I need help writing an Objective-C function that will take in an array of UIImages/PNGs, and return/save one tall image of all the images stitched together in order vertically. I am new to this so please go slow and take it easy.
My ideas so far: Draw a UIView, then addSubviews to each one's parent (of the images) and then ???
The normal way is to create a bitmap image context, draw your images in at the required position, and then get the image from the image context.
You can do this with UIKit, which is somewhat easier, but isn't thread safe, so will need to run in the main thread and will block the UI.
There is loads of example code around for this, but if you want to understand it properly, you should look at UIGraphicsContextBeginImageContext, UIGraphicsGetCurrentContext, UIGraphicsGetImageFromCurrentImageContext and UIImageDrawInRect. Don't forget UIGraphicsPopCurrentContext.
You can also do this with Core Graphics, which is AFAIK, safe to use on background threads ( I've not had a crash from it yet). Efficiency is about the same, as UIKit just uses CG under the hood. Key words for this are CGBitmapContextCreate, CGContextDrawImage, CGBitmapContextCreateImage, CGContextTranslateCTM, CGContextScaleCTM and CGContextRelease ( no ARC for Core Graphics). The scaling and translating is because CG has the origin in the bottom right hand corner and Y inscreases upwards.
There is also a third way, which is to use CG for the context, but save yourself all the co-ordinate pain by using a CALayer, set your CGImage ( UIImage.CGImage) as the contents and then render the layer to the context. This is still thread safe and lets the layer take care of all the transformations. Keywords for this is - renderInContext: