formula for resizing???

[artmaker]

OK, I need a math major here. There has got to be a formula for figuring this.

Q. When you resize an image, lets say you have a very large image. Oh about 17" wide but a very low resolution. (My friends digital camera does this. huge images but very low res.) I know you can resize the image to about 5" and increase the resolution so not to lose any quality.
BY how much?

Same goes in reverse. Very small image scanned or saved at a very high resolution as the case may be with slide scanners, you can scale the image up and decreace the resolution. Again, what is the formula for how much? There has to be one.

[Doug Nelson]

Just calculate your total width in pixels. Then you can divide the total number of pixels by your desired width (in inches) and get your final resolution (assuming you turn off resampling).

[thomasgeorge]

Another option is to invest in the program " Genuine Fractals" which does an exceptional job at this sort of thing and avoids most of the artifact problems associated with other methods. If you do a lot of that type of work, that program is a real time saver. Tom

[Doug Nelson]

GF is a plugin with an alternative resampling method. Since this question was about NOT resampling, it's good information to remember, but not really appropriate in this instance.

[artmaker]

Thanks Doug,
I knew it had to be something simple like that.

Just one more thing. What exactly is "resampling?"

So much to this program. I love it but I dont' think I'll ever know all there is to know about it.

[Doug Nelson]

Resampling simply means it looks at the pixels, looks at the destination, and decides if they need to be merged or split, and how to do that. Photoshop has "nearest neighbor", which basically just clones them or cuts them in half, or "bi-cubic", a more accurate (looking) mathematical way. Genuine Fractals and some other programs have even more advanced mathematical ways, but they're still just dividing or merging pixels.

[Ed_L]

Hello Artmaker,

Welcome to the site. If you resample upwards (adding more pixels), Photoshop (or whatever) "guesses" what the new pixels should look like. It can do this in different ways, and as Doug suggested, bi-cubic (if Photoshop) offers the best quality. It's not bad, but it's not "real" information. Hope that helps clear things up. If you ever get to the point where you do know everything there is to know about Photoshop, you won't have to worry about where tomorrow's meal is coming from.

Ed

[Stephen M]

My links page has some good info on Resampling and Resolution:

http://members.ozemail.com.au/~binaryfx/PSTV_links.html#R

Hope this helps,

Stephen Marsh.

[artmaker]

Thanks to all of you. This site is great! Real Photoshop experts here.

I kinda had an idea what resampling was. Never needed it but it's nice to fill in as many blanks as I can. It's very similar to anti aliasing right??? Sort of? Just a way to fix pixels to fake it. Yea! That's about as techno as I get.

Hey Ed, I bet you don't live far from me. Not to get off topic here. Just noticed your location to the side. I'm in SW lower Michigan. Small world huh?

Stephan your links page is cool too. I found some nyfty tidbits about resizing for print. Not that my beginner photoshop classes will ever need to know that, again I like to learn new things.

OK, I think I have what I needed. That math formula above from Doug hit the mark.

Just a note if interest, my own college math instructor took my last Photoshop class. Would have been nice to impress HIM with this math formula. Oh well.

Thanks all.