If I have an iPad app with lots of images, is PNG still the best option?

Question

I am working on an iPad application which has hundreds of photo-quality images. I would have naturally assumed to store these images as JPEGs so as to optimize the app file size. However, Apple's guidelines state:

Use the PNG format for images. The PNG format provides lossless image content, meaning that saving image data to a PNG format and then reading it back results in the exact same pixel values. PNG also has an optimized storage format designed for faster reading of the image data. It is the preferred image format for iOS.

However, if I store the same images as JPEGs at 100% quality, the size of them drops to about half that of the PNG lossless versions.

Is there really that much of a performance hit to use JPEG instead of PNG? If I am viewing these images in a carousel or gallery style, do I really need to worry about the performance and use PNGs instead?

Thanks!

Answer 1

Regarding the quality PNG is good for application kind of images, but JPEG is preferred for photos. Choose the lowest JPEG quality that gives good enough quality for your images.

Regarding speed, size also matters. I have no IPad to test with, but the smaller file size to read from flash or network might very well out weight any additional decompression cost. The only way to find out is to measure on your actual device.

Answer 2

There is a performance consideration but while PNG is preferred for quality, given your application, I'd suggest JPEG would be preferable.

Pure performance isn't the only factor of interest or concern; an iPad has only a finite space available to it, and filling that up with image data that most users are not going to need or want seems preferable to using more computational power for most cases.

One other thing to consider - on a gallery, you are strongly recommended to generate thumbnails which give you the best of both worlds: the smaller, more accessible image for general use and the full original image for 'best'.

If in doubt, benchmark with both and see how big the difference is in your application - and if the difference is something you can live with versus the space saving, go with JPEG.