The best and easiest way to get all of the screenshot sizes you need with a single keystroke or call to the script!
Run in the Editor or in a Standalone build.
Presets included for all of the resolutions you need for the iOS App Store and Google Play.
Add or remove resolutions with ease and save them as a preset for later use.
Unity 5 (or newer) and Unity Pro can use Render Texture (GUI elements must use the main camera as the render camera) for optimum resolution.
Alternatively you can use the standard rendering method which uses bilinear scaling
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