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"16-bit" mode.



 
 
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  #81  
Old November 21st 04, 02:32 PM
Mike Engles
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Kennedy McEwen wrote:

In article , Mike Engles
writes

Hello

Is astronomical image processing done in a linear space or a gamma
space?

Normally in linear space, Mike - because you are primarily interested in
physical quantities and their quantitative results.

After that has been achieved, the representation for human consumption
is created - either from the source data or the processed data depending
on the objective required.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)



Hello

Is the same true for imaging from spacecraft, interplanetary or
otherwise or is gamma encoding done before transmission?

Mike Engles
  #86  
Old November 21st 04, 09:04 PM
Mike Russell
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Dave Martindale wrote:
[re Photoshop's 16 bit representation]

The Pixar Image Computer used tricks like this many years ago. The
memory was 12 bits per component, with 2048 representing a value of
1.0. Values up to 3071 were brighter than white, up to 1.5. The range
3072-4095 represented negative values in [-0.5, 0]. When a 12-bit
value was loaded into the processor, it was automatically extended to
16 bits in a way that preserved the positive or negative meaning,
then the arithmetic was all 16/32 bit.

Dave


It did indeed, and I'm astonished that you remember these details so long
after I, who worked on this stuff for years, have forgotten them!

You touch on another important aspect of having displayable values occupy
only a subset of the total 16 bits. This is the ability to represent
negative
intermediate values.

Negative numbers are an important component of most
graphic calculations, and the ability to represent negative values in place
saves,
storage, and the time required to convert between storage formats, while
retaining the ability to make calculations that require per pixel negative
values. An example would be the subtraction of two channels, or the Pirl
function, which uses negative terms in calculating the theoretically perfect
resampling filter.

OTOH, Adobe's weird 16 bit format makes it more difficult to interface to
other graphics libraries, requiring additional passes to convert to and from
16 bit mode.
--

Mike Russell
www.curvemeister.com
www.geigy.2y.net


  #87  
Old November 21st 04, 09:10 PM
Mike Russell
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Mike Engles wrote:
....
[re linear encoding of specialized pixel data values]

Is the same true for imaging from spacecraft, interplanetary or
otherwise or is gamma encoding done before transmission?


Yes. Gama encoding compresses some data values, and there is no reason to
do this to raw data from a spacecraft.

Here's an article that may interest you, by Alvy Ray Smith, on the
distinction of work and display color spaces.
http://alvyray.com/Memos/MemosMicros...rAlphaQuestion
--

Mike Russell
www.curvemeister.com
www.geigy.2y.net


  #88  
Old November 21st 04, 09:10 PM
Mike Russell
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Mike Engles wrote:
....
[re linear encoding of specialized pixel data values]

Is the same true for imaging from spacecraft, interplanetary or
otherwise or is gamma encoding done before transmission?


Yes. Gama encoding compresses some data values, and there is no reason to
do this to raw data from a spacecraft.

Here's an article that may interest you, by Alvy Ray Smith, on the
distinction of work and display color spaces.
http://alvyray.com/Memos/MemosMicros...rAlphaQuestion
--

Mike Russell
www.curvemeister.com
www.geigy.2y.net


  #89  
Old November 22nd 04, 12:54 AM
Chris Cox
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In article ,
wrote:

In message ,
Chris Cox wrote:

Without your original data to test, I can't even guess what went wrong.


You don't need my original data.

Any image in "16 bit greyscale" mode has all kinds of numbers between 0
and 32768 missing,


Only if you started with an image that had numbers missing.
The representation is 0..32768 -- all numbers are possible.


and not possible no matter hown much you blur or
interpolate. "16 bit greyscale" is about 13.5 bit greyscale.


No, that is not even remotely correct.

Chris
  #90  
Old November 22nd 04, 01:07 AM
Chris Cox
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In article , Matt Austern
wrote:

Chris Cox writes:

I've tried. Their engineer insists that it's 30x faster to work with
15 bit quantities than 16 bit ones.


Which is correct (for 0..32768 representation versus 0..65535
representation).


Perhaps this is offtopic, and perhaps you can't answer it without
revealing proprietary information, but can you explain why 15-bit
computation should be so much faster than 16-bit? (If there's a
publication somewhere you could point me to, that would be great.)
I've thought about this for a few minutes, I haven't been able to
think of an obvious reason, and now I'm curious.


1) Because a shift by 15 (divide by 32768) is much faster than a divide
by 65535.

One of the most common operations is (value1*value2 + (maxValue/2)) /
maxValue

With 0..255 we can pull some tricks to make the divide reasonably fast.
For 0..65535 the tricks take quite a bit more time (and serialize the
operation), or we have to use a multiply by reciprocal
For 0..32768, we can just use a shift.


2) A lot fewer overflows of 32 bit accumulators

This is still a problem.
When 64 bit processors become the norm (and the @#!^&$ OS allows a
fully 64 bit application), then that becomes less of a problem.


3) The 2^N maximum value also has some benefits when dealing with
subsampled lookup tables that require interpolation.


4) the 2^N maximum value also has benefits to blending operations that
need a middle value (for 0..255 it was pretty random whether 127 or 128
was used for the middle).


Chris
 




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