);
glVertexAttribPointer (2, 3, GL_FLOAT, GL_FALSE, 0, (void*)0);
You use;
glEnableVertexAttribArray (vs_v_data);
glBindBuffer (GL_ARRAY_BUFFER, vertexBuffer_2);
glBufferData(GL_ARRAY_BUFFER, size, _data_on_cpu_2, GL_DYNAMIC_DRAW);
glVertexAttribPointer (vs_v_data, 3, GL_FLOAT, GL_FALSE, 0, (void*)0);
This should let you access a float3 inside your vshaders called v_data that has whatevers stored in vertexBuffer_2, presumably secondary vertex information to lerp between for animation.
A simple shader for this that simply repositions vertices based on an input tick
#version 120
attribute in float tick;
attribute in vec3 v_data;
void main()
{
gl_Vertex.xyz = mix(gl_Vertex.xyz, v_data, tick);
}
If you want per entity data instead of/in addition to per vertex data, you should be doing that via texture buffers.
If your trying to access vertex buffer obj data inside kernels you need to use a bunch of functions;
cudaGraphicsGLRegisterBuffer() This will give you a resource pointer to the buffer, execute this once after you initially setup the vbo.
cudaGraphicsMapResources() This will map the buffer (you can use it in CUDA but not gl)
cudaGraphicsResourceGetMappedPointer() This will give you a device pointer to the buffer, pass this to the the kernel.
cudaGraphicsUnmapResources() This will unmap the buffer (you can use it in gl, but not CUDA)
I've just read through a tutorial about Vertex Array Objects and Vertex Buffer Objects, and I can't work out from the following code how OpenGL knows the first VBO (vertexBufferObjID[0]) represents vertex coordinates, and the second VBO (vertexBufferObjID[1]) represents colour data?
glGenBuffers(2, vertexBufferObjID);
// VBO for vertex data
glBindBuffer(GL_ARRAY_BUFFER, vertexBufferObjID[0]);
glBufferData(GL_ARRAY_BUFFER, 9*sizeof(GLfloat), vertices, GL_STATIC_DRAW);
glVertexAttribPointer((GLuint)0, 3, GL_FLOAT, GL_FALSE, 0, 0);
glEnableVertexAttribArray(0);
// VBO for colour data
glBindBuffer(GL_ARRAY_BUFFER, vertexBufferObjID[1]);
glBufferData(GL_ARRAY_BUFFER, 9*sizeof(GLfloat), colours, GL_STATIC_DRAW);
glVertexAttribPointer((GLuint)1, 3, GL_FLOAT, GL_FALSE, 0, 0);
glEnableVertexAttribArray(1);
Edit: Thanks to Peter's answer, I found the following two lines of code which hook up each VBO with the shaders (indices 0 & 1 correlate to the VBO index):
glBindAttribLocation(programId, 0, "in_Position");
glBindAttribLocation(programId, 1, "in_Color");
It doesn't, you have to tell it which is which in the shader.
layout(location = 0) in vec3 position;
layout(location = 1) in vec3 color;
void main()
{
/* ... */
}
glVertexAttribPointer is only meant to be used with shaders. Therefore you, as the shader writer, always know what each attribute index is supposed to do, as it maps to a certain variable in your vertex shader. You control the index to variable map with commands glBindAttribLocation or glGetAttribLocation, or from special keywords inside GLSL.
If you're just using the fixed function pipeline (default shader), then you don't want to use glVertexAttribPointer. Instead for the fixed function pipe the equivalent commands are glVertexPointer and glColorPointer, etc. Note that these are deprecated commands.
Also note that for fixed function glEnableVertexAttribArray is to be replaced with glEnableClientState.
I am just starting to learn OpenGL today from this tutorial: http://openglbook.com/the-book/
I got to chapter 2, where I draw a triangle, and I understand everything except VAOs (is this acronym OK?). The tutorial has this code:
glGenVertexArrays(1, &VaoId);
glBindVertexArray(VaoId);
While I understand that the code is necessary, I have no clue what it does. Although I never use VaoId past this point (except to destroy it), the code does not function without it. I am assuming this is because it is required to be bound, but I don't know why. Does this exact code just need to be part of every OpenGL program? The tutorial explains VAOs as:
A Vertex Array Object (or VAO) is an object that describes how the vertex attributes are stored in a Vertex Buffer Object (or VBO). This means that the VAO is not the actual object storing the vertex data, but the descriptor of the vertex data. Vertex attributes can be described by the glVertexAttribPointer function and its two sister functions glVertexAttribIPointer and glVertexAttribLPointer, the first of which we’ll explore below.
