To create a glowing text effect in Krita, following the following steps:
1: ensure that you have a dark background, or it will be difficult to see the effect of the glow.
2: start by creating a clone layer of the layer or group layer that you want to apply the effect to.
3: place the clone above the clonee.
4: select the clone layer and add a blur filter mask to it. There are two options that you can choose from: Gaussian Blur an Lens Blur. Gaussian Blur is rounder and is better for neon-like glow, while Lens Blur can be for a "city lights" glow effect as the glow will follow a pentagonal shape by default.
5: change the blend mode of the clone from Normal to one of the lighten-type blend modes. You have various options to choose from, pick the one that looks best for you, e.g. Addition, Fog Lighten, Linear Dodge, and Linear Light, etc.
Important: some blend modes, like Addition, can only make color lighter, which means that the glow becomes invisible when placed against a white background. Other blend modes, like Hard Light, can make color both lighter and darker, so they don't have this problem. If you can't find a blend mode that works in all cases, you have the option to create a second clone layer with a different blend mode to make the colors darker before making them lighter.
Making Your Glow High Quality
By default, a new document created in Krita has a color depth of 8 bits and a gamma-corrected sRGB color profile. These settings work for most cases, but when we work with certain filters like blurs they aren't optimal as they are too low quality and introduce banding artifacts to our blur. On top of that, using Addition with gamma-corrected sRGB will perform the addition in gamma-corrected space, which will look very different from how additive light colors work in real life.
To solve this, first you should use a higher color depth, normally 16 bit integer will suffice, and, you should also use a different color profile as well. Note that regardless of what profile you use, the image needs to be converted to your monitor profile in order to be displayed. However while Krita is processing the image it can use the selected color space.
Because 16 bit provides so many more colors compared to 8 bit, it's a good idea to use a wide gamut profile instead of sRGB. A wide gamut standard that was created to replace sRGB already exists, called Rec. 2020, and Krita ships with this color profile as well.
To change your color space, click on Image -> Convert Image Color Space on the menubar, and select 16-bit integer/channel and Rec2020-elle-V4-g10.icc (linear wide gamut) or sRGB-elle-V2-g10.icc (linear sRGB gamut). The "g10" means the gamma is 1.0, which means it's linear.
If you change the color space your image may look different. If you don't want this to happen, select the same color space you had previously (most likely sRGB-elle-V2-g10.icc).
Observation: if you don't change the color space but change the depth, Krita will automatically select a linear color profile, that's why changing the depth appears to change the colors at first glance. What changes the colors is changing the color profile.
Linear sRGB might look brighter than you want, but that's simply how light actually works, and therefore how things that glow should actually work. Humans are more sensitive to changes at darker levels of brightness, so what we perceive as "mid grey" is actually on the lower end of brightness. Conversely, if something is already bright, linear sRGB makes it much more difficult to make it even brighter, as the amount of light that is required to perceive a change greatly increases.
Making the Glow Softer or Stronger
There are a few methods we can use to make our glow softer or brighter.
Changing Opacity
The simplest method to make our glow softer is to change the Opacity of the clone layer.
Doubling Opacity
To make the glow stronger, simply create a clone of the clone layer and make it Addition blend mode as well.
Alternatively, create a second glow effect with a different blur radius to create a softer falloff.
If you do this a few times, you will eventually get a bright white color, so long as the source of your effect isn't pure red, green, or blue. If you want the color to become white eventually, and you are using the Addition blend mode, it needs to be a bit grey because pure red plus pure red is always going to be pure red.
Controlling the Gamma Manually
Now that we have stopped using the 2.2 gamma color space and switched to an 1.0 gamma color space we can reintroduce gamma to our glow effect by creating the gamma function ourselves.
To do this, follow the following steps:
1: select the clone layer that has a Gaussian blur already added to it.
2: add a new filter mask. Select Color Adjustment.
3: change Channel from RGBA to Alpha.
4: drag the curve around to change the strength of the glow.
Tip: you don't need to limit yourself to a single control point. You can in fact create all sorts of weird glow shapes if you wish!
Tip: on linear color space it can be extremely difficult to make dark colors to create a soft glow using this curve. A trick you can use it to simply add a second color adjustment curves to further adjust already adjusted alpha! You can in fact do this very quickly by duplicating the filter mask, which will create a second filter mask that applies the same adjustment to the glow effect.
How Do I Make The Effect Colorful?
If you want to make a colorful glow, just use a gradient fill for the text instead of a flat color.