I know using images with different DPI is a fairly common practice in Android. But I don’t really see the purpose of it, why can’t we just use a ultra huge image and then scale it in the program? I know we probably need to do extra calculation if we don’t images in different DPI, but besides that, is there’s any other reason?
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Tim and a.d. have the correct reasoning for you. Here’s a visual example if you’d like to see a comparison between something rescaled or drawn at the correct size. Effects are always more noticeable if there are very small details.
The first is a large 200×200 (we’ll call it xhdpi), and the second is simply the same image resampled for mdpi (75×75). The third is redesigned to the same specs but starting at 75×75. As you can see, the text on the second is significantly blurred, and there are some artifacts along the edges of the shapes where rescaled.