InDesign: Placing 300 ppi image at 300% for intended 100 ppi output?

Question

I've worked with print graphics for many years, and never place a 300 ppi image into InDesign at any size larger than 120%. However, I have been working on some large display banners, and the printer tells me they need only 100 ppi. So I have been creating 100 ppi versions of my images and placing them at 100%. Question: Instead of creating new versions of the images for this purpose, could I place my 300 ppi images at no larger than 300% for effective resolution of at least 100 ppi? In other words, can I "trust" InDesign to effectively mimic what I'd be doing in Photoshop? I don't want to end up with visible pixels or too much blurring, but I'd also love to save myself some extra work if it's not needed. Thanks.

Answer 1

You can....

However..... I'm sure you are aware that your question is kind of a "special" case.

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_{(You may know this, but it doesn't hurt to post it for others who may be reading. I don't intend to be condescending.)}

In general....

If you resample in Photoshop, then Photoshop interpolates pixels to try and keep quality the same or at least not degrade the image. In other words, Photoshop determines new pixel data (color etc) when resizing to achieve the best possible appearance.

Resizing placed images in InDesign does not interpolate their pixel data. It's a straight scale without any interpolation in InDesign.

You want good interpolation if maintaining the pixel density (quality) of an image is important upon resizing that image.

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All that being posted.... For a 1:1 output after scaling to a larger size, there's probably no need for much interpolation. You aren't asking the app to maintain pixel density, which is where interpolation is really necessary.

What many do is merely work at scale within InDesign - i.e. use 300ppi images at a 1/3 scale. Then output at 300%... which is essentially the same as enlarging the 300ppi image 300% within InDesign then outputting at 100%.

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So, yes. I would be comfortable with enlarging in InDesign and outputting at 100%. The "effective ppi" is indeed what InDesign is going to output at 100%.