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Partially Resident Textures: Not Your Father’s Megatexture

John Carmack’s id Software may not be the engine licensing powerhouse it was back in the Quake 3 days, but that hasn’t changed the revolutionary nature of his engine designs. The reason we bring this up is because there’s a great deal of GPU technology that can be directly mapped to concepts Carmack first implemented. For id Tech 4 Carmack implemented shadow volume technology, which was then first implemented in hardware by NVIDIA as their UltraShadow technology, and has since then been implemented in a number of GPUs. For id Tech 5 the trend has continued, now with AMD doing a hardware implementation of a Carmack inspired technology.

Among the features added to Graphics Core Next that were explicitly for gaming, the final feature was Partially Resident Textures, which many of you are probably more familiar with in concept as Carmack’s MegaTexture technology. The concept behind PRT/Megatexture is that rather than being treated as singular entities, due to their size textures should be broken down into smaller tiles, and then the tiles can be used as necessary. If a complete texture isn’t needed, then rather than loading the entire texture only the relevant tiles can be loaded while the irrelevant tiles can be skipped or loaded at a low quality. Ultimately this technology is designed to improve texture streaming by streaming tiles instead of whole textures, reducing the amount of unnecessary texture data that is streamed.

Currently MegaTexture does this entirely in software using existing OpenGL 3.2 APIs, but AMD believes that more next-generation game engines will use this type of texturing technology. Which makes it something worth targeting, as if they can implement it faster in hardware and get developers to use it, then it will improve game performance on their cards. Again this is similar to volume shadows, where hardware implementations sped up the process.

In order to implement this in hardware AMD has to handle two things: texture conversion, and cache management. With texture conversion, textures need to be read and broken up into tiles; AMD is going with a texture format agnostic method here that can simply chunk textures as they stand, keeping the resulting tiles in the same format. For AMD’s technology each tile will be 64KB, which for an uncompressed 32bit texture would be enough room for a 128 x 128 chunk.

The second aspect of PRT is managing the tiles. In essence PRT reduces local video memory to a very large cache, where tiles are mapped/pinned as necessary and then evicted as per the cache rules, and elsewhere the hardware handles page/tile translation should a tile not already be in the cache. Large tomes have been written on caching methods, and this aspect is of particular interest to AMD because what they learn about caching here they can apply to graphical workloads (i.e. professional) and not just gaming.

To that end AMD put together a technology demo for PRT based on Per-Face Texture Mapping (PTEX), a Disney-developed texture mapping technique that maps textures to polygons in a 1:1 ratio. Disney uses this technique for production rendering, as by constraining textures to a single polygon they don’t have to deal with any complexities that arise as a result of mapping a texture over multiple polygons. In the case of AMD’s demo it not only benefits for the reasons that Disney uses it, but also because when combined with tessellation it trivializes vector displacement, making art generation for tessellated games much easier to create. Finally, PRT fits into all of this by improving the efficiency of accessing and storing the Ptex texture chunks.

Wrapping things up, for the time being while Southern Islands will bring hardware support for PRT software support will remain limited. As D3D is not normally extensible it’s really only possible to easily access the feature from other APIs (e.g. OpenGL), which when it comes to games is going to greatly limit the adoption of the technology. AMD of course is working on the issue, but there are few ways around D3D’s tight restrictions on non-standard features.

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  • CeriseCogburn - Sunday, March 11, 2012 - link

    We'll have to see if amd "magically changes that number and informs Anand it was wrong like they did concerning their failed recent cpu.... LOL
    That's a whole YEAR of lying to everyone trying to make their cpu look better than it's actual fail, and Anand shamefully chose to announce the number change "with no explanation given by amd"... -
    That's why you should be cautious - we might find out the transistor count is really 33% different a year from now.
    Reply
  • piroroadkill - Thursday, December 22, 2011 - link

    Only disappointing if you:

    a) ignored the entire review
    b) looked at only the chart for noise
    c) have brain damage
    Reply
  • Finally - Thursday, December 22, 2011 - link

    In Eyefinity setups the new generation shines: http://tinyurl.com/bu3wb5c Reply
  • wicko - Thursday, December 22, 2011 - link

    I think the price is disappointing. Everything else is nice though. Reply
  • CeriseCogburn - Sunday, March 11, 2012 - link

    The drivers suck Reply
  • RussianSensation - Thursday, December 22, 2011 - link

    Not necessarily. The other possibility is that being 37% better on average at 1080P (from this Review) over HD6970 for $320 more than an HD6950 2GB that can unlock into a 6970 just isn't impressive enough. That should be d). Reply
  • piroroadkill - Friday, December 23, 2011 - link

    Well, I of course have a 6950 2GB that unlocked, so as far as I'm concerned, that has been THE choice since the launch of the 6950, and still is today.

    But you have to ignore cost at launch, it's always high.
    Reply
  • CeriseCogburn - Thursday, March 08, 2012 - link

    I agree RS, as these amd people are constantly screaming price percentage increase vs performance increase... yet suddenly applying the exact combo they use as a weapon against Nvidia to themselves is forbidden, frowned upon, discounted, and called unfair....
    Worse yet, according to the same its' all Nvidia's fault now - that amd is overpriced through the roof...LOL - I have to laugh.
    Also, the image quality page in the review was so biased toward amd that I thought I was going to puke.
    Amd is geven credit for a "perfect algorythm" that this very website has often and for quite some time declared makes absolutely no real world difference in games - and in fact, this very reviewer admitted the 1+ year long amd failure in this area as soon as they released "the fix" - yet argued everyone else was wrong for the prior year.
    The same thing appears here.
    Today we find out the GTX580 nvidia card has much superior anti-shimmering than all prior amd cards, and that finally, the 7000 high end driver has addressed the terrible amd shimmering....
    Worse yet, the decrepit amd low quality impaired screens are allowed in every bench, with the 10% amd performance cheat this very site outlined them merely stated we hope Nvidia doesn't so this too - then allowed it, since that year plus ago...
    In the case of all the above, I certainly hope the high end 797x cards aren't CHEATING LIKE HECK still.
    For cripe sakes, get the AA stuff going, stop the 10% IQ cheating, and get our bullet physics or pay for PhysX, and stabilize the drivers .... I am sick of seeing praise for cheating and failures - if they are (amd) so great let's GET IT UP TO EQUIVALENCY !
    Wow I'm so mad I don't have a 7970 as supply is short and I want to believe in amd for once... FOR THE LOVE OF GOD DID THEY GET IT RIGHT THIS TIME ?!!?
    Reply
  • slayernine - Thursday, December 22, 2011 - link

    Holy fan boys batman!

    This comment thread reeks of nvidia fans green with jealousy
    Reply
  • Hauk - Thursday, December 22, 2011 - link

    LOL, Wreckage first!

    Love him or hate him, he's got style..
    Reply

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