Two images composition in iphone app

Question

Hi here is what I want to do in my app. Suppose I have two UIImage, the same size. At the beginning, I show the first one. The user start to use their finger touch and move on the screen. For all the locations users touched, I would like to display the correspondent pixels from the second image. How can I do it?
Thanks,
ff

Answer 1

In your scenario, you can just use a regular 2x image and use UIImageView to kick-in and scale-down images 50% during runtime. This will work at the API end and your apps will have no problems getting approved. People do this.
But, the drawback:
You should ideally determine the device resolution at launch and scale down accordingly to avoid untidy or pixelated images
Some smaller graphics will suffer - like buttons, icons, etc... - as Apple doubles pixels in each direction. Considering this, using just a 2x image approach may not be the best way of ensuring 'crisp' images
On the other hand, you could simply use both 1x and 2x images and let the device pick up the image that is optimal, depending on its resolution. While this will certainly result in a larger sized app, it wont be a memory crunch on the device since the API picks up the relevant image and only loads that at runtime. It will not load the other image that is not optimal for the device, depending on where it is run.
On a sidenote, there's a beautiful article here that I think you should read: http://bjango.com/articles/designingforretina/
I want to include as many excellent images in our apps as we can include in the 50 meg size limit
Yes, Apple increased an app-size suitable for 3G download to 50MB from the older 20MB since they knew most existing apps would at least double in size due to the 'retina' optimized images which are twice the original resolution.
However, do note that this 50MB limit applies only for downloading via your cellular operator on 3G/ LTE. Users can still download the app from App Store via Wi-Fi if your app exceeds 50MB. One more thing to consider is that with the data prices going skywards everywhere globally, users will anyway be cautious with download large apps and most of them will use Wi-Fi (I do and I know many of my friends do) when they want to get an app that is considerably larger in filesize.
Trust this helps.