You can set the overall resolution of all the images in your publication in InDesign CS5.5. in the EPUB Export Options dialog (File > Export > EPUB). Click on the Image tab, and set the Resolution in ppi.
If you want to customize the rasterization options for an individual image, select it, then choose Object > Object Export Options > EPUB and HTML. Set the Resolution.
You could also use the later dialog box to purposely rasterize an object (for example, group all the objects on the first page and rasterize them for a cover image).
Thanks Steve, I’m aware of those options. Since I created the images at 300 in Photoshop, that’s what I specified in the Export Options dialog. Hence my puzzlement that the image quality is so much worse on viewing. Is there anywhere else I can tell InDesign to leave the images alone?
That does not match my experience when making EPUBs. I'd suggest making a sample image and choosing different variations of resolution (using Object > Object Export Options) to see the difference in output. I don't have much faith in ADE I'm afraid for image quality. I'd view the EPUB in the reader itself. For example, I usually copy the EPUB file into iTunes and view it in iBooks.
If you want to export your images at 300 dpi, InDesign does that. Check the images in the generated file (it's virtually mandatory to know how to do that, by the way). It is a safe bet that you will find that, indeed, your images are stored in there at "300 dpi" (since you don't know the resolution of the device people will use to view the file with, that is not really "the" resolution of your image -- if you think it is, then you are wrong).
All you are finding out now is that you are at the mercy of THE SOFTWARE that you are using to VIEW it with. What does it do with your super-sized images? Scale down? Using what algorithm? "Fast-but-reliable" would be my guess, since e-readers are not really well known for their incredible CPU capabilities.
Adobe's ADE is not really setting any high standards for e-pub viewers either. If you are using the beta, that is rather good (if you're not, you should), but the real touchstone is viewing your book on a REAL device.
With all said and done, why would anyone insert 300 dpi images in an epub? That's pretty ... optimistic ... at any rate.
My thanks for your responses.
Very good point to look at the images within the epub file. They are 72 dpi images, so it would seem that InDesign is indeed lowering the resolution. I had thought I tried every variation of the available settings within InDesign to avoid this but it would seem to not be the case. If you have any specific suggestions about what setting is causing this, that would be great.
Viewing the images in the target device is of course ideal. I'm creating this for a wide range of online bookstores, so what device it will be viewed on can't be known. Since ADE drives more than 50 devices, I had hoped that would at least prove to be a reliable base of some sort -- disappointing to hear that ADE can not be trusted in that regard. I had assumed I could proof the book in that before converting to mobi and proofing on a Kindle.
Why would anyone insert 300 dpi images, you ask? In its publishing guidelines, Amazon says: "To future-proof the content, save images in 300 dpi" Is this then bad advice on their part? Elizabeth Castro echoes this recommendation in her excellent book on the subject, by the way. It's rather difficult to know just what to do, I must admit. But I guess we're still in the early days of eBook creation, with best practices still being in a state of flux.
Once again, my thanks for sharing your experience.
Very good point to look at the images within the epub file. They are 72 dpi images, so it would seem that InDesign is indeed lowering the resolution.
If these images are PNGs, well, resolution is not stored in that file format (I'm not sure about JPEGs). Hence my befuddledness, also towards Amazon -- surely an 1" x 1" image at 300 dpi will not display at that size *on any device*? (Currently on the market, Februari 2012, etc.) It is my understanding all devices and software so far will display bitmap images at their own screen resolution, that is, iBooks will display this hypothetical 300 dpi image not at a size of 1 inch but rather as 300 x 300 pixels (however big or small those are).
You mention Amazon, and I believe they have their own file format -- was that Kindle? How does a Kindle handle resolution settings? And remember that if ID saved your images as PNGs, ALL resolution info already has been lost.
The images are JPEGs. I can't answer your questions regarding image display I'm afraid. I wish I could.
Yes, Amazon does not use the ePub format, complicating life somewhat. It provides information about its publishing program here.