Tap on the onion icon in the layer to enable the Onion Skin to see your previous and next drawings. 
Touch and hold the onion icon on the layer to open a window with onion skin options.
These parameters are accessible via the settings panel with more options.
To access more onion skin options, go to the settings panel and go to the Onion Skin tab. You can see up to 8 drawings (sheets) before and after your current drawing (sheet).
You can choose to display the onion skin as Colors or Images for either previous drawings or next drawings.
When choosing Colors, you can access the color wheel to set the color you want for either previous or next drawings.
When choosing Images, you can see previous and next drawings in their original colors. It is best to use the Images option when you have images without transparency.
Switch the sliders on and off with the buttons next to each slider. The values of the sliders can be changed independently.
The lock allows you to move all the 8 sliders at once. The option Render Position allows you to choose if the onion skin image is displayed over (Front) or under (Back) the current pixels.
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