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Old Apr 21, 2003, 8:55 AM   #1
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Default Irfanview, resizing & file size questions.

Greetings,

Iím new to digital cameras & resizing.

I understand that pixels divided by DPI equals inches. I also understand that I can reduce the dimensions of a photo, but keep the pixel size the same and the DPI goes up.

What I donít understand is when I reduce a 1600x1200, 72 dpi, 22x16 inch photo in VuePrint to a 1600x1200, 266dpi, 6x4 inch photo, the file size stays the same (423kb to 423kb).

However, when I do the same thing in IRFANVIEW the file size goes from 423kb to 391kb. Whatís happening and why?

Iím looking for a bulk resizing program that will reduce the image size, bump up the DPI and keep the file size the same. Any suggestions?

Thanks in advance, Brad
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Old Apr 21, 2003, 9:09 AM   #2
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This all depends on whether you are working with a compressed file format like jpg or with a non-compressed format like tif.

IrfanView, like most programs, has the ability to compress data to differing degrees in the jpg format. This is set by going into "options" and moving a slider. Only if you keep the identical level of compression presently set in the image will the file size remain constant when changing display or print dimensions. The difficulty then becomes knowing what the camera is using in terms of compression. This too is variable depending on whether you have quality set at fine, normal, etc'.

What you could do is experiment with IrfanView's compression until you find the level which matches your camera's output. Then changing dimensions without changing either compression degree or resampling, will preserve file size.

Lin
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Old Apr 21, 2003, 9:23 AM   #3
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I'm using a Olympus D-520Z camera. The pictures are in jpg format at the second from the top setting.

If I lose file size, I'm losing picture quality, right?

What's jpg2000 ? I saw that as an option in Irfanview.
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Old Apr 21, 2003, 9:41 AM   #4
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You are not necessarily loosing image quality, a lot depends on what was there to begin with. You really should use the best quality available to shoot in if you don't want to loose image quality when resizing.

When going with smaller print sizes than the defaults for your original pixel dimensions (the defaults are image dimension in horizontal and vertical divided by dpi desired), pixels must necessarily be removed. This is called resampling. When printing larger than the defaults allow, pixels must be added. This is also called resampling, and the process by which it is accomplished is called interpolation. It's done by a math algorithm whereby adjacent sampling site values (from which pixels are created) are examined and intermediate values are calculated and a pixel representing these values created.

There is actually only one print size at a given dpi which can be made without either adding or subtracting pixels. It's usually "not" the print size you want, so the best way to handle this is to use a print software which handles it all for you transparently. My suggestion is to use IrfanView to do your batch resizing for sending images over the internet, etc., and to purchase an inexpensive but powerful print program to do your printing. My personal favorite is Mike Chaney's Qimage - link below:

http://www.ddisoftware.com/qimage/

jpg 2000 is a new type of compression which has these attributes:

*Better image quality at the same file size
*25-35% smaller file sizes at comparable image quality
*Good image quality even at very high compression ratios, over 80:1
*Low complexity option for devices with limited resources
*Scalable image files -- no decompression needed for reformatting. With JPEG 2000, the image that best matches the target device can be extracted from a single compressed file on a server. Options include:
*Image sizes from thumbnail to full size
Grayscale to full 3 channel color
*Low quality image to lossless (identical to original image)
Progressive rendering and transmission through a layered image file structure. Example: from a single 100 KByte image file of a 512x512 original image, a low resolution 32x32 pixel thumbnail image can be transmitted by sending only 10Kbytes. Sending an additional 15 KBytes increases the resolution to 64x64 pixels, and so on. Other layerings provide for progressive transmission and rendering based on quality, color component and spatial location in the image - See:

http://www.aware.com/products/compre...peg2000_g.html
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