Ask HN: How should I optimize images for iPhone/iPad app? | Hacker News Reader

Ask HN: How should I optimize images for iPhone/iPad app?

2 pointssixQuarks8 comments

I'll be publishing my first iPhone/iPad app early next year - in the design phase right now.

It's basically a photo gallery app. I have lots of unique photos that the user will be flipping through.

I need to know what size and format I should send the photos to my developer.

Can they just be JPEG photos at 72 dpi? The retina display specs kind of confuse me.

thanks

The iPhone retina display is 640×960 pixels and 326 ppi. So think about how many photos you expect to display at a time, with some space between them, and divide your photo dimensions accordingly. Also note that computers are generally better at displaying images crisply at dimensions that are powers of two so e.g. 64 pixels on a side is better than 63 or 65.

If you need to support older iPhone non-retina displays, the area is exactly 1/4 as large in roughly the same physical space. So to display the same number of images on an older iPhone display, make each image file 1/4 the area (e.g. a 64 pixel square on a retina display should be a 32 pixel square on older iPhones; this will be the same physical size but less detailed). You can send the developer files at both sizes (normal and 1/4-area), it is not difficult to install both with an app and let the device choose the best one.

"Also note that computers are generally better at displaying images crisply at dimensions that are powers of two"

Do you mean multiples of 2 or powers of 2?

Powers of two. If an image needs to be scaled down in hardware it will be successively split in half, so 256 becomes 128 becomes 64, etc. The machine can handle any image size, but doing so requires it to simulate divided pixels with new colors and this "error" becomes more noticeable as the image is scaled repeatedly. Powers of two divide cleanly.

I'm actually looking to have each photo display full size on the iphone. So should I make each photo 640x960 at 326 dpi in photoshop? Or would 72 dpi be fine?

Also, is JPG better or PNG?

72dpi is fine. Dpi doesn't really matter in most cases. And PNG is definitely the best bet. I'm not even sure iPhone apps support JPG. But 99% of people use PNG, so use that.

I believe that JPEG is the best format for making a good photo, whereas PNG is better overall for any other kind of image.

The OS actually supports a huge variety of images natively, including PNG, JPEG, TIFF, GIF, several Windows formats and X-Windows bitmaps.

PNG is definitely the smallest and fastest to render, especially when compiled by Xcode (Apple does some hacking to the file format so that the device doesn't have to do as many calculations at runtime).

Seems like this is something your developer should be answering for you.

I haven't hired them yet. I just want to get the photos ready first.

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