[Nouveau] Status of s3tc patent in respect to open-source drivers and workarounds

SpliFF spliff at warriorhut.org
Sun Mar 28 04:47:12 PDT 2010

Hi radeonhd, nouveau, mesa3d developers,

Firstly, thank you all very much for all the important work you do.

I've been working as a part-time developer on the "Spring RTS" project
(open-source game engine) which runs on linux (and other os). Some time
ago I tried the engine on the open-source ATI radeonhd driver, which I
understand to be partly based on mesa 3d, and all textures were white. I
originally assumed it was an engine bug but after some investigation
tracked it down to s3tc texture (de)compression not being integrated in
the open driver for legal reasons. It seems the same issue applies to
Mesa3d and Nouveau - that is s3tc being patented and the patent owner
refusing to comment or provide cover for open-source projects
implementing the techniques discussed in US Patent 5956431

I recently took inspiration from an article/talk presented by Andrew
Trigell regarding patents where he claims developers often overestimate
the scope of patents
(http://news.swpat.org/2010/03/transcript-tridgell-patents/) because
they fixate on the title and summary instead of the actual claims.

The key point he was making is that in order to infringe a patent you
apparently have to infringe on ALL ELEMENTS of a PRIMARY claim. If you
don't do something in the primary claim then by definition you aren't
doing the things in secondary claims that reference it (ie, where the
claim reads "... in claim X ..."). In short if you workaround the
primary claims you probably aren't infringing. Wikipeadia, with some
jurisdictional hand-waving, appears to agree: "If all of the claim's
elements are found in the technology, the claim is said to "read on" the
technology; if a single element from the claim is missing from the
technology, the claim does not literally read on the technology and the
technology does not infringe the patent with respect to that claim."

So I took a look at the 4 primary claims that apply to DECODING s3tc
textures. All other claims either rely on these claims or involve
encoding, which Mesa 3D / Xorg drivers should not need to do. Finding a
workaround for these primary claims is necessary to avoid infringement
for an s3tc decoder. I'll break these into seperate elements because a
workaround (non-infringement) for ANY element is a workaround for the
ENTIRE claim itself. The elements are seperated by semicolons and I've
added line-breaks to make them clearer).

To avoid confusion I've removed the preamble from each claim, they don't
count as elements of the claim.

In short, if anyone on these lists can see a way to decompress an s3tc
image without doing just 1 of the items from EACH of the following 4
claims then a legal s3tc decoder should be possible.

an encoded image decomposer,
coupled to receive encoded image data file having at least one
compressed image block,
for breaking the encoded image data file into individual compressed
image blocks,
each compressed image block having at least one associated codeword,
each codeword generated through selecting a block type for an original
image block comprising the compressed image block,
computing an analog curve for the block type,
selecting a partition along the analog curve for the computed analog curve,
and computing the set of codewords for the partition;

at least one block decoder,
coupled to the encoded image decomposer,
for decompressing the compressed image blocks;

and an image composer,
coupled to each block decoder,
for ordering the decompressed image blocks in an output file.

receiving the encoded image data;

decomposing the encoded image into the modified header and the
individual encoded image blocks;

reading the modified header to generate an output header;

decoding each individual encoded image block to generate a decoded image
including receiving each individual encoded image block, including a
plurality of codewords,
and a bitmap of at least one pixel,
the plurality of codewords derived through selecting a block type for an
original image block of the encoded image block,
computing an analog curve for the block type,
selecting a partition along the analog curve for the computed analog curve,
and computing the plurality of codewords for the partition;

and composing the output header and the individual decoded image blocks
to generate an output file of the original image.

a block address computation module, coupled to receive each codeword
from the header information, for computing an address of an encoded
image block having the identified pixel;

a block fetching module, coupled to receive the encoded image block
portion and the computed address, for fetching the encoded image block
having the identified pixel;

and a block decoder, coupled to receive the fetched encoded image block,
for decoding the image block to generate a quantized color associated
with the identified pixel.

computing an address for an encoded image block having the identified
the address computed from the at least one codeword for the encoded
image block;

fetching the encoded image block using the computed address;

computing quantized color levels for the fetched encoded image block;

and selecting a color of the identified pixel from the quantized color
levels to output.


One thing that stands out to me are that none of the primary claims seem
to describe the technical format itself (ie, no algorithms, keywords,
block sizes, byte-alignment, etc..) This means the claims at issue seem
to be *methods* of data handling, (and not particularly inventive ones
in my opinion). That suggests to me that with a little thought it should
be possible to generate the same results via a method that doesn't
infringe one of the 4 specific claims listed above. To clarify, despite
all the technical mumbling in the abstract the claimed invention does
not appear to be a "format" as such, but a method of pulling "blocks"
and a "header" out of an undefined format and generating "color pixels".
Even the abstract claims all the presented examples of keywords, values
and structure for s3tc are not the actual invention being claimed.
Another important point is that some of the wording in the claims may
not mean what it appears to mean (ie, the generally accepted
definition), the full patent may redefine some common software terms in
the wrapper.

Some possible avenues of investigation, straight off the top of my head are:

* Determine the patents' definition of a "unit" and/or "module", and use
something else (ie, a monolithic function)
* Determine the patents' definition of a "block", and use something
else. (characters, a bytestream, ...)
* Determine the patents' definition of "compose / decompose", and do
something else.
* Do we need to "compose a header" as a seperate stage (use a footer,
temporary vars, registers)
* Does the curve have to be "analog"? What about a lookup table with
discreet points that approximate a curve?
* Can the algorithms work on groups of blocks or sub-blocks instead of
"individual blocks".

More for DRI projects than Mesa, but I'll ask anyway:
* Do we need to decode anything; could an X11 driver force-feed the raw
bytes to hardware instead?

Claim 22 seems like a pretty important step and possibly a hard one to
workaround but it also appears to be claiming the invention of using a
key lookup to fetch values with some vague notion of quantising them. Is
it just me or are they claiming to have invented indexed color?

PS. If some of the points above seem pedantic that's deliberate. My
understanding is that patent claims are actually intended or required to
be NARROW in scope as broad claims are usually rejected. That means a
workaround that doesn't do PRECISELY the things in each claim above is,
by definition, a new invention. I'm hoping an invention that members of
this community are capable of finding before someone else patents it.

I've CC'ed the EFF on this because I'd like to see it become a part of
their patent-busting efforts. Fear of this patent is blocking an
extremely important component of 3D gaming under linux (the use of DDS
textures under an open-source driver). Not that I'm a fan of DDS, just
that it's the format of choice for commercial and non-commercial games
until hardware supports something better, and even then still a
requirement for running many classic games under WINE +
radeonhd/mesa3d/nouveau. It seems absurd that I can buy a product with a
patent license granted (NVIDIA/ATI card) but then be denied the
protection of that license because I don't use part of the product (the
official driver). I really hope that as a community we can either invent
around this patent or bust it (at least in US / EU where it is most
problematic) without losing hardware support or requiring the
reformatting/conversion of digital assets.


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