Understanding Faces in Emacs

Oct 13, 2025 by miodeya.com 28 views

Introduction

Emacs is a powerful text editor that offers a wide range of customization options to enhance the user experience. One of the key features of Emacs is its ability to create custom faces, which can be used to style text in various ways. However, when it comes to customizing the underline color of faces, many users encounter difficulties. In this article, we will explore the issue of making the underline color of faces different from the foreground color in Emacs.

Understanding Faces in Emacs

Faces in Emacs are used to style text in various ways, including font, color, and underline. Each face is defined by a set of attributes, such as foreground color, background color, and underline style. When a face is applied to a piece of text, Emacs uses the attributes defined in the face to style the text.

The Problem: Customizing Underline Color

When creating custom faces in Emacs, users often encounter difficulties when trying to make the underline color different from the foreground color. The issue arises because the underline attribute in Emacs faces is not a separate attribute, but rather a property of the foreground color. This means that when you set the foreground color of a face, you are also setting the underline color.

Defface Syntax and the Issue

The defface command in Emacs is used to define custom faces. The syntax for defface is as follows:

(defface face-name
  '((t (:foreground "color" :underline "underline-color")))
  "Face description")

In this syntax, face-name is the name of the face, color is the foreground color, and underline-color is the underline color. However, as mentioned earlier, the underline attribute is not a separate attribute, but rather a property of the foreground color. This means that when you set the foreground color, you are also setting the underline color.

Workaround: Using a Separate Underline Face

One workaround for this issue is to create a separate face for the underline, and then apply that face to the text. This can be achieved using the defface command with a separate face name for the underline.

(defface face-name
  '((t (:foreground "color")))
  "Face description")
(defface face-name-underline
'((t (:underline "underline-color")))
"Underline face description")

In this example, we define two faces: face-name and face-name-underline. The face-name face has a foreground color, but no underline color. The face-name-underline face has an underline color, but no foreground color. We can then apply the face-name face to the text, and then apply the face-name-underline face to the text to create the underline effect.

Example Use Case

Here is an example use case for the workaround:

(defface my-face
  '((t (:foreground "blue")))
  "My face description")
(defface my-face-underline
'((t (:underline "red")))
"My underline face description")
(set-face-attribute 'my-face nil :underline t)
(set-face-attribute 'my-face-underline nil :inherit 'my-face)

In this example, we define two faces: my-face and my-face-underline. We then apply the my-face face to the text, and then apply the my-face-underline face to the text to create the underline effect.

Conclusion

Customizing the underline color of faces in Emacs can be a challenging task, but there are workarounds available. By creating a separate face for the underline, and then applying that face to the text, we can achieve the desired effect. This workaround requires some additional effort, but it provides a flexible and customizable solution for styling text in Emacs.

Additional Tips and Tricks

To apply a face to a specific piece of text, use the face-apply function.
To apply a face to a specific region of text, use the face-apply-region function.
To create a custom face with a specific underline style, use the defface command with the :underline attribute.
To create a custom face with a specific underline color, use the defface command with the :underline attribute and a specific color value.

References

Emacs Lisp Reference Manual: Faces
Emacs Lisp Reference Manual: Defface
EmacsWiki: Faces
EmacsWiki: Defface
Frequently Asked Questions: Customizing Face Underline Colors in Emacs ====================================================================

Q: Why can't I change the underline color of a face in Emacs?

A: The underline attribute in Emacs faces is not a separate attribute, but rather a property of the foreground color. This means that when you set the foreground color of a face, you are also setting the underline color.

Q: How can I create a custom face with a different underline color?

A: One workaround for this issue is to create a separate face for the underline, and then apply that face to the text. This can be achieved using the defface command with a separate face name for the underline.

Q: How do I apply a face to a specific piece of text in Emacs?

A: To apply a face to a specific piece of text, use the face-apply function. For example:

(face-apply 'my-face "Hello, World!")

This will apply the my-face face to the text "Hello, World!".

Q: How do I apply a face to a specific region of text in Emacs?

A: To apply a face to a specific region of text, use the face-apply-region function. For example:

(face-apply-region 'my-face (region-beginning) (region-end))

This will apply the my-face face to the region of text between the beginning and end of the region.

Q: How do I create a custom face with a specific underline style?

A: To create a custom face with a specific underline style, use the defface command with the :underline attribute. For example:

(defface my-face
  '((t (:underline t)))
  "My face description")

This will create a face called my-face with an underline style.

Q: How do I create a custom face with a specific underline color?

A: To create a custom face with a specific underline color, use the defface command with the :underline attribute and a specific color value. For example:

(defface my-face
  '((t (:underline "red")))
  "My face description")

This will create a face called my-face with an underline color of red.

Q: Can I use a variable to set the underline color of a face?

A: Yes, you can use a variable to set the underline color of a face. For example:

(defvar my-underline-color "red")
(defface my-face
  '((t (:underline my-underline-color)))
  "My face description")

This will create a face called my-face with an underline color that is set by the my-underline-color variable.

Q: How do I remove the underline from a face in Emacs?

A: To remove the underline from a face in Emacs, use the set-face-attribute function with the :underline attribute set to nil. For example:

(set-face-attribute 'my-face nil :underline nil)

This will remove the underline from the my-face face.

Q: Can I use a custom face as a underline face?

A: Yes, you can use a custom face as a underline face. For example:

(defface my-face
  '((t (:underline t)))
  "My face description")
(defface my-underline-face
'((t (:underline "red")))
"My underline face description")
(set-face-attribute 'my-face nil :underline t)
(set-face-attribute 'my-underline-face nil :inherit 'my-face)

This will create a face called my-face with an underline style, and a face called my-underline-face with an underline color of red. The my-underline-face face will be used as the underline face for the my-face face.