png on EPUB

On thinking about this a bit more, I note two possiblities:

1. That despite all attention given to the PPI, the exported file is simply too small, in actual pixel width, to carry this detailed content. At a nominal screen density of 144PPI (which is the typical effective resolution of most EPUB readers and apps, regardless of actual screen rez or other factors), make sure this image is at least 144 x [desired screen display width]. At a nominal six inches, this image should be at least ~900 pixels wide.
   
   
2. That because the image is too small, Apple reader is expanding it to fit screen space and thus compounding the blur issue. That is, the effective PPI is 100, 75, or less and the blur thus no suprise.

I'd first make sure the image is exporting to at least 1000 pixels in width, with exact values to be adjusted by the export PPI (150 or 300).

I'd then — as sauggested before — try another, vanilla reader that does not automatically try to expand all images to screen width, which is what I suspect is the real problem here. Without that "feature," your problem would have been a small image, and the solution much more obvious.

If I may be so bold in stating the following: both the question and the answers here need clarifying.  🙂

First, a couple of ground rules must be defined in regard to how InDesign treats images in documents that have a digital platform intent:

• PPI is not important in Illustrator or Photoshop when preparing images for digital output. This includes desktop screens, mobile devices, tablets, and so forth. We work with pixels, not PPI or DPI. Only for print jobs do we deal with PPI/DPI.
• PPI is however important to InDesign because the way InDesign calculates things it "thinks" in PPI even when selecting a digital intent document template. So while the rest of the world disregards PPI for digital bitmap-based imagery, InDesign must use a standard PPI setting to make sense of the size of imported bitmaps.
• InDesign assigns therefore an arbitrary 72 PPI to images that translate 1:1 to the digital document template that a user picked.
• In InDesign all document templates that are listed under Web and Mobile fall in the category of digital intent.
• When the effective PPI of a placed image equals 72PPI that image's resolution matches the device's native resolution one on one 1:1. 1 pixel in your bitmap image matches 1 pixel in the document canvas.
• To publish an epub with bitmap images at 100% native resolution to our main digital target platform we must export our epub at a PPI resolution of 72PPI. (NOT 150PPI. NOT 300PPI !!!)

If this all sounds rather obscure to you, then you find yourself in a right state of mind. 😛

A simple example to explain how this works.

We create a new document meant to be published to regular iPads as the highest quality digital platform. We can still use the produced epub on lower resolution devices, of course (but you have to realize that images may have double the resolution actually required for those displays).

An iPad features a retina screen with a resolution of 2048 by 1536 pixels. InDesign interprets this as 2048px * 1536px @ 72PPI.

When we import an image and place it we should check the effective PPI in the Links panel. If the placed image (after transforming to the required width, height, and cropping) is much lower than 72PPI we have a problem. Ideally it should be 72PPI or higher!

To prepare bitmap images for our iPad output platform, we completely disregard PPI. We ONLY look at the pixels involved:

• If we require a full width image in portrait mode: in Photoshop (or equivalent image editor) the resolution should be a minimum of 1436px.
• If we require a full screen image in a landscape page: 2048px by 1536px.
• If we require a quarter width image at landscape resolution: 1024px width.

...and so on. DO NOT WORK WITH PPI in Photoshop or Illustrator. Calculate pixels!!!

It is actually not necessary to go through these steps. While ideally we provide InDesign with a finely crafted bitmap, as long as the effective PPI of the placed bitmap is equal to or higher than an effective PPI of 72, we can always export the epub with an image resolution setting set to 72 PPI in the Conversion Settings.

Here I imported a PNG bitmap in an iPad document. The original PNG's resolution is 2008px by 1782px. Too high for the iPad Retina in portrait orientation, and that does not matter! The effective PPI is 99, and that is more than 72PPI. So when we export to an epub, we simply ensure that the conversion is set to PNG and 72PPI. This will convert our imported bitmap to and EXACT 1:1 version at the native resolution of the iPad retina (because it is placed aligned to the default margins in portrait mode, the final pixel resolution of the width will be 1536 - 36px - 36px = 1464px. The height is inconsequential in a reflowable Epub (up to an extent: depending it will be displayed scaled down to fit...)

So. Back to your @Hosun image prep workflow.

You prepare the vector illustration / diagram in Illustrator. FORGET PPI. Only worry about pixels.

