Let's start with an image:
As you can see from the image above, Krita has multiple different categories of tools. I'd even say it has too many tools, because this toolbox doesn't fit within screen when it's a single vertical line; I need to make it a bit larger, so it's two vertical lines, to access all the tools quickly.
Configuring the Toolbox
The toolbox itself can be dragged and moved around by clicking and dragging the handle at the top of the toolbox, and it can be resized by dragging the corners and edges.
In particular, it's possible to make the toolbox horizontal instead of vertical by dragging it to the upper and lower edges of the canvas.
Although this may sound a bit controversial—since many people prefer the single-window mode in GIMP, for example—I personally prefer to have the toolbox and dockers floating over the main window instead of docked. That's when they're floating, you can make them overlap, saving precious screen space.
Changing the Tool Icon Size
It's possible to change the size of the icons in the toolbox by right clicking on it. A context menu will appear with various size options.
Changing Alignment of Icons
You can also make the icons align to the right instead of the left by tight-clicking on the toolbox.
Hiding the Toolbox
It's possible to change the visibility of the toolbox by clicking on Settings -> Dockers -> Toolbox. But this is probably not a good idea, though.
A better idea would be to click on View -> Show Canvas Only on the menubar (keyboard shortcut: tab) to hide all dockers and display only the canvas. You can make them appear back by pressing the tab key again.
Common Tool Properties
Keyboard Shortcuts: some tools have default keyboard shortcuts. It's possible to customize the keyboard shortcuts of any of the tools through Settings -> Configure Krita -> Keyboard Shortcuts -> Tools -> Tool Shortcuts.
Mode: there is only one tool active at a time, and there is always one tool active.
Tool Options: many tools have advanced settings that can be customized in the Tool Options docker when they are active. It's displayed by default, and found on the top-right corner as a tab. You can change the visibility of this docker by clicking on Settings -> Dockers -> Tool Options.
Modifiers: some tools that activate when clicking have different effects when a modifier key is held before clicking (Ctrl, Alt, or Shift). Some tools that work by clicking and dragging have may also be affected by a modifier key being pressed after clicking.
Context: most tools operate on the currently active layer, which is highlighted in the "Layers" docker, and they're restricted by the current selection. An exception are tools that don't modify layers at all, such as sampling tools. Paint tools also depend on the currently selected tool, color, blending mode, whether erase mode is toggled, and other settings found in the toolbar.
Vector Tools
This section was moved:
Drawing Tools
This section was moved:
Resize & Move Tools
Next we have a few tools to resize and move things.
Transform Tool
Tthis tool lets you translate, rotate, scale, and shear layers, or selected areas.
It actually has a many settings in its tool options that would be separate tools in other applications. In fact, I don't know why THESE aren't separate tools in Krita but rectangle and circle are. For example, you can switch modes to perspective transform, cage transform, mesh transform, warp transform, and even liquify is a hidden tool in the tool options of this tool.
Holding Shift constraints the axes and angles while modifying transforms with this tool.
This tool is necessary to edit transform masks you create in the Layers docker.
Move Tool
This tool lets you move layers on the canvas.
Holding Shift lets you move only horizontally or vertically.
In its tool options you can specify the position of the layer as X, Y coordinates, which is useful if you are working with graphics copy-pasted from Inkscape and you want to place them at the exact same coordinates they were in Inkscape.
Crop Tool
This tool lets you crop the image, canvas, layer, or animation frame, depending on its settings in the tool options. I'm not exactly sure of what the difference between these options are.
This is mainly used to set the size of the document after you draw something on it.
Sample & Fill Tools
Krita has a few tools that can be used to sample parts of the canvas and fill parts of the canvas with a specified color or pattern.
Gradient Tool
This tool creates a gradient effect on the image. A gradient in this sense is an effect that starts in one color and slowly fades to the other color. You use it by dragging a line from a starting point to where the gradient ends.
As usual, it's possible to use the Shift key to constraint the angle of the line, and make the gradient perfectly horizontal or vertical.
It's possible to configure more complex gradients through the toolbar.
In the tool options, we can make the gradient pattern repeat, and change its shape from linear to radial, among other options.
One problem you may have working with gradients is a "banding" effect that occurs when colors slightly change from one to the next. This occurs when you work with an image that only has a color depth of 8 bits per pixel per color channel. This can be solved by changing the color depth of the image to 16 bit integers, or by enabling the dithering option in the gradient tool options.
Eyedropper Tool
This tool is used to sample a color from the canvas, replacing the current foreground color with the sampled color. By default, it samples the the color from the single pixel that you click on, and it will sample the color combined from all visible layers (i.e. the color you see on the canvas).
Its tool options allow you to sample the average color of a radius, and to sample the color of only the currently selected layer.
There's a setting to add to a palette the sampled color. This could help quickly create palettes from colors found in an image.
A common use case for this are digital artists who work with a layer for the colors of an illustration underneath a line art layer. To color an area with the same color you used in another area, you can quickly sample it and switch to a painting tool.
Note: in some applications, it's possible to sample the background color by holding a modifier key or by right-clicking. Krita appears to lack this functionality.
Colorize Mask Tool
This strange tool is used with Krita's colorize masks to quickly add colors to layers. It's a strange tool because the tool itself doesn't actually seem to do anything that the normal brush tool doesn't do. The only difference is that its tool options has controls to manipulate a colorize mask layer, such as updating the colorize mask, showing and hiding its output, and removing colors, although some of these functions are also available in the Layers docker.
It seems that some brushes work differently with this tool. From my observations, the colorize mask keeps a set of internal color channels for each distinct color you use on it, and displays the colors used in a palette in the tool options of the colorized mask tool. What's impressive about this is that if you draw with green color, for example, and then you draw with red color over it, you can "remove" the red stroke, and the green stroke will remain as if you never had drawn over it with red, which means they are actually separate "internal layers" for each color instead of a single layer.
It's actually kind of vexing because this is a great UI if you work mainly with flat colors, but it's employed in a completely different tool. For example, in both FireAlpaca and CSP, it's possible to have 8-bit layers with a color, so you can paint the whole thing one color, and then easily change to another color just by changing the color of the layer. These are tools designed for flat color styles, and they are subpar compared to this. OpenToonz, a free animation software, actually has a proper implementation of using colors as separate layers, and Krita is tantalizing close to having it, except it's in the wrong tool. The colorize tool is used to algorithmically fill areas of a layer with a color. It's not designed for flat-color coloring. Crucially, it seems there is no way to change the color of a stroke after you created it, even though it's apparent that Krita is keeping separate colors as separate layers internally, because you can't use brushes like "blur" and "smear" which would output multiple colors in a single layer, and texture brushes that change in hue also work differently with the colorize mask tool and colorize masks.
Smart Patch Tool
This tool lets you draw over an object on a layer and have an algorithm try to remove the object from the image by replacing the area where the object is with something similar to the image surrounded the object.
It doesn't work very well for objects that occupy large portions of the image, and it can take a few seconds to run, but it can quickly get rid of smaller objects. I'm actually very impressed with this tool. I tested it with a photo of a person standing on a beach, and it was able to completely remove the person's silhouette adding sand, water, and sky to where you would expect it. I don't think GIMP has a tool like this, which is kind of weird because generally GIMP has better tools for this sort of thing.
Bucket Fill
This tool works like the gradient tool, but fills an area with the foreground color or with a pattern, as configured through the toolbar.
[Work in progress.]
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