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Re: [Public WebGL] Generic compression formats (and a texImage2D API issue)



Hi Kenneth,

On Tue, May 11, 2010 at 07:01, Kenneth Russell <kbr@google.com> wrote:
> The topic of texture compression has been discussed in the working
> group. The current Web APIs do not provide an efficient way to
> download such compressed texture data.

Hmm, contrary to specific-format compression, format-independent
compression hinting (like ARB_TEXTURE_COMPRESSION)
is about NOT requiring web developers to provide textures in one or,
worse but realistic for interoperability, multiple compressed formats;
this is about letting implementations or drivers compress image data
to the best locally available format at texture upload whenever the
developer has hinted that one texture is eligible for compression
(generally, a texture with low-detail and/or compression artifacts are
tolerated).

As such, there is no need for Web APIs to download any compressed
texture data over the network, PNG/JPEG should be sufficiently compact
for this
purpose, it is only about storing textures efficiently in video memory
(and the associated performance benefits).

I agree that might be better discussed as a post-1.0 feature - though
a hint does not require full support on every implementation early on
-
but we should make sure that it can be introduced later without
introducing source-level incompatibilities (the lack of internalformat
parameter
in commonly used texImage2D helpers unfortunately guarantees that).


>> Regardless texture compression, this made me notice an issue with the
>> current texImage2D helper signatures
>> (having a HTMLImageElement, HTMLCanvasElement or HTMLVideoElement parameter) :
>> they do not have a (internal) format parameter, moreover potential
>> usage for compression (and/or extensions), it can be useful to
>> have such anyways as it would allow, for instance, to load only the
>> alpha channel from a PNG into an alpha mask texture,
>> without having to perform per-pixel processing in JavaScript using an
>> intermediate canvas etc.
>
> This has come up on this mailing list recently and was discussed at
> length. Please search the archives for the thread "Retrieving number
> of texture channels". At this point this is a request for enhancement.

Thanks for the reminder, I didn't have this thread in mind.
After reading it again, I think Chris also supported that
internalformat parameter should be added to tex2Image2D helpers.
Even if we do not support "conversion" initially (e.g use only
alpha-channel from a PNG), it deserves a higher priority
than RFE imho since not having the parameter in 1.0 will introduce
source-level incompatibilities later and also limit potential
extensions doing specific compression and/or conversions.


Regards,
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