I realize that there are a lot of questions like mine and I read them all, but something is fighting in my head trying to find the connection between the written answers and my actual photoshop's behavior. So everywhere people say that when you change the PPI without "resample" checked - you will change the size of the document and opposite - if you change it with "resample" checked - you will change the amount of pixels (add or subtract pixels). Now maybe I am really distracted and not paying attention to what is written BUT when I uncheck "resample" and change the PPI the image doesn't change at all, but when I do check it and change the PPI - the image size changes so it's opposite to what is written in every blog... I am sure that there is something I don't logically understand in all this.
Resample = Interpolate pixels to add additional data
With resample checked. . .
Adjusting the PPI setting - the image dimensions do not change. Photoshop interpolates the pixel data to add or subtract pixels based upon your PPI adjustment while maintaining the existing width and height of the image. In short, Photoshop adds pixel data when needed.
Adjusting the image dimensions - the image dimensions obviously change. Photoshop interpolates the pixel data and adds or subtracts NEW pixels based upon your dimension adjustment - but the PPI setting remains the same. So Photoshop interpolates in an attempt to retain the existing PPI setting based upon your new dimensions. Again, Photoshop adds pixel data when needed.
Without resample checked . . .
Pixel data (PPI value) is tied to the image dimensions. You can't change one without altering the other.
Adjusting the PPI setting - The image dimensions will change. Photoshop does not interpolate the pixel data. That is to say it won't add or subtract pixel data to match your adjustment. It uses the existing pixel data and either condenses or expands it based upon your desired PPI - therefore, for a higher PPI value the existing pixels are condensed resulting in smaller image dimensions. For a lower PPI value, the existing pixels are "stretched" or expanded resulting in larger image dimensions. Photoshop adjusts existing pixel data without adding new data.
Adjusting the image dimensions - The PPI setting will change. Photoshop does not interpolate the pixel data and again, won't add or subtract pixel data to match your adjustment. It uses the existing pixel data and either condenses or expands the data based upon your desired dimensions. So, if you input a smaller dimension, Photoshop condensed the pixel data to fit that dimension, resulting in a higher PPI value. If you input larger dimensions, Photoshop expands or "stretches" the existing pixel data to fill your new dimensions, resulting in a lower PPI value. And again, Photoshop adjusts existing pixel data without adding new data.
Ok, here's the
Image Size dialogue:
Notice how the settings are divided into groups?
Pixel Dimension group contains the
Height in pixels. These are the only real attributes of the image. The actual pixels saved in the image.
Document Size group contains
Height in physical dimensions and the
Resolution. These are only settings/instructions and can be seen as the "recommended" size at the "recommended" resolution. When you place the image in InDesign with a single click, this is the size the image will get. But you can choose to ignore the "recommended" size and scale the image as you please.
Resample Image is ticked off, the
Document Size can be set to what ever you want. It won't change the actual pixels of the image. You can say that the
Document Size has changed, but not the image itself.
Resample Image is ticked on and the
Document Size is changed, the pixels of the image will change.
Ok scott pretty much nailed the technical details let us look at WHY it works like this. Realize that PPI is not a feature of an image, but a order to the production system. But see theres many (and any number of mixed and confusing) ways of using PPI:
PPI is used to make physical units meaningful. Then use real units to define what you are doing. So you as a designer are thinking that ha this line is X units long or image should be Y units wide.
As a naive quality metric for the output quality (that is often misused).
It is simply irrelevant in your production pipeline.
Likewise you can have different interpretations of what image size means:
- Number of pixels in the image
- Size of the document produced by a printer
- How much disk space you use
Now becasue of this you can have different goals and reasons for doing the scale. Now there are many possible scenarios possible here some of them might go like this:
Client wants to produce a bigger poster, and wants you to submit a production instruction to just make the image bigger. Understanding that well the bigger poster would most likely be observed from further away would more than offset the quality drop and the client inst really paying much anyway you just spread the pixels on a bigger area. So you just override the metadata of your image to produce a bigger print area without altering the document at all, so you uncheck interpolate.
Yes, the image didn't change! Only the preparation order changed for the downstream system which just makes the image bigger or smaller with the data that you got*. And thus the size reported for the images physical dimension changes, because you changed the interpretation of the images size.
You have draws a small thumbnail becasue its quicker or it just happened to be a interesting ink spot at the edge of your canvas. Now you want really to add the details into the image. But to do more details you need more pixels. So you would do the scaling or PPI change with interpolation checked.
This gives you some space to work with. So you can now use your time to fill in the details to finish the artwork. You now have more pixels, but that is what you wanted more data!
So in the first case you are interested in just outputting a image bigger this does not necessarily mean the image needs more pixels. See the PPI just ties the physical dimensions into what you think the image is going to look like in production.
Now in the second case you needed more pixels. Whether or not you really cared for how BIG the image is going to be when produced is irrelevant. Or indeed if your production system cares for the PPI dimensions at all. Its simply irrelevant. It could have been your target, yes then the dialog helped you calculate that. If not then you'd just switch the unit to pixels and percent and ignore the others settings.
*This may or may not be optimal, but that is a much harder question ti answer so lets not go there.
Resize nor change DPI does without resampling not affect to pixel dimensions, but they affects to the size in millimeters or inches which is invisibly stamped inside the file. There are plenty of graphic software which reads that size and place the image on a page just in the written size.
InDesign users remember well they have placed a high resolution image - say 5000 pixels high - to a page and only a small portion of the image fits to screen, because the image has wrong physical dimensions. It was probably 72 or 75 DPI and physically over 1,7 meters.
I have taken a habit to walk through all photos to be placed into a printwork and in that work also set all to 300 DPI without resampling. This prevents the annoying zoom-outs to see what I was placed and also reveals low-resolution images which I nearly allways get from the customers among the usable ones.