I am trying to understand the theory behind OpenGL and I'm studying VBOs at the moment.
This is what I understand so far: when we declare a series of vertices, let's say 3 vertices that form a triangle primitive, we basically store those nowhere, they're simply declared in code.
But, if we want to store them somewhere we can use a VBO that stores the definition of those vertices. And, through the same VBO we send all that vertex info to the Vertex Shader (which is a bunch of code). Now, the VBO is located in the GPU, so we are basically storing all that info on the GPU's memory when we call the VBO. Then the Vertex Shader, which is part of the Pipeline Rendering process, "comes" to the GPU's memory, "looks" into the VBO and retrieves all that info. In other words, the VBO stores the vertex data (triangle vertices) and sends it to the Vertex Shader.
So, VBO -> send info to -> Vertex Shader.
Is this correct? I'm asking to make sure if this is the correct interpretation, as I find myself drawing triangles on screen and sometimes letters made up of many triangles with a bunch of code and functions that I basically learned by memory but don't really understand what they do.
To break it down:
// here I declare the VBO
unsigned int VBO;
// we have 1 VBO, so we generate an ID for it, and that ID is: GL_ARRAY_BUFFER
glGenBuffers(1, &VBO)
// GL_ARRAY_BUFFER is the VBO's ID, which we are going to bind to the VBO itself
glBindBuffer(GL_ARRAY_BUFFER, VBO)
// bunch of info in here that basically says: I want to take the vertex data (the
// triangle that I declared as a float) and send it to the VBO's ID, which is
// GL_ARRAY_BUFFER, then I want to specify the size of the vertex
// data, the vertex data itself and the 'static draw' thingy
glBufferData(...).
After doing all that, the VBO now contains all the vertex data within. So we tell the VBO, ok now send it to the Vertex Shader.
And that's the start of the Pipeline, jsut the beginning.
Is this correct? (I haven't read what VAOs do yet, before I get to that I'd like to know if the way I deconstruct VBOs in my mind is the right way, or else I'm confused)
I think you are mixing up lots of different things and have several confusions, so I'm try to work through most of them in the order you brought them up:
when we declare a series of vertices, let's say 3 vertices that form a triangle primitive, we basically store those nowhere, they're simply declared in code.
No. If you store data "nowhere", then you don't have it. Also you are mixing up declaration, definiton and initialization of variables here. For vertex data (like all other forms of data), there are two basic strategies:
You store the data somewhere, typically in a file. Specifying it directly in source code just means that it is stored in some binary file, potentially the executable itself (or some shared library used by it)
You procedurally generate the data through some mathematical formula or more general by some algortihm
Methods 1. and 2 can of course be mixed, and usually, method 2 will need some parameters (which itself need to be stored somewhere, so the parameters are just case 1 again).
And, through the same VBO we send all that vertex info to the Vertex Shader (which is a bunch of code). Now, the VBO is located in the GPU, so we are basically storing all that info on the GPU's memory when we call the VBO.
OpenGL is actually just a specification which is completely agnostic about the existence of a GPU and the existence of VRAM. And as such, OpenGL uses the concept of buffer objects (BOs) as some continuous block of memory of a certain size which is completely managed by the GL implementation. You as the user can ask the GL to create or destroy such BOs, specify their size, and have complete control of the contents - you can put an MP3 file into a BO if you like (not that there would be a good use case for this).
The GL implementation on the other hand controls where this memory is actually allocated, and GL implementations for GPUs
which actually have dedicated video memory have the option to store a BO directly in VRAM. The hints like GL_STATIC_DRAW are there to help the GL implementation decide where to best put such a buffer (but that hint system is somewhat flawed, and better alternatives exist in modern GL, but I'm not going into that here). GL_STATIC_DRAW means you intent to specify the contents once and use the may times as the source of a drawing option - so the data won't change often (and certainly not on a per-frame basis or even more often), and it might be a very good idea to store it in VRAM if such a thing exists.
Then the Vertex Shader, which is part of the Pipeline Rendering process, "comes" to the GPU's memory, "looks" into the VBO and retrieves all that info.