I don't understand how the VAO describes the vertex attributes. I have not described them in any way. Does it get the information from the glVertexAttribPointer? I guess this must be it. Is the VAO simply a destination for the information from glVertexAttribPointer?
On a side note, is the tutorial I am following acceptable? Is there anything I should watch out for or a better tutorial to follow?
"Vertex Array Object" is brought to you by the OpenGL ARB Subcommittee for Silly Names.
Think of it as a geometry object. (As an old time SGI Performer programmer, I call them geosets.) The instance variables/members of the object are your vertex pointer, normal pointer, color pointer, attrib N pointer, ...
When a VAO is first bound, you assign these members by calling
glEnableClientState(GL_VERTEX_ARRAY); glVertexPointer...;
glEnableClientState(GL_NORMAL_ARRAY); glNormalPointer...;
and so on. Which attributes are enabled and the pointers you supply are stored in the VAO.
After that when you bind the VAO again, all the those attributes and pointers also become current. So one glBindVertexArray call is equivalent to all the code previously needed to set up all the attributes. It's handy for passing geometry around between functions or methods without having to create your own structs or objects.
(One time setup, multiple use is the easiest way to use VAOs, but you can also change attributes just by binding it and doing more enable/pointer calls. VAOs are not constants.)
More info in response to Patrick's questions:
The default for a newly created VAO is that it's empty (AFAIK). No geometry at all, not even vertexes, so if you try to draw it, you'll get an OpenGL error. This is reasonably sane, as in "initialize everything to False/NULL/zero".
You only need to glEnableClientState when you set things up. The VAO remembers the enable/disable state for each pointer.
Yes the VAO will store glEnableVertexAttribArray and glVertexAttrib. The old vertex, normal, color, ... arrays are the same as attribute arrays, vertex == #0 and so on.
I always think about VAO as an array of data buffers used by OpenGL. Using modern OpenGL you will create a VAO and Vertex Buffer Objects.
//vaoB is a buffer
glGenVertexArrays(1, vaoB); //creates one VAO
glBindVertexArray(vao.get(0));
glGenBuffers(vbo.length, vbo, 0); //vbo is a buffer
glBindVertexArray(vao.get(1));
glGenBuffers(vbo1.length, vbo1, 0); //vbo1 is a buffer
glBindVertexArray(vao.get(2));
glGenBuffers(vbo2.length, vbo2, 0); //vbo2 is a buffer
The next step is to bind data to a buffer:
glBindBuffer(GL_ARRAY_BUFFER, vbo[0]);
glBufferData(GL_ARRAY_BUFFER,vertBuf.limit()*4, vertBuf, GL_STATIC_DRAW); //vertf buf is a floatbuffer of vertices
At this point OpenGL Sees:
Now we can use glVertexAttribPointer to tell OpenGL what the data in the buffer represents:
glBindBuffer(GL_ARRAY_BUFFER, 0); //bind VBO at 0
glVertexAttribPointer(0, 3, GL_FLOAT, false, 0, 0); //each vertex has 3 components of size GL_FLOAT with 0 stride (space) between them and the first component starts at 0 (start of data)
OpenGL now has the data in the buffer and knows how the data is organized into vertices. The same process can be applied to texture coordinates etc but for texture coordinates there would be two values.
glBindBuffer(GL_ARRAY_BUFFER, vbo[1]);
glBufferData(GL_ARRAY_BUFFER,coordBuf.limit()*4, coordBuf, GL_STATIC_DRAW);
glVertexAttribPointer(0, 2, GL_FLOAT, false, 0, 0);
Next you can bind texture and draw arrays, you will want to create a Vert and Frag shader, compile and attach it to a program (not included here).
glActiveTexture(textureID); //bind our texture
glBindTexture(GL_TEXTURE_2D, textureID);
glDrawArrays(GL_TRIANGLES,0,6); //in this case 6 indices are used for two triangles forming a square
Vertex Array Objects are like macros in word processing programs and the like. A good description is found here.
Macros just remember the actions you did, such as activate this attribute, bind that buffer, etc. When you call glBindVertexArray( yourVAOId ), it simply replays those attribute pointer bindings and buffer bindings.
