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Image size reduction question


RichardC

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Hi folks,
Is there a ?best way? to reduce the image size from a digital camera to something acceptable for posting on the web without destroying the image quality?
My camera gives me pics that are 1200 x 1600 pixels at 72 ppi. What?s the best way to get it down to the 300 or 400 range? All my attempts are really bad }P
Thanks in advance..............RC
 

Welles

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Hi Richard,

Try opening the image in Photoshop in a full screen mode. Then in the navigator reduce the screen viewing size to 50%, 25%, or 12.5% and use a screen capture followed by Save For Web...

Don't use 66.7%, 33%, 16.5% or any other odd percentage as they introduce visual distortion.
 

RichardC

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That seems to be an improvement but it's still not very good :(
Maybe I'm not doing it right....... [doh] Duhhh.......when you say "screen capture" is that what I call taking a picture (Cmd/shift/4 on a Mac)? ;\
'Making some progress anyway.
Thanks Welles....RC
 

ooptea

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now i might be way off here but my camera takes pictures at about the same siz as yours and i just use the image size function in PS and have never had a problem with quality loss. change the size save as jpg on high quality . hope im not way off but there is my 2 cents
 

Welles

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Nah, you're right on, ooptea. I was just providing a different option because Richard C had reservations about his results. The only thing I'd suggest is to experiment while resizing with different interpolation algorithms, Bicubic Sharper, for example. Even then, maybe a bit of sharpening after the resize.

Richard C,

Could you post one of your resized images. There's always the possibility that you are expecting too much from a fairly small screen image. ;)

BTW, when you use the Save For Web... option with the intention of using it on a web site, generally speaking you should never go above 60% and generally 50% is plenty. I remember when I was first learning about saving for web it was before the era of broadband and the real experts suggested to reduce the quality of JPEGs until you absolutely couldn't stand it and then squeeze it slightly more. File size and bandwidth were vital considerations!
 

Erik

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You can downsample. But you must understand that you inevitably will loose quality.
First because you take jpg pics, which are already a "let's act as if" solution as jpg reduces filesize by flattening differences between pixels. This loss cannot be undone.
Second because you will reduce the number of pixels dramatically (you'll keep one out of three/one out of four) and each pixel contains information that is either lost when you downsample in the 50%-25% etc scale as PS will simply discard one out ouf two/three out of four pixels) or approximated when you choose % that cannot be divided and hence have to be calculated.
And third because you're going to reduce the quality even more by resaving as jpg, meaning you'll flatten out even more.

I don't know how you see your own photographic skills (attention to dark shadows, ...) but it's always best to start with file that does not have enough contrast. Enhancing is easier that, say, trying to fill in burned out highlights.

Best is to take your pics also at the size for the web you intend to use, and then upload them directly as they were taken. These extra kB are normally not the problem.

So if you need to downsample, do it, in your case, in four steps, say, 1230wide, then 913, then 610 wide, then 400.
Use bicubic as that way PS calculates a mean value of all pixels surroinding every pixel, which gives best quality.

Otoh: many people don't see quality loss as visually it is not always obvious (depending on subject and other factors). But as you say you get bad results, it looks like you want high standard. In that case there is no easy way.
You can create an action for this as you're going to use it several times.
 

RichardC

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Erik said:
So if you need to downsample, do it, in your case, in four steps, say, 1230wide, then 913, then 610 wide, then 400.
Thanks Erik.......can you elaborate a little more on this? Are these just random numbers that you picked (out of the "void") to use as an example?

Thanks also to Welles and ooptea........yes I guess my expectations are too high. Your input has been helpful......RC
 

Erik

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When you have an image of 1600x1200pixels and you reduce it to 400x300pixels, Photoshop discards three out of four pixels.
To be honest: I didn't actually see it do this, but I read it in one of the "classic" books (bible, Gary Bouton, can't exactly remember which one. I'll look it up if you want to).

When you force PS to calculate and you choose bicubic, it takes into account every pixel that surround each pixel and calculates a mean value for this. This gives a better result.

But if you downsample in one big leap, then PS still has to discard too many pixels in one time. Which is why it is better to do it in several smaller steps. And to take care that none of the steps makes it easy for PS by taking, say, 50%. Better is to use values that are near 3/4, 1/2 and 1/4 (in your case, as you want to go from 1600 to 400) but not exactly 75%, 50%, 25%.

Hence the values I gave. These are "random" but I quickly calculated that they cannot be calculated easy.