I think one could put it that way, although some GPUs have a dedicated "vertex fetch" hardware stage which actually reads the vertex data which is then fed to the vertex shaders. But that's not a really important point - the vertex shader needs to access each vertex' data, and that means the GPU will read that memory (VRAM or system memory or whatever) at some point before or during the execution of a vertex shader.
In other words, the VBO stores the vertex data (triangle vertices)
Yes. A buffer object which is used as source for the vertex shader's per-vertex inputs ("vertex attributes") is called a vertex buffer object ("VBO"), so that just follows directly from the definition of the term.
and sends it to the Vertex Shader.
I wouldn't put it that way. A BO is just a block of memory, it doesn't actively do anything. It is just a passive element: it is being written to or being read from. That's all.
// here I declare the VBO
unsigned int VBO;
No, you are declaring (and defining) a variable in the context of your programming language, and this variable is later used to hold the name of a buffer object. And in the GL, object names are just positive integers (so 0 is reserved for the GL as "no such object" or "default object", depending on the object type).
// we have 1 VBO, so we generate an ID for it, and that ID is: GL_ARRAY_BUFFER
glGenBuffers(1, &VBO)
No. glGenBuffers(n,ptr) just generates names for n new buffer objects, so it will generate n previously unused buffer names (and mark them as used) and returns them by writing them to the array pointed to byptr. So in this case, it just creates one new buffer object name and stores it in your VBO variable.
GL_ARRAY_BUFFER has nothing to do with this.
// GL_ARRAY_BUFFER is the VBO's ID, which we are going to bind to the VBO itself
glBindBuffer(GL_ARRAY_BUFFER, VBO)
No, GL_ARRAY_BUFFER is not the VBO's ID, the value of yourVBO variable is the VBO's ID (name!).
GL_ARRAY_BUFFER is the binding target. OpenGL buffer objects can be used for different purposes, and using them as the source for vertex data is just one of them, and GL_ARRAY_BUFFER refers to that use case.
Note that classic OpenGL uses the concept of binding for two purposes:
bind-to-use: Whenever you issue a GL call which depends on some GL objects, the objects you want to work with have to be currently bound to some (specific, depending on the use case) binding target (not only buffer objects, but also textures and others).
bind-to_modify: Whenever you as the user want to modify the state of some object, you have to bind it first to some binding target, and all the object state modify functions don't directly take the name of the GL object to work on as parameter, but the binding target, and will affect the object which is currently bound at that target. (Modern GL also has direct state access which allows you to modify objects without having to bind them first, but I'm also not going into details about that here).
Binding a buffer object to some of the buffer object binding targets means that you can use that object for the purpose defined by the target. But note that a buffer object doesn't change because it is bound to a target. You can bind a buffer object to different targets even at the same time. A GL buffer object doesn't have a type. Calling a buffer a "VBO" usually just means that you intent to use it as GL_ARRAY_BUFFER, but the GL doesn't care. It does care about what is buffer is bound as GL_ARRAY_BUFFER at the time of the glVertexAttribPointer() call.
// bunch of info in here that basically says: I want to take the vertex data (the
// triangle that I declared as a float) and send it to the VBO's ID, which is
// GL_ARRAY_BUFFER, then I want to specify the size of the vertex
// data, the vertex data itself and the 'static draw' thingy
glBufferData(...).
Well, glBufferData just creates the actual data storage for a GL buffer object (that is, the real memory), meaning you specify the size of the buffer (and the usage hint I mentioned earlier where you tell the GL how you intend to use the memory), and it optionally allows you to initialize the buffer by copying data from your application's memory into the buffer object. It doesn't care about the actual data, and the types you use).
Since you use GL_ARRAY_BUFFER here as the target parameter, this operation will affect the BO which is currently bound as GL_ARRAY_BUFFER.
After doing all that, the VBO now contains all the vertex data within.
Basically, yes.
So we tell the VBO, ok now send it to the Vertex Shader.
No. The GL uses Vertex Array Objects (VAOs) which store for each vertex shader input attribute where to find the data (in which buffer object, at which offset inside the buffer object) and how to interpret this data (by specifying the data types).
Later during the the draw call, the GL will fetch the data from the relevant locations within the buffer objects, as you specified it in the VAO. If this memory access is triggered by the vertex shader itself, or if there is a dedicated vertex fetch stage which reads the data before and forwards it to the vertex shader - or if there is a GPU at all - is totally implementation-specific, and none of your concern.
