Optimizing UI Image Asset-Android has categorized the mobile displays into various density buckets to ease the image optimization workflow.
Density Bucket Scaling Factor Resolution
mdpi 1.0 360 x 640 px
hdpi 1.5 540 x 960 px
xhdpi 2.0 720 x 1280 px
xxhdpi 3.0 1080 x 1920 px
xxxhdpi 4.0 1440 x 2560 px
Now suppose you have a Mobile UI designed in Photoshop (or whatever program you prefer) at the dimensions of 720 x 1280 px and you have an image asset that you need to export for all the densities that has a pixel dimension of 100 x 100 px. You already know that it falls under the density bucket xhdpi(2.0). Now head over to http://androidpixels.net and in the Pixels column in front of xhdpi enter the value 100. It will make other values change and you’ll have the pixel dimension for other densities that you need to create the image asset at. This scenario works when you manually need to create image assets and when the image asset is not in a square ratio.
There is another scenario in which you might need to extract all the image assets in a design. If you follow the above steps it will take a lot of time depending on the number of image assets and will definitely lead to frustration.
I have figured out a way around this problem. You just need to follow the below mentioned steps.
You already know that your design fall under xxxhdpi. Suppose one of your image asset is at 200 x 200 px. Enter this value in front of xxxhdpi and next to it you’ll get it’s DP value or optical size. In this case it’s 50 DP.
Now head over to Android Asset Studio and click on the 4th link that says Generic Icons.
Now on this screen set all the options as illustrated.
Navigate to your Downloads folder and find the zip file, Extract it and you’ll find all the image assets placed in the right folders in respect to the different density buckets.