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Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha

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On 2/6/2015 4:54 PM, Mark Nelson wrote:
> On Thursday, February 5, 2015 at 10:59:30 PM UTC-5, Thomas Richter wrote:
>> JPEG XT is a whole backward compatible extension to the well-known JPEG
>> compression standard for full images, that is, all images compressed in
>> this form will be reconstructable by any legacy JPEG decoder.
>
> Here I was imagining there was nothing left to do for JPEG.
>
> This is pretty cool, and making it backwards compatible must have taken a fair amount of ingenuity.
>

luckily, the JPEG format has "some" room for extension (unlike some 
other formats), namely in the form of APPn markers. JPEG decoders can 
safely ignore them if they don't care what they contain, and there are 
some general conventions for avoiding "false positives", namely using 
some sort of marker/tag (such as an ASCIIZ string or FOURCC or similar) 
to identify the type of marker.

so, if you make the format extensions as optional markers, then normal 
decoders can reasonably safely ignore them, and if the basic image looks 
about like what they expect, then it is possible for them to still 
decode the base image.

> Can't wait to see some implementation.
>

dunno about JPEG XT.


I have my own extended JPEG variant (BTJPG), which does "basically 
something similar" (HDR, alpha channel, ...).

for the most part though, I have just ended up using it as a sort of 
lossy PNG alternative.

in my case, it uses a few markers, namely:
APP11, "CompLayer", "Alpha", 0
APP12, FCC("LDAT"), Image Data...
APP12, FCC("LDAT"), ...
...

where the sub-image is basically a mono JPEG in this case, but is FF 
escape-coded, and broken up into pieces and stuffed into the JPEG (APPn 
markers are limited to around 64kB due to only having a 16-bit size).

the APP11 marker there basically indicates that the following image 
(given in the "LDAT" markers) represents an alpha image. otherwise, the 
markers may also be used to give other information.


additonally, there may be an optional "RGB" CompLayer marker, where a 
decoder which uses this format will use this image instead of the base 
image for the RGB components (this is a more friendly way to do lossless 
images in the format, leaving the base image as a lossy version. the 
base image may also be encoded losslessly, but in conventional JPEG 
decoders, the image colors come out slightly-off and a little 
over-saturated due to the use of a different color-space, namely RCT 
instead of YCbCr).


> - Mark
>

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JPEG announces availablility of CD Text for JPEG XT lossless and alpha Thomas Richter <thor@math.tu-berlin.de> - 2015-02-06 04:59 +0100
  Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha BGB <cr88192@hotmail.com> - 2015-02-06 15:52 -0600
    Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha Thomas Richter <thor@math.tu-berlin.de> - 2015-02-07 10:28 +0100
      Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha BGB <cr88192@hotmail.com> - 2015-02-07 11:14 -0600
        Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha Thomas Richter <thor@math.tu-berlin.de> - 2015-02-08 12:54 +0100
          Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha BGB <cr88192@hotmail.com> - 2015-02-08 08:18 -0600
  Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha Mark Nelson <snorkelman@gmail.com> - 2015-02-06 14:54 -0800
    Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha BGB <cr88192@hotmail.com> - 2015-02-06 18:01 -0600
    Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha Thomas Richter <thor@math.tu-berlin.de> - 2015-02-07 10:24 +0100
  Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha "Dex" <admin@127.0.0.1> - 2015-02-08 11:36 +0800
    Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha Thomas Richter <thor@math.tu-berlin.de> - 2015-02-08 06:20 +0100
      Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha "Dex" <admin@127.0.0.1> - 2015-02-08 15:36 +0800
      Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha BGB <cr88192@hotmail.com> - 2015-02-08 02:18 -0600
  Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha Aaron Boxer <boxerab@gmail.com> - 2016-01-26 08:02 -0800
    Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha Thomas Richter <thor@math.tu-berlin.de> - 2016-01-27 16:04 +0100
      Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha Aaron Boxer <boxerab@gmail.com> - 2016-01-28 05:02 -0800
      Re: JPEG announces availablility of CD Text for JPEG XT lossless and alpha Aaron Boxer <boxerab@gmail.com> - 2016-01-28 05:02 -0800

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