And that's the start of the Pipeline, just the beginning.
Well, depends on how you look at things. In a traditional rasterizer-based rendering pipline, the "vertex fetch" is more or less the first stage, and vertex buffer objects will just hold the memory where to fetch the vertex data from (and VAOs telling it which buffer objects to use, and which actual locations, and how to interpret them).
It all boils down to this: when you work in "normal" programs, all what you have is the CPU, caches, registers, main memory, etc.
However, when you work with computer graphics (and other fields), you want to use a GPU because it is faster for that particular task. The GPU is an independent computer on its own, with its own processor, pipeline and main even memory.
This means your program needs to somehow transfer all the data to the other computer and tell the other computer what to do with it. This is no easy task, so OpenGL simplifies things for you. Thus they give you an abstraction (VBO) that represents a buffer of vertices in the GPU, among many other abstractions for other tasks. Then they give you functions to create that resource (glGenBuffers), fill it with data (glBufferData), "bind it" to work with it (glBindBuffer), etc.
Remember, it is all a simplification for your benefit. In truth, the details of how everything is performed underneath is way more complex. Having abstractions like VBOs for vertices or IBOs for indexes makes it easier to work with them.
To create a uniform buffer object in OpenGL, why must I call both glBindBuffer and glBindBufferRange? According to documentation,
Think of glBindBufferRange as binding the buffer to two places: the particular index and the target. glBindBuffer only binds to the target, not the index.
So then glBindBuffer seems superfluous to me if you are already binding a buffer to that same target using glBindBufferRange.
I am reading Learning Modern 3D Graphics Programming and chapter 7 shows how to make uniform buffer objects. The code used in the book:
glGenBuffers(1, &g_GlobalMatricesUBO);
glBindBuffer(GL_UNIFORM_BUFFER, g_GlobalMatricesUBO);
glBufferData(GL_UNIFORM_BUFFER, sizeof(glm::mat4) * 2, NULL, GL_STREAM_DRAW);
glBindBuffer(GL_UNIFORM_BUFFER, 0);
glBindBufferRange(GL_UNIFORM_BUFFER, g_iGlobalMatricesBindingIndex, g_GlobalMatricesUBO, 0, sizeof(glm::mat4) * 2);
This code works until I comment out the call to glBufferData, in which case I get a black screen. This is surprising to me because afterwards we are binding g_GlobalMatricesUBO to the GL_UNIFORM_BUFFER binding target anyways. The documentation linked to earlier also says:
Do note that this does not replace standard buffer binding with glBindBuffer. It does in fact bind the buffer to the target parameter, thus unbinding whatever was bound to that target. But usually when you use glBindBufferRange, you are serious about wanting to use the buffer rather than just modify it.
Which seems to touch on what I'm confused about, but I do not understand this explanation. It says it unbinds whatever was previously bound to the target, which seems to reinforce my (wrong) idea that binding a buffer to the target beforehand is unnecessary.
glBufferData does not only initialize the buffer, it creates the buffer object's data store with the specified size. You can think about it like memory allocation in c++.
glBufferData creates a data store for the buffer object which is currently bound to the specified target. In this case it creates a data store for the named buffer object g_GlobalMatricesUBO, because it is bound to the specified target GL_UNIFORM_BUFFER.
glBindBufferRange doesn't create any data store, it use the data store which has to exist. The buffer object with the data store is specified by the 3rd parameter.
Am I allowed to bind a single opengl buffer to multiple indexed targets (of the SSBO target) simultaneously?
For instance, suppose my shader has two different uniform blocks with different binding indexes. If the information I need is located in the same buffer, am I allowed to use glBindBufferRange, and bind different ranges of the same buffer to these two binding indexes, simultaneously?
Another use case I see is, for instance, if I have a shader with two uniform blocks, again with different binding indexes, but this time, the only data member both uniform blocks have is an open array (with unspecified size). Am I allowed to use glBindBuffer to bind the same buffer to both uniform blocks, and guarantee by code, to only access the array indexes within the proper range in the buffer?
I believe it's fine to do so.
