Talk:Texture Cache

GIF

We can safely avoid the GIF format. It's 256 colors only. it use unisys patent LZW (the good old patent used in the good old GIF expired, but unisys repatent'd it because of some "improvments" in the algorithm.). Unisys suddenly claimed they wanted royalties to encode GIF file, and just for that we shouldn't use it. source : License Information on GIF and Other LZW-based Technologies -- kerunix Flan

The last of the patents expired in October 2006. GIF is no longer patent encumbered. But PNG is still better. Keep in mind by recompressing the image to cache it you're trading CPU usage for disk space. Compression is usually a lot slower than decompression. Just storing uncompressed might be overall faster. In the end, we need to assume nothing and benchmark, benchmark, benchmark. Seg Baphomet 22:45, 23 March 2007 (PDT)

The GIF format is not suitable. It only supports 256 colors per frame. It does not support semi-transparent pixels. PNG is a superior format. Has anyone considered the time required to re-encode images in the target format?

Databases

There appears to be support with APR for a portable interface to common databases, namely Berkely-DB and SQL. Dzonatas Sol 18:23, 23 March 2007 (PDT)

Stats

It would be useful to get stats on current and new hardware profiles from LL to get a sense of what people are using and where the focus should lie. Iron Perth 03:00, 23 March 2007 (PDT)

TGA

I can't find it in the spec, but i think this format give the warranty that images of a given size will always have the same file size. (without RLE of course). This may help for some optimization.

Additionaly, the "developper area" of the TGA file format could be used to store some usefull information.—The preceding unsigned comment was added by kerunix Flan

Converting images into the TGA format for cache storage has the advantage that the client already supports rendering TGA images. It would be one of the easiest solutions. Images with the same dimensions would all be the same but otherwise would be different in size. Strife Onizuka 02:22, 24 March 2007 (PDT)

Faster Formats

Contribution from Tofu Linden from the email discussion

RLE is cheap but also not very effective at all for many, many textures; wherever I might have used RLE in ye olden days I'd now use trusty old zlib or blingy liblzf, the latter of which is more or less on the same order of speed as RLE but a LOT more effective on data (images) with redundancy not necessarily manifesting as runs of pixels.

Iron Perth 02:30, 24 March 2007 (PDT)

And as said on the mailling, the destructive effect of JPEG2000 probably killed any chance to have a good compression rate using RLE. RLE is fast and the only compression supported by the Targavision specification. If we don't use it (probably because it's useless) then don't use compression at all. One of the advantage of using RLE like said in the specification is that we don't have to decompress anything to read the header, developper area, and footer. Kerunix Flan 08:02, 24 March 2007 (PDT)

Current Caching

Both Tofu and Steve have made the point they feel cache hit rate issues are a priority..

Contribution from Steve Linden from email discussion:

2. The way we calculate texture pixel coverage (and therefore desired discard level) is as follows:

a. When anything using a texture is rendered. set its pixel coverage to MAX(current pixel coverage, rendered pixel coverage)

b. Once a frame, reduce the current pixel coverage of all textures by 10%.

This way, everything rendered recently has an accurate pixel coverage, and everything not rendered degrades over time. gImageList.mForceResetTextureStats merely tells the code "assume that all textures are no longer visible" and resets the pixel area of all texture to 0. As soon as they are rendered their pixel coverage is updated.

3. Even if a textures pixel coverage gets set to 0, we do not immediately discard the texture data. We only discard the data if we need room for more textures (in which case we discard the textures with the lowest priority), or if no object in view is using the texture *and* 30 seconds have elapsed.

do that mean that, even if we have a lot of VRAM available, the texture will be removed from cache if we don't see the texture for 30s ? and then, moving our avatar in the sim will make the client redecoding the texture from cache to load it in VRAM again ? Why not discarding the texture only when we're out of memory ? (please delete/replace this comment by the answer) Kerunix Flan 08:18, 24 March 2007 (PDT)

4. Tateru is absolutely correct in that caching textures to disk mostly just saves on network bandwidth. Unless you have a slow network connection, it can be almost as fast to load textures across the network as from a disk, especially if that disk is not especially fast or is fragmented. Decoding the textures takes quite a bit longer. However, if you have multiple CPUs and enable Client > Rendering > Run Multiple Threads, the decoding will be done on the second CPU and go much faster.

I can't believe loading a texture from a remote cache isn't much slower than loading a texture from a local cache. And if you add the usual packet loss and high ping (for non-us user) it's *certainly* much slower. Kerunix Flan 08:18, 24 March 2007 (PDT)

He also makes some interesting comments about teleporting around and filling up the texture cache. Check out his original email here:

https://lists.secondlife.com/pipermail/sldev/2007-March/001133.html

Iron Perth 02:44, 24 March 2007 (PDT)

Home Cache

If a resident has a home, why not have a separate texture cache for it, a Home Cache. This cache would only be explicitly cleared down and would be accessed first by the client when the resident logs on or teleports home.

Once populated with textures, each logon/teleport to home would not involve network traffic, as they would be available locally, on the client.

Benja Kepler 02:52, 24 March 2007 (PDT)