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adison
31-Jan-2005, 08:18
Hi, I am asking your help with a problem wich seem to be simple at a first look: I want to change the size of a tiff file, but without changing the dimensions of the image itself, and without converting the image into any compressed file format as jpg, gif, etc.

So let`s take the practical situation: I have a tiff image file of 20 Mb (megabytes). The dimension of this image is: 3000 x 2000 pixels.

I want to change this 20 Mb tiff image file into a 10 Mb tiff image file, but without converting it to file formats that could compress the image like jpg, loosing quality. Is allowed only converting in BMP or other formats wich doesn`t compress the data of the file.

Is anyone of you who have an ideea to solve this problem? Or this is impossible.

Please help me.

thank you in advance,

Adison.

isaac
31-Jan-2005, 08:41
You need to differentiate between lossy and lossless compression. A lossy compressed format like jpeg will, as you note, lose quality because it throws out information. A lossless compression algorithm will lose no quality whatsoever, since all it will do is pack the data better. What you want is compression by definition, but you want the lossless type.

If you open up your tiff in Photoshop, for example, you should be able to save it as a tiff with lzw or zip compression. Both of these are lossless and could save you a factor of two in the file size. Exactly how much will depend on the image. Other file types like gif and, I believe, png are also compressed in a lossless fashion.

-isaac

Ellis Vener
31-Jan-2005, 08:53
The only way to make a 20mb TIFF into a 10mb TIFF is to change the resolution --which will mean throwing away 50% of your data! What are you actually trying to achieve? Save storage space?

adison
31-Jan-2005, 09:03
the story goes like that:

I have a jpg file of 2,5 Mb (3000x2000 pixels). If I convert it to tiff via Corel Photopaint or Adobe photoshop, results a 9 Mb tiff file (3000x2000 pixels). So far so good ! But, if I convert the same jpg file via Nikon Capture 4 ( or 3) the resulting tiff file is 18 Mb (3000x2000 pixels).



My problem starts here, becouse I need to processing the file with Nikon Capture, but this ends in very large file than I process the files in Corel or Adobe.

The only solution is to find a way to decrease size of tiff files....

....A solution that I cannot imagine at this moment. Perhaps some of you can.

thank you for understanding.

chris jordan
31-Jan-2005, 09:03
If you want to keep it as a 3000x2000 pixel file, but sized down to 10 MB, then you will have to use one of the compression algorithms. I would recommend JPEG, at the highest quality setting. I don't know how the compression happens at "12" quality JPEG, but the pixels are not affected-- you can res it back up and they are all the same (or so close that it doesn't matter).

If you want to resize the file to a smaller pixel dimension, then open the file in Photoshop, go to "image size" and resize the image to the dimensions you want. When you're done, apply a small amount of unsharp masking to account for the fuzziness caused by resizing; then do a "save as" a TIFF in the new size.

Glenn Kroeger
31-Jan-2005, 12:13
Peter:

A 2000 x 3000 tiff file has a natural size of 18MB (3 bytes/pixel * 6MP). Sounds like both Adobe and Corel are compressing the tiff with a lossless compression scheme such as LZW compression. You could edit the file in Nikon software at full size, then open it and resave with LZW compression in PS.

Bill L.
31-Jan-2005, 16:45
Are you doing RGB rather than grayscale? If so, one may be 16 bit depth and the other may be 8 bit depth. Photoshop CS will tell you what the bit depth is on the top of the image. If your image has been edited and is ready for printing, you won't lose anything by saving it at 8 bits per channel, but if you still need to edit it, I would try and keep it in 16 bit. As noted above, if your program is using LZW compression, it is lossless compression (though I tend not to use it as I have found it slow to open for large files).

Cheers!
Bill

Daniel Geiger
31-Jan-2005, 23:56
Also check for color space used. Most digital cameras and scanners capture in RGB. If the file **somehow** gets converted to CMYK, you increase file size by 1/3. I think Lab stays in 3 channels, what happens in index colors is yet an other story. The 9 to 18 MB jump sounds like 8 to 16 bit color.

Never modify an image in jpeg. As you save, and re-save the image, the lossy compression algorithm is re-applied each time to the image, so you progressively lose image detail. LZW compression is fine. For a science journal I work as an associate editor we ask authors to prepare all files as tiffs, and only for e-mail attachments of ftp-transfer to convert the files to hightest quality jpegs. The point there is reduced file size to make network transfer faster, with acceptable loss. But if you have the bandwidth, transferring LZW compressed tifs is better. At the printer's end, it gets immediately back converted to tif.

18 MB files are not that large, Photoshop should have no problems with them. If you are on an older Mac, increase memory allocation to that particular application (3-4x of file size), or get some more RAM. OSX does memory allocation for you. I think PCs also allocate RAM dynamically. 256 or 512 MB RAM is pretty cheap these days.

Good luck Daniel