I kinda sorta get a grip what texture arrays are about, but nowhere on the internet I can find info about what they actually are. Are they like one big texture atlas (singular continuous block of memory divided in parts of equal dimensions), or are they like texture units (completely separate textures that you can bind all at once)? The consequence of this question is, what are the size limits of a single texture array, and how to use them efficiently? If, for example, my GPU handles 4096x4096 textures and 64 units, can I create a texture array of 64 4096x4096 textures? Or is the actual limit 4096*4096 pixels per array? Or maybe something else? How to query the limit?
They are arrays of images (the concept is generally referred to as layered images, because there are multiple types of textures that actually store multiple images). An array texture is special in that it has equal dimensions for each of its layers. Additionally, though the memory is allocated the same way as a 3D texture, filtering across image layers is impossible for array textures and the mipmap chain for a 2D array texture remains 2D.
The very first form of array texture ever introduced was the cube map, it had 6 2D layers... but it also had a special set of lookup functions, so it was far from a generic 2D array texture.
Nevertheless, the limit you are discussing is per-image dimension in your array texture. That is, if your maximum texture size is 4096, that means a 2D texture is limited to 4096x4096 per-image for the highest detail mipmap level.
Layers in your array texture are not like Texture Image Units at all. They are separate images, but they all belong to a single texture object, which can be bound to a single Texture Image Unit.
The maximum number of layers in your array texture can be queried this way:
GLint max_layers;
glGetIntegerv (GL_MAX_ARRAY_TEXTURE_LAYERS, &max_layers);
On the topic of using array textures efficiently, you may consider a new feature called Sparse Array Textures (via GL_ARB_sparse_texture).
Say you wanted to reserve enough storage for a 4096 x 4096 x 64 array texture but only actually used 4 or 5 of those layers initially. Ordinarily that would be horribly wasteful, but with sparse textures, only the 4 or 5 layers in-use have to be resident (committed). The rest of the layers are handled like demand-paged virtual memory in operating systems; no backing storage is actually allocated until first use.
Related
For texture sizes greater than GL_MAX_ARRAY_TEXTURE_LAYERS, I have to use an array of single textures instead of one texture array. No problem for me.
I'm just wondering whats the reason behind making GL_MAX_ARRAY_TEXTURE_LAYERS so much smaller than GL_MAX_TEXTURE_SIZE?
edit: this can be deleted
For texture sizes greater than GL_MAX_ARRAY_TEXTURE_LAYERS
I believe you have mis-understood what this limitation means. This is not like GL_MAX_3D_TEXTURE_SIZE, which is a limitation on all axes of a 3D texture. It is a limitation on the number of array elements in a 2D array texture. When you call glTexImage3D/TexStorage3D, the limitation applies to the depth parameter only, not the "size".
The width/height limit is still governed by GL_MAX_TEXTURE_SIZE.
I recently re-wrote some code to use Shader Storage Buffer Object in OpenGL, to send in dynamic sized arrays into GLSL, vastly increasing performance when drawing many procedurally generated objects.
What I have is a couple of thousand points, and for each point I render a procedurally generated circular billboard. Each one can in turn have different colors and radius, as well as a few other characteristics (represented as bools or enums)
I fill a vector with these positions, packed together with the radius and color. Then I upload it as a Shader Storage Buffer Object with dynamic size. I create a dummy VAO, containing 0 vbos, but call the draw command with the same amount of points that I have.
Inside the shader, I then iterate through this array, using the gl_VertexID, and generate a quad (two triangles) with texture coordinates, for each point.
I'm looking for a way of doing the same in Vulkan. Is there some way in Vulkan, to pass a dynamic sized array into a shader? Reading about Shader Storage Buffer objects in Graham Seller's Vulkan book, it only mentions them being read-write, but not capable of dynamically sized arrays.
Edit: It seems that storage buffers are in fact capable of dynamic sized arrays, based on Sasha Willems particle example. Is there a way of doing the same thing via uniforms?
I may be misunderstanding your question. SSBOs have identical behavior and functionality between the two APIs. That's why they're named the same. An unbounded array at the end of a storage block will have its length defined at runtime, based on what data you provide.
The size of a buffer descriptor is not hard coded into the descriptor set layout; it's something you set with VkWriteDescriptorSet. Now unlike the offset, you cannot change a descriptor set's size without changing the descriptor itself. That is, you don't have an equivalent to vkCmdBindDescriptorSets's pDynamicOffsets field. So you have to actually update the descriptor in-situ to change the length.
But that just requires double-buffering your descriptor set; it shouldn't be a problem.
Is there a way of doing the same thing via uniforms?
Again, the answer is the same for Vulkan as for OpenGL: no.
Currently, in my rendering engine, I have a VBO for each mesh data (1 VBO for vertices, 1 VBO for normals, 1 VBO for texture coordinates, 1 VBO for tangents and 1 VBO for bitangents) and all of them are bound together with a VAO.