Most probably, afterwards the image will be a bit blurry. Yet, if you sharpen it the usual way (unsharp mask,...) the stairsteps of the jpg might become more visible.
Sharpening has in fact little or nothing to do with sharpening. What is done is in fact nothing else than accentuating the edges by adding contrast to what is lighter and what is darker.

Therefore, an edge-mask is best to get good sharpening.

The procedure and settings come from Deke McLelland. But, seen the fact you can find them on every photoshop site on the net, I don't think I do something illegal by giving the method here.

1/ Open the Channels pallette and duplicate the channel with the best contrast.
2/ With only this active, Filter>FindEdges
3/ Image>Adjust>Invert (Ctrl/Cmd+I). You now have whitish lines on a dark background.
4/ Filter>Noise>Median This averages the pixels. Radius of about 2 is perfect.
5/ Filter>Other>Maximum This makes the whites broader. Radius here appr. 4
6/ Filter>Blur>Gaussian Blur. Same value as Maximum filter
This is your edge mask

7/ Now return now to composite view, but stay in the Channels Pallette
8/ Ctrl/Cmd-click the name of the mask channel you just created. This loads the mask as a soft selection.
9/ When you use Unsharp Mask now, it'll only enhance the contrast in pixels that the mask allows, and these are the edges or, in other words, the transitions of tonal values that were different enough to be accepted in the settings we chose. You can now set Unsharp Mask to maximum aount, and some 2 pixels Radius.
Forget about threshold.

I won't be able to answer more today, but in some 24h I can be back if you have questions.

Hope this helps. I know it's not the easiest way out, but that's the way we really learn.

But, realy: if you intend to post photographs on the web and you can, it's best to take that extra pic at the size you will upload. A few kB, and the best quality you can get.
 

Welles

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I rather disagree with the stairstep theory of interpolation...if you are using Photoshop CS or CS2. In PS CS new algorithms were created which allow one to resample images in one step as well as any other more laborious technique.

There was a long thread on the Adobe forums a year or so ago with dozens of opinionated people chiming in with their favorite techniques of resampling images. The general consensus was stimulated by Chris Cox who, in the beginning of several hundred posts, asserted that the new algorithms were as good as you could do. I came to believe him and use, almost exclusively, Bicubic Sharper and Smoother.
 

RichardC

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[oops] I guess I should have mentioned in the beginning that I'm using PS6.
Thanks again everyone.......RC
 

Welles

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RichardC said:
[oops] I guess I should have mentioned in the beginning that I'm using PS6.
Thanks again everyone.......RC
:rofl: Haw! In that case, Erik is giving you absolutely the best information possible. Ignore everything I said.
 

Erik

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Last before I go to bed.
I still use PS7 and CS and CS2 are pure skiffy to me.

So all I wrote is limited to PS6 and 7 (which are more or less the same, except for brushes).
 

RichardC

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[confused] OK Welles. I take it from all this that an upgrade to the latest PS would mostly cure my reduction woes?

Erik.....I'm looking forward to experimenting with all the info that you just gave me here [excited]

Thanks again folks.....RC
 

Welles

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RichardC said:
[confused] OK Welles. I take it from all this that an upgrade to the latest PS would mostly cure my reduction woes?
Hmmm... I wouldn't count on it. We'd have to experiment and see. I'm worried that you might be hoping for too much no matter what technique was used. :\
 

RichardC

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:D I didn't mean to put you in an awkward spot there Welles. I appreciate your honesty 8))
RC
 

RichardC

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Erik said:
1/ Open the Channels pallette and duplicate the channel with the best contrast.
.
Hi Erik,
I?ve been intrigued with your ?edge mask? suggestion but I?m not sure what you mean by ?best contrast? or how to find the channel with the best contrast. Can you help again or maybe direct me to a good toot or book about this? [doh]
 

RichardC

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Man...things have changed here since i last posted! I see no one has answered my last question (I don't think) :{ [doh]...are you still there Erik? If not can someone else please help?
Thanks to anyone.......RC
 

Welles

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Hi RichardC,

When you look at the three channels which make up an RGB image, one of them has noticeably more contrast, 'the whites are whiter and the blacks are blacker' and more definition is observable. While such a choice is somewhat subjective, one channel almost always stands out.

Does that help?
 

RichardC

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Thanks Welles.....I see now that you guys are referring to the "most" contrast. as the "best" contrast. It's my own burned out analytical photographer's thinking that has me all screwed up here. [confused] :) I shall overcome this! Thank you........RC
 

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