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Thread: Map Texture Objects

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  1. #1
    Senior Member OpenGL Pro
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    Map Texture Objects

    We've currently got the capability to map buffer objects, which enables loading of data into buffers without needing to use any intermediate system memory arrays or otherwise do a memory copy. We don't have that ability with textures - true, we can do something similar with PBOs but it involves more round-tripping.

    Being able to map a texture object has the following uses:


    • The ability to implement "texture orphaning" for dynamic textures which must be fully replaced each frame, without needing a round-trip through a PBO.
    • The ability to more directly get data into a texture which can provide advantages for both creation and updating use cases, and without needing any intermediate steps (this can facilitate any kind of texture streaming scheme).
    • The ability to read data from a texture without the driver first having to do a copy back to system memory.


    This suggestion is to overload the glMapBuffer, glMapBufferRange, glFlushMappedBufferRange and glUnmapBuffer calls to also accept a texture target as the target parameter. This target can be any that is valid for any of the glTexImage/glTexSubImage calls. Behaviour should otherwise be identical, and the texture must be unmapped before it can be used in a draw call.

    Issues I can see include:


    • Is it only allowed to map mip level 0, or - if not - how to specify mapping a mip level other than 0? Suggest to perhaps use glMapBufferRange with an appropriate offset?
    • glBindBuffer or glBindTexture?
    • What happens if glTexImage or glTexSubImage is called while a texture is mapped?

  2. #2
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    Being able to map a texture object has the following uses:
    OK, so... how do you define what you get when you map it?

    It's easy to say that you just "map a texture", but textures are opaque to allow drivers to hide their particular implementations. That's why pixel transfers are complicated, while buffer uploads are simple byte copies. It hides details like swizzling, the specific bit-pattern of formats, etc.

    So now you want to directly expose the vagaries of the range of OpenGL hardware. There are three ways to go about it:

    1. Force the driver to use a single, specific standard across vastly different hardware. Say goodbye to cross-platform portability, let alone future-proofing.

    2. Extend the query API to tell you how to interpret the data for a particular format, thus allowing different hardware to expose its particular eccentricities to you. Of course, since most existing sources of streamed data (FFMpeg buffer writing, DirectShow, etc) will export to their own format, you have to use an intermediate buffer. They must write to some memory, you convert it to the hardware's version in the mapped space. In short: no different from having those APIs write to a mapped PBO (assuming they can).

    Also, I'm just guessing, but I'm fairly sure NVIDIA's not going along with that. They're seem really protective of their IP and implementation details.

    3. Split the difference and allow the user to tell OpenGL that a particular texture will need to adhere to a particular structure. That is, it should be mappable. Of course, now you have to make glTexStorageMappable functions for 3 separate types (1D, 2D, 3D). Either that, or you're going to have to create a bunch of new image formats that force the texture to use a specific format. Or some kind of texture parameter or something.

    Even assuming that there would be a particular structure that all hardware could support.

    The ability to implement "texture orphaning" for dynamic textures which must be fully replaced each frame, without needing a round-trip through a PBO.
    Doesn't glInvalidateTexImage give us that already? You upload to it, use it, invalidate to it, and write again. It seems much easier than mapping a texture just to invalidate it.

    The ability to read data from a texture without the driver first having to do a copy back to system memory.
    I'm not sure how useful that ability is. For most GPU situations, that memory is across a (relatively) slow bus. You'd be better off issuing a DMA and then doing something else until it's over, which we effective have with PBOs.

    Also, there has never been a guarantee that mapping means you're talking to GPU memory.

    This suggestion is to overload the glMapBuffer, glMapBufferRange, glFlushMappedBufferRange and glUnmapBuffer calls to also accept a texture target as the target parameter.
    Absolutely, not!

    Ignoring the question of whether this is good and useful functionality, that is not the way to implement it. OpenGL has enough confusing and overloaded functions that do twenty different things to pointlessly add another 4 to that list.

    glMapBufferRange means glMapBufferRange. We're only recently getting to the point where we don't have to call glVertexAttribPointer with an argument that we have to pretend is a pointer, and it really fetches some of its data from somewhere else. There's no need to screw up that important progress just to avoid adding a few new API functions.

    The ARB is not running out of functions. There is not some hard limit that they're coming up on, such that OpenGL can't have more functions. There's absolutely no reason to overload a perfectly good API when you can just have glMapTexSubImage et. al.

    If this is going to happen, then it gets its own API. Don't screw up APIs that actually make sense just to shoehorn in new functionality. That road leads to stupidity like AMD_pinned_memory (good functionality, Godawful API).

  3. #3
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    Hmmm - as rants go I'd give it maybe a 2. It would have been a 6 or 7 but it blatantly contradicts other things you've ranted in favour of (or against, as appropriate) in the past.

  4. #4
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    Really? Because I don't remember the time I argued that IHVs should put their texture format on display for everyone to see. Or when I said that OpenGL's API needs to get worse and more confusing by senselessly overloading functions. Or when I said that mapping memory assured you of getting GPU access.

    But whatever it takes to avoid addressing the substance of my argument, right? Because the best way to present your case is to dismiss any inconvenient facts.

