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image sizing vs. resampling

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image sizing vs. resampling

Postby Peru » Tue Jun 30, 2015 5:28 pm

I found this to be a good explanation:
http://www.photoshopessentials.com/esse ... esampling/
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Re: image sizing vs. resampling

Postby sidd finch » Tue Jun 30, 2015 5:33 pm

Image Resizing: Changing the size the image will print without changing the number of pixels in the image.


Sounds like resizing is the best way to keep the maximum resolution of the photo.

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Re: image sizing vs. resampling

Postby Bob » Wed Jul 01, 2015 12:14 am

Here's a quote from the article:

Image Resizing: Changing the size the image will print without changing the number of pixels in the image.
Image Resampling: Changing the number of pixels in the image.

See? You already know enough so that the next time someone refers to changing the number of pixels in the image as resizing the image, you can look them proudly in the eyes and say, "I think what you really meant to say there, Bob, is that you're going to be resampling the image, not resizing it." Assuming, of course, that the person's name is Bob. And assuming you don't want Bob to like you much anymore because you think you know it all.


Well, my name is Bob. And, if you say that to me, I won't dislike you more, I'll simply explain to you that yes, I am resizing the image and resampling is the method I am using to do it. What the author of that article is calling resizing is actually another resizing method called scaling. An image can be resized by resampling, by scaling, or by a combination of both resampling and scaling. You an also resize by cropping or extending the canvas, but the article didn't address those so I won't either.

Resampling changes the number of pixels in an image and that affects the size whether viewed on a monitor, used in a video, or printed. Every thing else being constant, If the number of pixels increase, the size will be larger. If the number of pixels decrease, the size will be smaller. For use in a video project, only the pixel dimensions matter and the resolution value is ignored. Consequently, when we talk about resizing for video, we are only talking about resampling.

Resolution is a scale factor used by a printer. If an image is 3000 pixels high and the resolution is 300 pixels/inch, the printer will print the image 10 inches high. If the resample checkbox is off, disabling resampling, changing the resolution will not change the number of pixels in the image. Only the size as printed will change. If the resample checkbox is on, it's a little more complicated as you are now allowing both scaling and resampling to occur and changing the resolution will also change the number of pixels in the image. This happens because Photoshop will attempt to keep the specified size as printed constant as the resolution is changed -- it can only do that by resampling the image and changing the number of pixels.

FYI, the resize dialog box shown in the article has been replaced in Photoshop CC. It now looks like this:

resize 1.JPG


Note that when Resample is unchecked, the Width, Height, and Resolution are all linked. Change one and the other two change proportionally since no pixels are changed. If we check the Resample box, the dialog box will change with only Width and Height having a link symbol (which can be unlocked reflecting the fact that you can resample asymmetrically). You can change the resolution without changing the print dimensions or change the print dimensions without changing the resolution, but in both cases the pixel dimensions will change. Here's what it looks like:

resize 2.JPG
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Re: image sizing vs. resampling

Postby Steve Grisetti » Wed Jul 01, 2015 6:25 am

Great discussion, guys!

That bicubic setting is all important in increasing or decreasing the size of a photo -- and it makes an incredible difference!

When increasing a photo's resolution, ALWAYS set the Bicubic setting to Bicubic Smoother.

When decreasing a photo's resolution (or making it smaller), ALWAYS set Bicubic to Bicubic Sharper.
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Re: image sizing vs. resampling

Postby momoffduty » Wed Jul 01, 2015 8:22 am

Thanks for this discussion. I was in a quandary on what to do for printing photos the other day. In Nik Collection I use the Output sharpener. You are to change the image size before using the Sharpener. For web, I would resize to the 2048 Long side X Constrain Proportions and use Display method in the Output sharpener (and adjust amount).

Now, for printing I am supposed to resize for the file size that is printed and then select the appropriate method and the lab's ppi. However, I went to the lab's info page and it said not to resize. They will resize using their methods. Since I had slightly sharpened overall in ACR and then in PS did selective sharpening, I did not use the Output Sharpener. I am using Mpix and curious how the photos will turn out. I have used the lab before and very happy with the results, but only did the sharpening/masking in ACR.
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Re: image sizing vs. resampling

Postby Bob » Wed Jul 01, 2015 2:57 pm

Regarding Steve's recommendation for using Bicubic Smoother and Bicubic Sharper, that's excellent advice when using the older version of Photoshop. But, Photoshop CC has added a new algorithm for enlargement called "Preserve Details" that's even better than Bicubic Smoother. Photoshop CC has also added an "Automatic" option for the resample method. Automatic will choose Bicubic Smoother if you are reducing the size and Preserve Details if you are enlarging. Pretty handy. If there is noise in the image, Preserve Details can make it more prominent. If you explicitly select Preserve Details, you will get a noise slider which you can adjust to smooth out and reduce the noise.

I really like the change in the Size dialog.

Here's a free except from a Lynda.com course by Deke McClelland which describes the new dialog box and shows Preserve Details in action.
[youtube]http://www.youtube.com/watch?v=S29pVImGNJE[/youtube]
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Re: image sizing vs. resampling

Postby Chuck Engels » Wed Jul 01, 2015 9:17 pm

That's awesome info, thanks Bob ! :TU:
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