I'm now thinking about changing the system to hold a single VBO containing all the mesh data (vertices, normals, etc.) but how mush will I gain from this? Speaking about speed and utility (because I may not have all the data and provide only vertices and normals if my mesh isn't textured).
You'll be seeking to reduce overall memory bandwidth. If your buffer object contains all of your attributes interleaved together then that means that your entire array object references only one single contiguous section of memory. Which is much easier for a memory subsystem to cache. It's exactly the same principle as for CPUs — the more local consecutive memory accesses are, the faster they're likely to be.
There is also a potential detriment: the general rule is that you should align your elements to whichever is the greater of the size of the element and four bytes. Which leads to some wasted space. But the benefit almost always outweighs the detriment.
Obviously the only thing that will be affected is the time the GPU takes to fetch the vertices. If you're tessellation or fill bound you won't immediately see any improvement.
We use buffer objects for reducing copy operations from CPU-GPU and for texture buffer objects we can change target from vertex to texture in buffer objects. Is there any other advantage here of texture buffer objects? Also, it does not allow filtering, is there any disadvantage of this?
A buffer texture is similar to a 1D-texture but has a backing buffer store that's not part of the texture object (in contrast to any other texture object) but realized with an actual buffer object bound to TEXTURE_BUFFER. Using a buffer texture has several implications and, AFAIK, one use-case that can't be mapped to any other type of texture.
Note that a buffer texture is not a buffer object - a buffer texture is merely associated with a buffer object using glTexBuffer.
By comparison, buffer textures can be huge. Table 23.53 and following of the core OpenGL 4.4 spec defines a minimum maximum (i.e. the minimal value that implementations must provide) number of texels MAX_TEXTURE_BUFFER_SIZE. The potential number of texels being stored in your buffer object is computed as follows (as found in GL_ARB_texture_buffer_object):
floor(<buffer_size> / (<components> * sizeof(<base_type>))
The resulting value clamped to MAX_TEXTURE_BUFFER_SIZE is the number of addressable texels.
Example:
You have a buffer object storing 4MiB of data. What you want is a buffer texture for addressing RGBA texels, so you choose an internal format RGBA8. The addressable number of texels is then
floor(4MiB / (4 * sizeof(UNSIGNED_BYTE)) == 1024^2 texels == 2^20 texels
If your implementation supports this number, you can address the full range of values in your buffer object. The above isn't too impressive and can simply be achieved with any other texture on current implementations. However, the machine on which I'm writing this answer supports 2^28 == 268435456 texels.
With OpenGL 4.4 (and 4.3 and possibly with earlier 4.x versions), the MAX_TEXTURE_SIZE is 2 ^ 16 texels per 1D-texture, so a buffer texture can still be 4 times as large. On my local machine I can allocate a 2GiB buffer texture (even larger actually), but only a 1GiB 1D-texture when using RGBAF32 texels.
A use-case for buffer textures is random (and atomic, if desired) read-/write-access (the latter via image load/store) to a large data store inside a shader. Yes, you can do random read-access on arrays of uniforms inside one or multiple blocks but it get's very tedious if you have to process a lot of data and have to work with multiple blocks and even then, looking at the maximum combined size of all uniform components (where a single float component has a size of 4 bytes) in all uniform blocks for a single stage,
MAX_(stage)_UNIFORM_BLOCKS *
MAX_UNIFORM_BLOCK_SIZE +
MAX_(stage)_UNIFORM_COMPONENTS * 4
isn't really a lot of space to work with in a shader stage (depending on how large your implementation allows the above number to be).
An important difference between textures and buffer textures is that the data store, as a regular buffer object, can be used in operations where a texture simply does not work. The extension mentions:
The use of a buffer object to provide storage allows the texture data to
be specified in a number of different ways: via buffer object loads
(BufferData), direct CPU writes (MapBuffer), framebuffer readbacks
(EXT_pixel_buffer_object extension). A buffer object can also be loaded
by transform feedback (NV_transform_feedback extension), which captures
selected transformed attributes of vertices processed by the GL. Several
of these mechanisms do not require an extra data copy, which would be
required when using conventional TexImage-like entry points.
An implication of using buffer textures is that look-ups inside a shader can only be done via texelFetch. Buffer textures also aren't mip-mapped and, as you already mentioned, during fetches there is no filtering.
Addendum:
Since OpenGL 4.3, we have what is called a
Shader Storage Buffer. These too provide random (atomic) read-/write-access to a large data store but don't need to be accessed with texelFetch() or image load/store functions as is the case for buffer textures. Using buffer textures also implies having to deal with gvec4 return values, both with texelFetch() and imageLoad() / imageStore(). This becomes very tedious as soon as you want to work with structures (or arrays thereof) and you don't want to think of some stupid packing scheme using multiple instances of vec4 or using multiple buffer textures to achieve something similar. With a buffer accessed as shader storage, you can simple index into the data store and pull one or more instances of some struct {} directly from the buffer.
Also, since they are very similar to uniform blocks, using them should be fairly straight forward - if you know how to use uniform buffers, you don't have a long way to go learn how to use shader storage buffers.
