[argyllcms] Re: Calibrating a digital photo frame

On Mon, Dec 17, 2007 at 06:16:46PM +0100, Fr?d?ric Mantegazza wrote:
> 
> Ok, I make some tests, using dispread (with the new -C option, to call a 
> script just waiting for a key pressed). I generated 100 patches with 
> printtarg, downloaded them to the photo frame, and launched dispread...
> 
> The results are not very good, and I don't understand why.

100 patches is not very many at all, especially for an uncalibrated LCD
device. In my experience the targen default of 836 patches produced a
reasonable profile for my laptop display, while increasing this to 2000
got me better results in awkward dark areas. See also the guidance on
patch counts in http://www.argyllcms.com/doc7/targen.html of course.

> About conversion, I used jpegicc, from lcms project, but it does not seem 
> to be a very good job: even with quality 100, the compression is really to 
> much, and I see artefacts. I tried cctiff, but I need to convert images 
> from jepeg to tif, than from tif to jpeg in order to download then in the 
> photo frame. Is there a Argyll tool to convert jpeg files from one space 
> to another?

I think (Graham can no doubt confirm) that there's just cctiff. The fact
there's a quality option on the lcms tool at all suggests that it's
decompressing the JPEG, operating on the resulting image and then
recompressing. This is the most straightforward way of doing it but will
inevitably lead to degradation unless knowledge of the structure of the
original compressed data is used.

I think you will probably get the best results possible by converting
the image to TIFF, applying the colour transformation with cctiff, and
then reconverting to JPEG using the same subsampling settings and
quantisation tables as were used in saving the original images. The
cjpeg tool from the libjpeg project will let you specify these settings,
which should be possible to determine somehow from the SOF and DQT
markers in the original image.


Martin

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