Undo in painting apps like Penultimate and iDraft

Question

In apps like iDraft and Penultimate, they perform undos and redos very well without any delay.

I tried many approaches. Currently, my testing app writes raw pixel data directly to a file after each undo using [NSData writeToFile:atomically:] but I am getting 0.6s delay.

Can anyone give some hints on it?

Answer 1

Keep your undo stack in memory. Don't write to disk for every operation. Whether you keep around bitmaps or vectors, your file ops shouldn't be on the critical path for every paint operation you do.

If your data model is full bitmaps, keep just the changed rect for undo/redo.

Answer 2

As previously said, you probably don't need to write the data to disk for every operation, also in a pixel based case, unless you are trying to undo a full screen filter all you need to keep is the data contained within the bounding rectangle of the brush stroke that the user performed.

You can double buffer your drawing, i.e. keep a copy of the image before the draw, draw into the copy, determine the bounding rect of the user operation, copy and retain the appropriate data from the original (with size and location information). On undo you take that copy and paste it over the modified area.

This method extends to redo, on undo take the area that you are going to be overwriting and store it.

Answer 3

I don’t know iDraft nor Penultimate, but chances are they have a simpler drawing model than you have. When writing a drawing app you can choose between two essential drawing representations: either you track raw pixels, or you track drawing objects like lines, circles and so on. (Or, in other words, you choose between pixel and vector representation.)

When you draw using vectors, you don’t track the individual pixels. Instead you know there should be line between points X and Y of given width, color and other params. And when you are to draw such a representation, you call Quartz to stroke the line. In this case the model (the drawing representation) consists of a few numbers, takes little memory and therefore you can have many versions of a single drawing in a memory, allowing for a quick and convenient undo and redo.