It's also absolutely worth browsing the Issues section of the corresponding ARB extension.
Performance Implications
Daniel Rakos did some performance analysis years ago, both as a comparison of uniform buffers and buffer textures, and also on a little more general note based on information from AMD's OpenCL programming guide. There is now a very recent version, specifically targeting OpenCL optimization an AMD platforms.
There are many factors influencing performance:
access patterns and resulting caching behavior
cache line sizes and memory layou
what kind of memory is accessed (registers, local, global, L1/L2 etc.) and its respective memory bandwidth
how well memory fetching latency is hidden by doing something else in the meantime
what kind of hardware you're on, i.e. a dedicated graphics card with dedicated memory or some unified memory architecture
etc., etc.
As always when worrying about performance: implement something that works and see if that solutions is fast enough for your needs. Otherwise, implement two or more approaches to solving the problem, profile them and compare.
Also, vendor specific guides can offer a great deal of insight. The above mentioned OpenCL user and optimization guides provide a high-level architectural perspective and specific hints on how to optimize your CL kernels - stuff that's also relevant when developing shaders.
A one use case I have found was to store per primitive attributes (accessed in the fragment shader with help of gl_PrimitiveID) while still maintaining unique vertices in the indexed mesh.
I am implementing a voxel raycaster in OpenGL 4.3.0. I have got a basic version going where I store a 256x256x256 voxel data set of float values in a 3D texture of the same dimensions.
However, I want to make a LOD scheme using an octree. I have the data stored in a 1D array on the host side. The root has index 0, the root's children have indices 1-8, the next level have indices 9-72 and so on. The octree has 9 levels in total (where the last level has the full 256x256x256 resolution). Since the octree will always be full the structure is implicit and there is no need to store pointers, just the one float value per voxel. I have the 1D indexing and traversal algorithms all set.
My problem is that I don't know how to store this in a texture. GL_TEXTURE_MAX_SIZE is way too small (16384) for using the 1D array approach for which I have figured out the indexing. I need to store this in a 3D texture, and I don't know what will happen when I try to cram my 1D array in there, nor do I know how to choose a size and a 1D->3D conversion scheme to not waste space or time.
My question is if someone has a good strategy for storing this whole octree structure in one 3D texture, and in that case how to choose dimensions and indexing for it.
First some words on porting your 1D-array solution directly:
First of all, as Mortennobel says in his comment, the max texture size is very likely not 3397, that's just the enum value of GL_MAX_TEXTURE_SIZE (how should the opengl.h Header, that defines this value, know your hardware and driver limits?). To get the actual value from your implementation use int size; glGetIntegerv(GL_MAX_TEXTURE_SIZE, &size);. But even then this might be too small for you (maybe 8192 or something similar).
But to get much larger 1D arrays into your shaders, you can use buffer textures (which are core since OpenGL 3, and therefore present on DX10 class hardware). Those are textures sourcing their data from standard OpenGL buffer objects. But those textures are always 1D, accessed by integer texCoords (array indices, so to say) and not filtered. So they are effectively not really textures, but a way to access a buffer object as a linear 1D array inside a shader, which is a perfect fit for your needs (and in fact a much better fit than a normal filtered and normalized 1D texture).
EDIT: You might also think about using a straight-forward 3D texture like you did before, but with homemade mipmap levels (yes, a 3D texture can have mipmaps, too) for the higher parts of the hierarchy. So mipmap level 0 is the fine 256 grid, level 1 contains the coarser 128 grid, ... But to work with this data structure effectively, you will probably need explicit LOD texture access in the shader (using textureLod or, even better without filtering, texelFetch), which requires OpenGL 3, too.
EDIT: If you don't have support for OpenGL 3, I would still not suggest to use 3D textures to put your 1D array into, but rather 2D textures, like Rahul suggests in his answer (the 1D-2D index magic isn't really that hard). But if you have OpenGL 3, then I would either use buffer textures for using your linear 1D array layout directly, or a 3D texture with mipmaps for a straight-forward octree mapping (or maybe come up with a completely different and more sophisticated data structure for the voxel grid in the first place).
EDIT: Of course a fully subdivided octree is not really using the memory saving features of octrees to its advantage. For a more dynamic and memory efficient method of packing octrees into 3D textures, you might also take some inspiration from this classic GPU Gems article on octree textures. They basically store all octree cells as 2x2x2 grids arbitrarily into a 3D texture using the interal nodes' values as pointers to the children in this texture. Of course nowadays you can employ all sorts of refinements on this (since it seems you want the internal nodes to store data, too), like storing integers alongside floats and using nice bit encodings and the like, but the basic idea is pretty simple.
Here's a solution sketch/outline:
Use a 2D texture to store your 256x256x256 (it'll be 4096x4096 -- I hope you're using an OpenGL platform that supports 4k x 4k textures).
Now store your 1D data in row-major order. Inside your raycaster, simply do a row/col conversion (from 1D address to 4k x 4k) and look up the value you need.
I trust that you will figure out the rest yourself :)