| JPEG2000 compression ratios? [message #41151] |
Wed, 29 September 2004 05:14  |
Matt Feinstein
Messages: 33
Registered: July 2002
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Member |
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There's probably an 'obvious' answer to this, but I'm in need of a
hint...
I'd like to figure out the possible image compression ratios that
result from using different quality layers in JPEG2000-- but I don't
see any obvious way of getting that out of the IDLffJPEG2000 object.
Matt Feinstein
--
There is no virtue in believing something that can be proved to be true.
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| Re: JPEG2000 compression [message #43457 is a reply to message #41151] |
Thu, 14 April 2005 07:41  |
Emmanuel Christophe
Messages: 11
Registered: March 2004
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Junior Member |
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Roberto Monaco a �crit :
> I have a question regarding JPEG2000:
>
> How do you control compression ratio when you save an image as JPEG2000
> using the IDL object?
>
> The following is curious: within a Photoshop plug-in I use to save JPEG2000
> images, "they" define a compression factor. If I save the same image with
> different compressions ratios (e.g. 1:1, 1:10, 1:40) I obviously get big
> differences in the resulting file sizes. But if you read these files into
> IDL objects, surprisingly all properties are exactly the same!! Same
> dimensions for the data array, same bit_depth, same number of layers, etc.
> ?? This does not make any sense to me ... I must be overlooking at something
> stupid?
>
> Thanks for any tips.
> Roberto
>
>
I got this problem before, I only manage to get lossless jpeg2000
compression directly with IDL. I went around this problem by calling one
free implementation of jpeg2000 (using spawn): jasper
;creating source j2k file from idl data (image)
im_jpeg=obj_new('IDLffJPEG2000',filenamesrc,/write)
im_jpeg->SetProperty, bit_depth=16, signed=1, reversible=1
im_jpeg->SetData, image
OBJ_DESTROY, im_jpeg
;compressing file
spawn,'jasper -f '+filenamesrc+'-F '+filenamenew+' -T jpc -O rate=0.2'
;reopening in IDL
im_jpeg2=obj_new('IDLffJPEG2000',filenamenew,/read)
imnew=intarr(ns,nl)
imnew=im_jpeg2->GetData()
Hope this help,
Emmanuel
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| Re: JPEG2000 compression [message #43461 is a reply to message #41151] |
Wed, 13 April 2005 16:27 |
Rick Towler
Messages: 821
Registered: August 1998
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Senior Member |
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Roberto Monaco wrote:
>>> I have a question regarding JPEG2000:
>>>
>>> How do you control compression ratio when you save an image as JPEG2000
>>> using the IDL object?
>>
>> Looks like the N_LEVELS property.
>
>
> Yes, from the documentation you would expect N_LEVELS to be responsible for
> compression.
>
> This is the code I am using:
> jp2 = obj_new('IDLffJPEG2000', jp2_file, /WRITE)
> jp2->SetProperty, N_LEVELS=15
> jp2->SetProperty, TILE_DIMENSIONS=[2048.,2048.]
> jp2->SetData, image, /ORDER
> obj_destroy, jp2
>
> If I set N_LEVELS to 5 in the second line (the default) and run it again for
> the same image (same input file) I get both times jp2 files of the same
> dimension (31 MB) ?!?! I am obviously missing something here.
Well, it works for me:
jp2 = obj_new('IDLffJPEG2000', 'test1.jp2', /WRITE)
jp2->SetProperty, N_LEVELS=0
jp2->SetData, BYTSCL(DIST(1600,1200))
obj_destroy, jp2
jp2 = obj_new('IDLffJPEG2000', 'test2.jp2', /WRITE)
jp2->SetProperty, N_LEVELS=15
jp2->SetData, BYTSCL(DIST(1600,1200))
obj_destroy, jp2
test1.jp2 is 614KB
test2.jp2 is 13KB
Further, querying the image works as advertised too:
jp2 = obj_new('IDLffJPEG2000', 'test1.jp2', /READ)
jp2->GetProperty, N_LEVELS=nl
PRINT, nl
obj_destroy, jp2
% Compiled module: $MAIN$.
IDL> .GO
0
jp2 = obj_new('IDLffJPEG2000', 'test2.jp2', /READ)
jp2->GetProperty, N_LEVELS=nl
PRINT, nl
obj_destroy, jp2
% Compiled module: $MAIN$.
IDL> .GO
15
>> Well, I know nothing about JPEG2000 but I would hope that all of the
>> properties that you have specified *would* be identical. The only
>> property that should change would be the compression factor. Don't know
>> where that would be though. Maybe the N_LEVELS property again? Don't
>> know if that would be set after reading a file or if it only applies to
>> writing.