§6.1 (...) While a buffer object is bound, GL operations on the target to which it is bound
affect the bound buffer object, and queries of the target to which a buffer object is
bound return state from the bound object. Operations on the target also affect any
other bindings of that object
emphasis mine - which would directly suggest it's OK.
§6.1.1. (...) Each target represents an indexed array of buffer object binding points, as well
as a single general binding point that can be used by other buffer object manipulation
functions, such as BindBuffer or MapBuffer. Both commands bind the
buffer object named by buffer to both the general binding point, and to the binding
point in the array given by index. If the binds are successful no change is made
to the state of the bound buffer object, and any previous bindings to the general
binding point or to the binding point in the array are broken
What I'd distill from that is that it's not explicitely forbidden to bind a buffer range to multiple places, and as such, I'd assume it's allowed. It won't break the other bindings in that array, which means the previously bound ranges should stay unchanged and valid.
That being said, if the ranges overlap and you're writing to them, you might need barriers.
I am confused when I bind a vertex buffer object, whether the previous vbo is unbound or stay together with the newly bound vbo?
And what about other kind buffer objects? Where can I find some specification about this?
There are no "kinds" of Buffer Objects. A buffer object is an unformatted array of arbitrary data. Just as there are not "kinds" of void*'s, there are not "kinds" of buffer objects.
There are different uses for buffer objects, but these do not represent separate "kinds". You can use a buffer object as the destination for a pixel transfer, then read the written data as source vertex data. Then you can overwrite the buffer's data with a transform feedback output, then read the feedback data as a texture to access in a shader.
Buffer objects are very flexible.
As for the bind question, this is true of all OpenGL objects: when you bind an object to a target, anything that was previously bound to that target is unbound. So if you bind a buffer to GL_ARRAY_BUFFER, anything that was previously bound to GL_ARRAY_BUFFER is unbound.
I am struggling to understand how exactly VAO is handling buffer mapping.
What I'm doing could be described in this pseudocode:
SetUp:
BindVAO
BindArrayBuffer
glBufferData(GL_ARRAY_BUFFER, ExpectedMaxCount, NULL, GL_DYNAMIC_DRAW);//Allocate storage
glEnableVertexAttribArray
glVertexAttribPointer
BindElementBuffer
Allocate storage (no data yet)
UnbindVAO
UnbindArrayBuffer
UnbindElementBuffer
Draw:
SubArrayAndElementDataIfNeeded
BindVAO
DrawElements
Is this correct that when DrawElements is called OpenGL uses bound VAO to resolve array and element buffer bindings? After a Draw call the bound array buffer is 0, but element buffer is still the one that was used to Draw.
Is it mandatory to allocate buffer memory during VAO setup? Would VAO be invalidated if BufferData was called after setup?
I am struggling to understand how exactly VAO is handling buffer mapping.
Be very careful when using the word "mapping" around "buffers"; that has a specific meaning when dealing with buffer objects, one that you probably don't intend.
Is this correct that when DrawElements is called OpenGL uses bound VAO to resolve array and element buffer bindings? After a Draw call the bound array buffer is 0, but element buffer is still the one that was used to Draw.
One has nothing to do with the other. A Vertex Array Object, as the name implies, contains all of the state that is necessary to pull vertex data from arrays. When you bind one, all of that state comes back into the context.
The reason the "bound array buffer" is 0 after the call is because it was 0 before the call. Draw calls do not change OpenGL state.
Furthermore, you seem to have fallen for the GL_ARRAY_BUFFER trap. The GL_ARRAY_BUFFER binding only matters to three functions: glVertexAttribPointer, glVertexAttribIPointer, and glVertexAttribLPointer (see a pattern?). These are the only functions that look at that binding. What they do is take the buffer that is bound at the time these functions are called and associates that buffer with the current VAO. GL_ARRAY_BUFFER is not part of the VAO's state. A buffer object becomes associated with a VAO only when you call one of those three functions. Once you make that call, you can bind whatever you want to GL_ARRAY_BUFFER.
GL_ELEMENT_ARRAY_BUFFER is part of the VAO's state.
Is it mandatory to allocate buffer memory during VAO setup? Would VAO be invalidated if BufferData was called after setup?
Technically no, but it's good form to not use a buffer until it has storage. Especially if they're static buffers.