Your options are:

1. To create a document at the required pixel resolution in Illustrator. Or to convert the document to the resolution that you need. Then export the PNG bitmap directly from Illustrator for import in InDesign.
   
   This will work. Personally I avoid this approach, because Illustrator is not very good at converting vector art to crisp bitmaps. Or:
   
   
2. create a document at TWICE the required pixel resolution in Illustrator (or convert the document to twice the required resolution in pixels). Then export the PNG bitmap, open in Photoshop and scale it down (down/resample) with Bicubic Sharper (reduction).
   This will create a more crisp looking bitmap. Then export as PNG from Photoshop, and import in InDesign.
   No need to fiddle around with Smart Objects. Which will not produce as good a conversion anyway.
   
   (PS unfortunately Photoshop does not feature more appropriate downsampling algorithms such as CatmullRom. I prefer this because it results in crisper looking vector to bitmap conversions. Alas, not possible in Photoshop...)
     
3. While (2) may give the best result, there is a much easier workflow that does not require us to prepare a bitmap at all! We just import the SVG from Illustrator (or the AI file)!
   
   I know: you @Hosun stated that SVG is not allowed. Have no fear, because there is a simple trick.

• Place your vector file directly in InDesign. Scale to the size you want on the page.
• Select the frame with the SVG, and select Object-->Object Export Options.

• Rasterise the content, select PNG as the format, and set the resolution to 72PPI.
• Click Done.

Now export your epub once again with the conversion setting of 72 PPI (actually, the value no longer is applied to this SVG because, you set up an object export override that already tells InDesign to export the SVG as a 72PPI PNG!).

Et Voila! The generated PNG version of your diagram graphic is export at the EXACT required bitmap pixel resolution for the iPad at that screen (document) orientation (landscape or portrait - your choice).

The proof is in the pudding: when I crack open that epub (best done with https://www.docdataflow.com/ecancrusher/ because plain unzipping is not quite the right approach) I discover a converted PNG image of that SVG at a resolution of 1465px (the extra 1px is a rounding error introduced by InDesign) because I set up an iPad document in portrait mode.

Finally, it is important to note that if I had used the Google Pixel 2 as the document template, the width in pixels of my exported images would be 1080px. Or 1920px in landscape orientation.
Similarly, the iPhone 8/7/6 template would have resulted in a lower 750px portrait width resolution. The generic Android platform in landscape mode would yield 1280px max width.

And so on.

It is important to decide which digital platform will be your highest end publication target: if the epub is prepared for Kindle Fire/Nook, the portrait resolution would be a width @ 600px. If you export this at 72PPI in the epub export conversion settings, the images will be too low resolution when viewed on an iPad retina screen, which can display bitmap images at more than twice that resolution in portrait mode.

A way around this is to increase the export conversion setting to 150PPI. That will ensure that in the above case a max width image imported in a Kindle document template in portrait orientation mode is exported at 2.0833333 the original size --> 600px * 2.08333 = 1250 px (rounded). This is still less than the 1536px max portrait orientation resolution of an iPad Retina, but will display much better.

BUT! For this to work all placed bitmaps would have to be at an effective minimum PPI of at least twice the 72ppi.

And it is still not possible to reach that iPad target resolution. The only option left would be to switch to 300PPI, but that is COMPLETE OVERKILL and may result in an error message stating that your images use too many pixels. Not to mention the impact on book file size and performance (it WILL slow down readers and reading devices!).

So in the end, as it turns out a much simpler and effective strategy it to simply target the highest end digital platform in InDesign from the very start, and just work with the standard 72PPI effective resolution and export to 72PPI as well. And outside of InDesign PPI is completely irrelevant for bitmap images and digital screen work. Always think in the resolution of the target platform's screen resolution when preparing bitmap images within this context.

Still with me? Pat yourself on your shoulder, and have a nice cup of tea. Or coffee, if that floats your boat. 😉

Anyway. I hope this clarifies some of the confusion here and how InDesign treats bitmap images in digital document templates.

(print templates always work with 300PPI, by the way.)

If it is a vector source like from Illustrator and the results should be files for screen like ePubs or screen pdf save your Illustrator as SVG and place that in InDesign. 

Neither PNG or SVG should be usedd in a print workflow. 

How your PNG looks like affter the export from PS - on you local drive?