How to Add a Filter in Krita

Let's start with the destructive menubar method, which is how it works in most applications.

Destructive Filters with the Menubar

To apply a filter destructively in Krita, follow the following steps:

1: on the "Layers" docker, select the layer that you want to apply a filter to.

Note: filters can only be applied destructively to a single layer of the paint layer type. You can't apply a filter to a whole group at once, and you can't apply it to multiple selected layers at once. They also won't work with text layers, vector layers, clone layers, etc. Only the normal, basic, paint layer.

2: on the menubar, you can find a "Filter" menu item between "Select" and "Tools." Clicking on it will reveal a submenu of items with that are categories of filters. Each category has its own menu items that are the actual filters. Clicking on one of these menu items will apply its respective filter. Some filters won't apply immediately and will instead open a dialog box so you can configure their effects. As per convention, menu items that open dialog boxes end in an ellipsis (...), e.g. Filter -> Enhance -> Sharpen applies a filter immediately, while Filter -> Enhance -> Unsharp Mask... opens a dialog box.

The dialog box has a few interesting parts.

On the top, you can find controls to manage filter presents. Under them, there are the settings of the current filter. On the bottom-right, the OK and Cancel buttons. If you press OK, the filter will be applied. On the bottom-left. there's a button to display the list pane of the list-details layout. If you click on it, a pane will appear on the left where you can select a different filter from the one you picked from the menubar.

All of these can also be found in the other methods to add filters that we will see. However, there are three things in this dialog only exist on this menubar method. We'll learn about them below.

Non-Destructive Filter Masks

To apply a filter non-destructively in Krita using a filter mask, follow the following steps:

1: select the layer that you what to apply a filter to. Unlike the destructive method, you can select a group, vector, text, or clone layer this time besides layers of the paint type.

2: on the "Layers" docker, there's a button to add a new layer with a "plus" icon. On its right, you'll find a down arrow button that opens a popup menu. Click on it to open the popup menu.

3: on the popup menu, select "Add Filter Mask...." The same dialog from the destructive method will appear. Clicking OK this time will create a filter mask and attach it to the currently selected layer.

Non-Destructive Filter Layers

To apply a filter non-destructively in Krita using a filter layer, just repeat the steps above, except this time you select "Add Filter Layer..." instead of a filter mask.

Differences

There's a few differences between these three methods.

Destructive vs. Non-destructive

Multiple Selection: it's not possible to use the destructive method with more than one layer at once. If you want to apply the destructive filter to several layers, you will need to merge the layers into one single layer, or use a non-destructive method.

Preview Filter: for some reason, only the dialog in the menubar method has a checkbox to to preview the effect of the filter. I assume this is an oversight. Perhaps a developer through it wasn't necessary to have a preview in the non-destructive methods, as they don't apply anything to begin with. But, well, you could also Ctrl+Z. The dialogs are modal, so you can't hide the filter layer or the filter mask while the dialog is open, which is kind of inconvenient if you want to compare what the filter does with the original image.

G'MIC: G'MIC filters are only available destructively. For example, G'MIC has a chromatic aberration filter which you can find on Filters -> Start G'MIC-Qt -> Degradations -> Chromatic Aberrations, but this isn't available for filter masks and filter layers. The reason for this is that G'MIC isn't part of Krita, but a separate program that is installed with Krita. You can use the same filters in GIMP, for example, but I think you need to install a plugin for that.

Filter Masks vs. Filter Layers

There's a few important different between filter masks and filter layers.

Scope: filter masks only apply to the layer they're attached to. If you attach a filter mask to a group, the filter applies to the entire group. On the other hand, a filter layer applies to everything under the layer. However, there is a caveat.

Behavior with Pass Through: layer groups have a property called "Pass Through" which affects how filter masks and filter layers work. This property can be toggled on and off by clicking a "brick wall" icon on the "Layers" docker. It's where the "lock alpha" icon would be for normal layers. When toggled on, the brick wall gains an arrow cross it downward. This property essentially means that the group is used purely for organizational purposes, and isn't considered by the program for blending purposes. Basically, when Krita processes the layers, every time there is a group, Krita creates an internal new image for that group and starts drawing on it its layers, and after processing all the layers, it will be like Krita has merged the whole group. This affects our non-destructive filters in two ways:

1. When pass through is on, filter masks don't work on groups. That's because filter masks work on the group's internal merged image, and if pass through is on, there is no such image being generated, so the filter can't be applied.
2. When pass through is off, filter layers don't work on layers below but outside that group. For example, if you create a group called "filters" and place it above a paint layer, and then put inside of it a blur filter layer, the blur won't be applied to the paint layer by default, because the paint layer is outside the group. More specifically, what's happening is that the filter IS being applied, but it's being applied to the image of the group, which is an empty image if the only thing inside the group is a filter layer. I assume many try this and just think you can't make it work, but you can have a "filters" group where you place just your filters if you enable the pass through setting.

Histogram: some filters, such as Adjust -> Levels, have a histogram as part of its settings. A histogram shows statistics about the image, in this case how much of the image has how much brightness. This histogram is only available in the filter mask, not the filter layer. I assume this is because it's trivial to calculate the histogram for a single layer (or layer group), but doing it for part of the layer stack would be more complicated. It's not impossible, so maybe this is just a missing feature or a bug.

Adding a Filter Twice

A couple more tips:

Apply Filter Again

If you used a destructive filter once, you can quickly reuse the same filter by clicking on Filter -> Apply Filter Again on the menubar.

Alternatively, if you want to apply the same filter but change a setting, the Reprompt option under the above will show the dialog again.

Copy Paste

You can copy paste filter layers and filter masks.

In particular, you could create a very complex set of filters in a "filters" group, then just copy it from one file to another.

Drag and Drop

If you drag and drop a layer or mask normally, you move it from one place to another in the layer stack. However, if you hold the Ctrl key while you drag it, when you drop the layer or mask it will create a copy instead of moving it.