    Why do you always make things personal? I don't seek you out; I barely take note of the fact that it's mhagan presenting an idea. I'm only "rant"ing at your ideas because you post a lot of them and don't put much thought into them. This idea is nothing more than, "wouldn't it be wonderful if we could map textures?" There's no consideration of the ramification of such a decision. No explanation for how this could work cross-hardware. Even your suggestion of API shows how little actual thought you put into it. The only bit of substance to this is the basic idea: map texture memory.

    If you're going to seriously present an idea beyond the basic concept of "let us do this somehow", then put some effort into it. Show that you're better than just throwing ideas against a wall and hoping that one sticks.

  5. #5
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    Ok then.

    First of all, the ability to Map (or Lock in older version) a texture is something that has been in D3D for an eternity (in D3D11 both textures and buffers even use the very same API call). So far as the hardware vendors are concerned, this is a complete non-issue. There are no deep, dark, proprietary internal representations going on here; textures are just the same as buffers - a stream of bytes.

    Now let's get one thing real clear before continuing. This is not about adding functionality to GL that D3D also has. This is about adding functionality that may be generally useful, irrespective of whether D3D has it or not. D3D is not relevant beyond this point.

    So point 1 is this: the argument that vendors may not want to put their internal texture formats on display is bogus.

    Point 2 is this: even on hardware that may have it's own funky internal representation, the whole point of OpenGL as an abstraction layer is to abstract that away from the programmer. This is something that already happens with e.g. a glTexSubImage call. Any hypothetical Map/Unmap of a texture can go through the very same driver code paths as are used for glTexSubImage to accomplish this. So even in such a case, this amounts to use of a problem that has already been solved as an argument against.

    Point 2 also exposes way number 4 to go about it: if the internal representation is already appropriate for exposure to the programmer, then just expose it as it. Give the programmer a pointer and be done with it. This case could be satisfied e.g. where the internal representaion matches the internalFormat param used when creating the texture. If the internal representation is not appropriate, then likewise give the programmer a pointer, but add a conversion step that happens in the driver - either at Map time (for reading) or Unmap time (for writing). As I said - this is something that already happens with glGetTexImage/glTexSubImage, the driver already contains code to do it, so arguments against it won't fly.

    Now onto specifics.

    glInvalidateTexImage? No; that just accomplishes one part of the requirement, which is to orphan the texture. It does absolutely nothing about the second part, which is to avoid round-tripping through PBOs or program-allocated system memory in order to perform the update. Mapping a texture solves that; instead of the round-trip and extra memory copies you write directly (or as directly as the driver allows).

    Overloading the buffer calls. Yes, it's ugly, yes it's confusing, yes, a set of extra entry points would be better. And to head this one off at the pass - there is no need for separate entry points for 1D/2D/3D textures; follow the pattern established by glInvalidateTexSubImage instead - one entry point that works with all types.

    Portability? You're going to need to come up with some compelling reasons as to why it's a problem for portability, rather than just waving the word around. No, endianness is not one; we already use floats, unsigned ints, ints, unsigned shorts and shorts in buffer objects; this is another problem that has already been solved; endianness as an argument against is also bogus.

    Specific utility of this suggestion? I thought I'd made it clear but let's restate it again. It's explicitly not a case of "wouldn't it be great if..."; it's to avoid round-tripping through intermediate storage and/or intermediate objects when loading data into a texture. In other words, to serve the same utility as glMapBuffer/glMapBufferRange. No, PBOs don't already provide this as there is still a requirement for the driver or GPU to copy data from the PBO to the texture. No, this wouldn't invalidate the utility of PBOs as there are still cases where you may want an asynchronous transfer. The functionality suggested is to enable transfer of data in a similar manner to mapping a PBO, but without the additional intermediate step of transferrring from a PBO to the texture.

    And moving on:

    I'd dismissed the argument against as frivolous and vexatious because it read as an argument against being made purely for the purpose of arguing against rather than for constructively discussing pros and cons. No, I don't see substance in it to be addressed; much of it is bogus and can be shown to be so.

    Do I post a lot of ideas? No - there were precisely two others in this section of the forum.

  6. #6
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    As a side note, it would make texture streaming a lot less painful, a lot less. Also, for those GL implementations that have a unified memory architecture, this would be great. Various platforms have various hackage ways to "write directly to a texture" but they are icky and horribly non-portable (OMAP3/4 I am talking about you)

    My take is that at texture creation (somehow ) a texture's representation is specified so that mapping makes sense. One way for that somehow to happen is to have for each acceptable internal format value GL_FOO create the enumeration GL_FOO_MAPABLE, where MAPABLE means that when it is mapped the texture data is in a format specified in the spec (bytes/pixels, pixel format, line padding, etc). Even compressed textures would be ok. My only take where this can go icky is the GL_RGB8 is stored as GL_RGBA8 (and analogous GL_RGB8UI is stored as GL_RGBA8UI). I am not crazy about querying the GL implementation about this packing and padding since it will make the code that uses the mapable texture jazz hard to test reliably beyond "just try it on several different boxes and hope for the best".

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