So your next call to draw uses whatever was bound by the VAO.
VAO's don't store vertex data. No. The vertex data is stored in a vertex buffer or in an array of client memory.
VAO is an object that represents the vertex fetch stage of the OpenGL pipeline and is used to supply input to the vertex shader.
You can create vertex array object like this
GLuint vao;
glCreateVertexArrays(1, &vao);
glBindVertexArray(vao);
First let' do a simple example. Consider such an input parameter in a shader code
layout (location = 0) in vec4 offset; // input vertex attribute
To fill in this attribute we can use
glVertexAttrib4fv(0, attrib); // updates the value of input attribute 0
Although the vertex array object stores these static attribute values for
you, it can do a lot more.
After creating vertex array object we can start filling in its state. We will ask OpenGL to fill it automatically using the data stored in a buffer object that we supply. Each vertex attribute gets to fetch data from a buffer bound to one of several vertex buffer bindings. For this end we use glVertexArrayAttribBinding(GLuint vao, GLuint attribindex, GLuint bindingindex). Also we use the glVertexArrayVertexBuffer() function to bind a buffer to one of the vertex buffer bindings. We use the glVertexArrayAttribFormat() function to describe the layout and format of the data, and finally we enable automatic filling of the attribute by calling glEnableVertexAttribArray().
When a vertex attribute is enabled, OpenGL will feed data to the vertex shader based on the format and location information you’ve provided with
glVertexArrayVertexBuffer() and glVertexArrayAttribFormat(). When
the attribute is disabled, the vertex shader will be provided with the static information you provide with a call to glVertexAttrib*().
// First, bind a vertex buffer to the VAO
glVertexArrayVertexBuffer(vao, 0, buffer, 0, sizeof(vmath::vec4));
// Now, describe the data to OpenGL, tell it where it is, and turn on automatic
// vertex fetching for the specified attribute
glVertexArrayAttribFormat(vao, 0, 4, GL_FLOAT, GL_FALSE, 0);
glEnableVertexArrayAttrib(vao, 0);
And code in a shader
layout (location = 0) in vec4 position;
After all you need to call to glDeleteVertexArrays(1, &vao).
You can read OpenGL SuperBible to understand it better.
I was trying to understand this as well and now that I think I do, it would be prudent to post a code example aimed at
people less familiar with OpenGL architecture, as I found the previous examples not very illuminating and most tutorials
just tell you to copy paste the code without explaining it.
(This is in C++ but the code can be easily translated to C)
In this example, we'll be rendering a rectangle, which has 4 vertices. Each vertex has a position (vec3, xyz), texture coordinate (vec2, uv) and color attribute (vec4, rgba).
I think it's cleanest to separate each attribute into their own array:
float positions[] = {
+0.5, +0.5, 0,
+0.5, -0.5, 0,
-0.5, -0.5, 0,
-0.5, +0.5, 0
};
float colors[] = {
1, 1, 1, 1,
1, 1, 1, 1,
1, 1, 1, 1,
1, 1, 1, 1
};
float tex_coords[] = {
0, 0,
0, 1,
1, 1,
1, 0
};
Our vertex array object will describe the four vertices with these properties.
First, we need to create the vertex array:
GLuint vertex_array;
glGenVertexArrays(1, &vertex_array);
Each vertex array has a number of buffers, these can be thought of as properties of the array. Each vertex array has an
arbitrary number of "slots" for the buffers. Along with which buffer is in which slot, it saves the CPU-side pointer to
the data for the buffer, and the CPU-side datas format. We need to make OpenGL aware of both which slot to use, where the
data is, and how it is formatted.
The buffers slots are indexed, so the first buffer is index 0, the second is 1, etc.