>
>
> These images were created ouside IDL, in Photoshop saving the same TIFF
> image as JP2 using different compression ratios each time. Afterwards when I
> read them within IDL I would expect to see differences in their properties,
> since one corresponds to 1:1 (~30 MB of data) and another to 1:40 (~3 MB of
> data). But N_LEVELS, as ALL the other properties, are exactly the same for
> all cases...
Why do you think it is a problem that "ALL the other properties" are
exactly the same if you have taken the same image and saved it multiple
times, only changing the "compression factor"? What other properties
should change?
My point was that I would assume that the image dimensions, color space,
n_layers, offset and the other properties *wouldn't* change if you only
changed the "compression factor" in photoshop.
> By the way there is no compression property in 'IDLffJPEG2000' object
> structure.
I know. I was relating N_LEVELS to what you called "compression factor"
in photoshop.
-Rick
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| Re: JPEG2000 compression [message #43466 is a reply to message #41151] |
Wed, 13 April 2005 12:55 |
Roberto Monaco
Messages: 11
Registered: August 2002
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Junior Member |
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>> I have a question regarding JPEG2000:
>>
>> How do you control compression ratio when you save an image as JPEG2000
>> using the IDL object?
>
> Looks like the N_LEVELS property.
Yes, from the documentation you would expect N_LEVELS to be responsible for
compression.
This is the code I am using:
jp2 = obj_new('IDLffJPEG2000', jp2_file, /WRITE)
jp2->SetProperty, N_LEVELS=15
jp2->SetProperty, TILE_DIMENSIONS=[2048.,2048.]
jp2->SetData, image, /ORDER
obj_destroy, jp2
If I set N_LEVELS to 5 in the second line (the default) and run it again for
the same image (same input file) I get both times jp2 files of the same
dimension (31 MB) ?!?! I am obviously missing something here.
Perhaps one could define N levels and somehow store only some of these
levels, discarding for example the 2 higher resolution levels? But I don't
see a way of doing something like this, in SetData or elsewhere.
>> The following is curious: within a Photoshop plug-in I use to save
JPEG2000
>> images, "they" define a compression factor. If I save the same image
with
>> different compressions ratios (e.g. 1:1, 1:10, 1:40) I obviously get big
>> differences in the resulting file sizes. But if you read these files
into
>> IDL objects, surprisingly all properties are exactly the same!! Same
>> dimensions for the data array, same bit_depth, same number of layers,
etc.
>> ?? This does not make any sense to me ... I must be overlooking at
something
>> stupid?
>
> Well, I know nothing about JPEG2000 but I would hope that all of the
> properties that you have specified *would* be identical. The only
> property that should change would be the compression factor. Don't know
> where that would be though. Maybe the N_LEVELS property again? Don't
> know if that would be set after reading a file or if it only applies to
> writing.
These images were created ouside IDL, in Photoshop saving the same TIFF
image as JP2 using different compression ratios each time. Afterwards when I
read them within IDL I would expect to see differences in their properties,
since one corresponds to 1:1 (~30 MB of data) and another to 1:40 (~3 MB of
data). But N_LEVELS, as ALL the other properties, are exactly the same for
all cases...
By the way there is no compression property in 'IDLffJPEG2000' object
structure.
Roberto
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| Re: JPEG2000 compression [message #43467 is a reply to message #41151] |
Wed, 13 April 2005 09:24 |
Rick Towler
Messages: 821
Registered: August 1998
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Senior Member |
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Roberto Monaco wrote:
> I have a question regarding JPEG2000:
>
> How do you control compression ratio when you save an image as JPEG2000
> using the IDL object?
Looks like the N_LEVELS property.
> The following is curious: within a Photoshop plug-in I use to save JPEG2000
> images, "they" define a compression factor. If I save the same image with
> different compressions ratios (e.g. 1:1, 1:10, 1:40) I obviously get big
> differences in the resulting file sizes. But if you read these files into
> IDL objects, surprisingly all properties are exactly the same!! Same
> dimensions for the data array, same bit_depth, same number of layers, etc.
> ?? This does not make any sense to me ... I must be overlooking at something
> stupid?
Well, I know nothing about JPEG2000 but I would hope that all of the
properties that you have specified *would* be identical. The only
property that should change would be the compression factor. Don't know
where that would be though. Maybe the N_LEVELS property again? Don't
know if that would be set after reading a file or if it only applies to
writing.
-Rick
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