scale raster image to meet defined ppi in Illustrator

Question

Background: I am using Illustrator to combine several raster images. I apply scaling and clipping masks to them. Next, I try - if possible - to avoid non-integer resampling steps during subsequent printing. I.e. if an image originally was e.g. 150 ppi and the publisher asks for 150 dpi, I aim for 150 (original dimensions) or 300 ppi (i.e. the size was downscaled by an integer factor of 2), since

• (a) most times I am very close to such integer factors anyway, or
• (b) picture dimensions need to be reduced to meet minuimum ppi/dpi without "upsampling".

Question: Is there a way to tell AI to scale image dimensions to meet a defined ppi (at least necessary in scenario b)?

E.g., an image which is 140 ppi should get slightly smaller to become 150 ppi (I am NOT looking for upsampling!). I know I can see the resolution via Document Info window / flyout embedded (or linked) images and manually calculate a scale factor (actual ppi / desired ppi), but I am looking for a solution that is automatic and has no rounding issues.

Question (optional part): Am I wrong to aim to avoid non-integer factors b/o quality issues in subsequent resampling steps where possible (scenario a)?

Edit for clarification: I am dealing with composite figures, where I need to fine-adjust the size (millimeters) of each individual subfigure (bitmaps, may be photographs, microscope images, MRI scans, ...). Subfigures are prepared at sufficient resolution that allow using them at integer multiples/fractions of original mm dimensions to meet publisher-desired ppi and to fit to the other subfigures. Once the subfigures are at an approximately useful size, I need to fine-tune widths/heighths to have them aligned in a tidy way. To this end, I prefer cropping (clipping mask) them since I was afraif of quality loss by resizing by non-integer factors. Since one needs to see how it looks like (how much I can crop, do sizes look balanced, ...) it is not useful to "calculate" the millimeter dimensions and do the rest in photoshop.

Answer 1

Photoshop > Image Menu > Image Size

Then if you must use Illustrator for some reason, place the newly adjusted image into Illustrator.

Answer 2

"If I increase the ppi in PS/new/from clipboard dialogue, the image seems bigger, but looks as pixelated"

YES!

There is a way to force Illustrator to keep the original size and resolution of your images, the same way InDesign and QuarkXpress does it when an image is updated.

That is, you still need to fix your resolution in Photoshop. But when you'll update your images, they won't show "low quality" as you described in one of the comment; they will be resized according to the real dimension and resolution you specified in Photoshop.

Your clipping masks might need to be adjusted though.

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1) Open Photoshop and change your resolution and size in the "Image Size" panel as you already did. Illustrator will prompt you to update your image when you open your file with that linked image.

2) Open your "Links" panel in Illustrator, select your image and go in the options of that panel.

3) Select the "Placement Options"; in the scroll down select "File dimensions."

Whenever an image resolution will change or the size of the original image, Adobe Illustrator will keep it at its original size.

Answer 3

No.

Illustrator does not interpolate any raster image and does not store "effective" ppi settings either. There is no feature, tool, or method to ask Illustrator to read, interpret, or show the ppi of an embedded or linked raster image therefore there is no mechanism for Illustrator to doanything regarding ppi on a raster image.

You may have some luck by looking into the Rasterino plug in from AstuteGraphics.com. It offers some additional raster image features within Illustrator.