These locations correspond to the layout defined in the vertex shader:
// vertex shader
std::string _noop_vertex_shader_source = R"(
#version 420
layout (location = 0) in vec3 _position_3d; // slot 0: xyz
layout (location = 1) in vec4 _color_rgba; // slot 1: rgba
layout (location = 2) in vec2 _tex_coord; // slot 2: uv
out vec2 _vertex_tex_coord;
out vec4 _vertex_color_rgba;
void main()
{
gl_Position = vec4(_position_3d.xy, 1, 1); // forward position to fragment shader
_vertex_color_rgba = _color_rgba; // forward color to fragment shader
_vertex_tex_coord = _tex_coord; // forward tex coord to fragment shader
}
)";
We see that the position property is at location 0, the color property at 1 and the tex coords at 2. We'll store these
for clarity:
// property locations from our shader
const auto vertex_pos_location = 0;
const auto vertex_color_location = 1;
const auto vertex_tex_coord_location = 2;
We now need to tell OpenGL the information about each buffer outlined above:
// bind the array, this makes OpenGL aware that we are modifying it with future calls
glBindVertexArray(vertex_array);
// create the position buffer
glGenBuffers(1, &position_buffer);
// bind the buffer so opengl knows we are currently operating on it
glBindBuffer(GL_ARRAY_BUFFER, position_buffer);
// tell opengl where the data pointer is
glBufferData(GL_ARRAY_BUFFER, sizeof(positions), positions, GL_STATIC_DRAW);
// tell opengl how the data is formatted
glVertexAttribPointer(vertex_pos_location, 3, GL_FLOAT, GL_FALSE, 0, (void*) 0);
// tell opengl that this slot should be used
glEnableVertexAttribArray(vertex_pos_location);
Here, we generate a buffer that will hold our position data. For glVertexAttribPointer, we choose the
correct location, 3 elements (as the positions are xyz coordinates), and no offset or stride.
Because we have a separate array for all our properties, we can leave both as 0.
Similar to the position, we generate and fill the buffers for the color and tex coord property:
// color
glGenBuffers(1, &color_buffer); // generate
glBindBuffer(GL_ARRAY_BUFFER, color_buffer); // bind
glBufferData(GL_ARRAY_BUFFER, sizeof(colors), colors, GL_STATIC_DRAW); // set pointer
glVertexAttribPointer(vertex_color_location, 4, GL_FLOAT, GL_FALSE, 0, (void*) 0); // set data format
glEnableVertexAttribArray(vertex_color_location); // enable slot
// tex coords
glGenBuffers(1, &tex_coord_buffer);
glBindBuffer(GL_ARRAY_BUFFER, tex_coord_buffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(tex_coords), tex_coords, GL_STATIC_DRAW);
glVertexAttribPointer(vertex_tex_coord_location, 2, GL_FLOAT, GL_FALSE, 0, (void*) 0);
glEnableVertexAttribArray(vertex_tex_coord_location);
Where we chose 4 elements for the colors because they are in RGBA format and 2 for the tex coords for obvious reasons.
The last thing we need to render a vertex array is an element buffer. These can be thought of as a list of
indices that define which order the vertices will be rendered in. For us, we want to render the
rectangle as two tris in a triangle fan, so we choose the following element buffer:
// vertex order
static uint32_t indices[] = {
0, 1, 2, 1, 2, 3
};
glGenBuffers(1, &element_buffer); // generate
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, element_buffer); // bind
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW) // set pointer
We do not need to enable the element buffers slot, it is separate from the vertex array. We don't have to specify the format of the elements buffer here, that will be done during glDrawElements in the render step.
So why all this? All these functions tell OpenGL where to look for the data for the vertices. Specifying the pointers to
the correct buffer data and their layout, if we now bind the vertex array during a render step:
glUseProgram(shader.get_program_id()); // shader program with our vertex shader
glBindVertexArray(vertex_array);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, element_buffer);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
Where 6 is the number of elements in the element buffer.
This is all that's needed to correctly update the in values in the vertex shader. OpenGL will move the data from
our CPU-side positions, colors and tex_coords into the correct locations 0, 1 and 2 of the vertex shader respectively.
We don't need to bind anything else, the vertex array remembers what we gave it and does it for us, which is why it's convenient and should be preferred in modern OpenGL.
In summary:
Each vertex array has n buffers for arbitrary properties and 1 element buffer. For each property / buffer, we need to
a) generate it (glGenBuffers)
b) bind it (glBindBuffer(GL_ARRAY_BUFFER)
c) tell OpenGL where the data is located in RAM (glBufferData)
d) tell OpenGL how the data is formatted (glVertexAttribPointer)
e) tell OpenGL to use that slot (glEnableVertexAttribArray)
for the element buffer, we only need to generate it, bind it to GL_ELEMENT_ARRAY_BUFFER, then tell opengl
where the data is.
Hopefully that helped shed some light on things. I'm almost positive there will be factual errors in this post as
I'm also mostly new to OpenGL but this was the way I conceptualized it to get my code working.