One nice thing about the Move tools in Paint.NET is that they implement something I call fine-grained history. When you go to edit a selection or move its pixels around, you get 1 history entry for each adjustment you make. This allows you to move it around, rotate it, etc. and then when you press “Undo” it only backs out of the most recent adjustment you made. Contrast this to the Gradient, Line/Curve, and Text tools which don’t keep track of each edit you make and can only undo things wholesale.

Recently I’ve been working on adding this to the Gradient tool for 4.0. This support will then make its way into the shape tools, and probably the Text tool as well (also, shapes will be undergoing an overhaul soon). The reason I’ve been working on the Gradient tool is that it is a fairly simple tool and this allows me to get the code for fine-grained history working without having to worry about as many “moving parts.”

Here’s a screenshot showing the Gradient tool having gone through it’s initial “draw” operation, followed by two “edit” operations:

(click for full resolution version)

Once this makes its way to the new Shapes tool (not yet written unfortunately), you’ll be able to: 1) draw a shape, 2) move, rotate, resize the shape, 3) change the colors, 4) change the stroke, 5) change any other properties, and then 6) finish/commit it to the layer. Currently the shape tools in v3.5 are fairly barbaric and don’t even let you change their color or other properties once you’ve pressed down the mouse button. It can be quite painful if you are consistently off by a few pixels and have to constantly redo your Rectangle until it’s just right.

There are some other important benefits to doing this. I’ll be able to significantly reduce memory usage, for starters. In Paint.NET v3.5, the tools are implemented in a rather cumbersome way. While you draw, it’s modifying the layer data so it contains what you’re drawing … then it draws that to the screen. Then, when you move the mouse it has to copy back the original contents of the layer before drawing the new stuff. When you’re done, it has to copy back the original pixels, save it to disk, then redraw everything one last time. This is implemented with a very carefully debugged protocol that requires all sorts of data to be copied around various buffers before, during, and after any edit operation. Any bug in the protocol for drawing, copying, and emitting history entries can lead to data loss.

With the new system, the gradient (et. al.) draws itself using a layer overlay, which only affects the rendering pipeline and not the image itself. Undoing the edit before it’s committed is a simple matter of throwing away the overlay. When it’s time to commit the drawing, the code just says “hey I changed this rectangular area” and then a simple diffing algorithm copies away only the parts of the layer that changed. There’s no need for me to manage complicated geometry masks and dirty regions any more (other than what’s needed for an active selection, of course). All the large and temporary (and permanent, ack!) buffers are simply no longer needed.

This is all very important! Reducing memory usage is very important for an application like Paint.NET, especially on 32-bit systems. This also removes another barrier toward migrating to a fully tile-based memory allocator, which then opens up doors for many other very, very cool features.

Another benefit to doing this is that it moves the tool rendering code off of the UI thread and into the rendering pipeline (which was already moved off the UI thread for 4.0). The UI will always be responsive while you’re drawing, even if the canvas is still “catching up” to what you’ve done. It’s pretty cool to be able to throw around the Gradient tool’s “handles” (aka “nubs”) and see them responding at a much higher framerate than the canvas itself. If you’ve ever had to work with an image that is very large, or has a lot of layers, or both, then you probably already know that this will be a great thing.

Also, right now the Move Selected Pixels tool is a monster (for me that is; not for you). While it does work perfectly well, the code is wildly complicated and fragile. Adding or changing any features would be a recipe for disaster. By migrating it to this much simpler system, I’ll be able to reduce memory usage further, increase performance and quality, and remove any “hitch” you may notice at the start of a move operation. The reason the “hitch” happens is that the history data (the layer’s pixels) must be saved off into a temporary buffer before any rendering can be done. Afterward, there’s a frightening dance of copying pixel data from the layer to a scratch area, and then transforming pixels from a 2nd copy back to the layer, and then undoing it all before redoing it, etc. The nightmare gets worse when history entries come into the mix. The code is basically the worst time travel paradox you could possibly imagine – I can’t honestly say I fully understand how it works, even though I wrote it. What I can say is that it works, but only because it’s been thoroughly and exhaustively tested, debugged, and bugfixed.

Oh, and you’ll notice in this screenshot that I’ve finished up the last bits of UI reorganizing. I’ve done away with the Window and Utilities menus. Instead, each of the floating windows has its own toggle button at the top-right. The gear icon opens up the Settings dialog, and the question mark icon gives you the traditional help menu. Anything that was in the Utilities menu has either been moved into the Settings dialog, or removed. I‘ve also increased the size of these icons to 24×24 because I found they were just too darn small at 16×16.

Within the Settings dialog I’ve added a Diagnostics page which gives you a peek at your system information as Paint.NET sees it (basically the information that’s printed at the top of a crash log, and more). It will also give you access to the crash logs folder and some other simple things that should make troubleshooting a bit simpler. Oh, and for those who don’t like the “dancing ants” (animated selection outline), I’ve included a checkbox to control whether this is on or off (not on the Diagnostics tab though).

In time, I hope to elaborate further on what I mean by “this stuff opening doors for other very cool features.” I’ve been throwing around ideas for the last few days and it’s exciting how much things are radically simplified by all this. I’m hoping that 4.0 will serve as a solid basis for many, many features over its lifetime. 4.0 won’t have all the features everyone wants, but it will enable me to finally add so many more of them!

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