From: Norman.Yee@launchpad.unc.edu (Norman Yee)
Subject: Re: JPEG "Standard"
Organization: University of North Carolina Extended Bulletin Board Service
Date: Wed, 13 Jan 1993 04:22:50 GMT

>tgl+@cs.cmu.edu (Tom Lane) writes:
>>kxj6@po.CWRU.Edu (Kijin Jung) writes:
>> I was concerned about the standardization of JPEG
>> between two programs I have, so I did some tests.
>>
>> ... I compressed them using ColorView 2.0 for DOS
>> and WinJPEG 1.6. (The reason why I used a quality of
>> 74 and 76 for Colorview was because it did not
>> offer 75 as an option; so I also used those settings
>> for WinJPEG.) Also, Cview only accepts BMP files.
>>
>> Cview's JPEG output is much smaller than those of WinJPEG
>> for a given Quality.
>
>I'm guessing that you proceeded by loading and displaying the image,
>then using "Save".  This is a bad move since both programs will
>quantize the 24-bit image down to 8 bits for display (assuming your
>display hardware is 8 bits).  JPEG is intended for compressing 24-bit
>data, not 8-bit; your JPEG files are larger and of lower quality than
>they would be if you had made them directly from the Targa or BMP file.
>
>WinJPEG has a "batch conversion" option which allows direct compression
>of Targa to JPEG format without a quantization step.  This is a much
>better means of compressing full-color images.  As far as I can tell,
>ColorView has no similar capability.
>
>The difference in file size is due to Colorview and WinJPEG using
>different quantization algorithms; hence the data fed to the JPEG
>compressor is not the same.  In particular, I'd bet you had dithering
>turned off in ColorView.

Kijin Jung loaded the 24-bit images with WinJPEG's "8-bit Bitmap" option
on so that they were quantized to 256 colors and dithered before they
were displayed.  As Edward Hee pointed out in a different message, the
dithering introduces noise which makes the image look grainy and does
not compress well.  The reason that WinJPEG produced a larger image than
ColorView is that Jung saved the dithered image to a JPEG file.  To
correctly convert the 24-bit image to a JPEG, Jung should have turned
off the "8-bit Bitmap" option before loading the image.  With the "8-bit
Bitmap" option off, the 24-bit image will be neither quantized nor
dithered.

>> I am very confused by WinJPEG's method of saving JPG's.
>> WinJPEG takes two original files of the same content but
>> different format, yet outputs JPEG files of different sizes
>> (with entropy optimizationon).
>
>That seems strange to me too.  Could it be that there is some history
>dependence in their color quantization algorithm?  Are you *sure* the
>BMP and Targa files are identical?

This is a quirk in WinJPEG; it produces different quantized and dithered
images from identical 24-bit images stored in BMP and Targa format.  In
case anyone is interested, the mistake was that the 24-bit Targa was
dithered from the top-down and the BMP was dithered from the bottom-up.
Since the dithering algorithm is dependent on the previously processed
pixel information, two different images are produced from the same BMP
and Targa.

This problem is corrected in version 2.0 which will be released soon.

  Norman Yee, one of the authors of WinJPEG
  nyee@osiris.ee.tufts.edu


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