Display details on the iPad

Perhaps you all know this already, but it was new to me. I discovered that it is important what size (pixel dimensions) I make JPEG images for display on my iPad.

In preparing a short iPad slide show for a presentation, I simply uploaded some images that were roughly 4000x3000 pixels. I chose these for no other reason than they were already done. Once uploaded, they looked okay, but a bit "crunchy" in the details. In particular, the text (these were folio page images) was a bit rough.

Because the file sizes were a bit large, the slide show transitions dragged a bit. I decided to downsize the images to 1024x768, the native resolution of the iPad screen. I was amazed how much better the images looked! The image details were more natural looking, the text was clean and readable, the gradations much smoother — and the smaller file size made the slide show play much faster.

It appears that when the iPad has to crunch a file to fit the 1024x768 screen dimensions, something's got to give and in tossing out or averaging or whatever it does, it ends up creating a screen rendition that can reduce the quality of the image. This reminds me about that business of using only whole number multiples when viewing Photoshop images*

Next up, I need to see if this is true in rendering PDFs, too.

 

* If you don't know about this, here's a quote from David Blatner:

When you're viewing at an integral multiple of 100% (25%, 50%, 200%, 400%, and so on), Photoshop displays image pixels evenly. At 200%, four screen pixels (two horizontal, two vertical) equal one image pixel; at 50%, four image pixels equal one screen pixel, and so on. However, when you're at any "odd" percentage, the program has to jimmy the display in order to make things work. Photoshop can't cut a screen pixel or an image pixel in half, so instead it fakes the effect using anti-aliasing. Magnifications lower than 100% can give you a distorted view of resolution-dependent effects, such as sharpening.

The moral of the story is that you should always return to the Actual Pixels (100%) view to get the most accurate view of your image.

 

I think something related to this is what is happening with my files in the iPad that are larger than the native screen resolution.

Comments

[Peter]

Here's another reference I found useful...

http://thedigitalstory.com/2010/04/will_the_ipad_squish.html

Bottom line: consider sizing photos at 2304 x 1536 to allow for zooming.

Peter.

[Godfrey DiGiorgi]

Hi Brooks,

Absolutely true. To scale very large (4000x3000 pixel) images down to 1024x768, a standardized algorithm has to lose something. Whenever I do that with LR or PS or whatever, I have to resharpen for that output pixel dimension to maintain the perceptual quality of the image and tune that sharpening to the particular image.

Similarly, scaling up has its losses too.

For a display that gives people the ability to "zoom in a small amount", what I've found is that scaling and finishing my work to 1168x876 pixel dimensions works well for the iPad with minimal losses. If you don't recognize the numbers, that's scaling up 1024x768 by the square root of 2, doubling the number of total pixels. The transform back to 1024x768 for full frame display nets little loss this way.

Experiment and see what works best for you. :-)