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This is a simple texture atlas generator. It will generate atlases for the following:

  • Fonts, including font metrics, using cl-freetype2 and cl-cairo2
  • Sprites, including metrics (texture position and frames) and sprite names, from PNG files.
;; This will produce a 128x128 png file with a rendition of the
;; specified characters at 18 points and 72 dpi.

(make-font-atlas-files "times.png" "times.met" 128 128
                       :dpi 72
                       :string "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789.,;:?!@#$%^&*()-_<>'\"$[] ")

Example output PNG.

;; This will produce a 96x96 png file as seen below.

(make-sprite-atlas-files "sprites.png" "sprites.met" 96 96
                         (directory "*.png"))

Example output PNG, generated for an upcoming game using cl-sdl2.


There are now two different systems and packages:

  •, via the system texatl-client, is a "client" library, the pure-CL functions and dependencies necessary to render fonts, assuming you have loaded them.

  • :texatl, via the system texatl, has foreign dependencies, but is able to generate textures on the fly.



The primary function for generating an atlas is TEXATL:MAKE-FONT-ATLAS:

                 &key (DPI 72) (STRING *default-characters*))


Surface is a cairo image surface of the specified size.

Texatl-font is a class with the following slots:

  • face-metrics: This provides the following 4 metrics which may be used for layout:
    • max-ascender: The max distance from the baseline to the top of any glyph. This may be used with x and top to determine the baseline.
    • max-descender: The max distance from the baseline to the lowest point of any glyph. This is expressed as a negative number.
    • height: This is the distance between baselines font. (Generally this is rather small and should be treated as a minimum rather than a practical value.)
    • max-advance: The maximum width of any glyph.
  • glyph-index: A hash table of character-to-index. Each index is the element of the array of glyph metrics for that character.
  • glyph-metrics: An array of metrics vectors. Each element in this array corresponds to one character. The elements of each metrics vector are as follows:
    • x, y: The first two elements are the pixel-offset of the top left of the glyph in the texture. You will likely use these directly as U,V when rendering. Not baseline-relative.
    • width, height: The width and height of the glyph. You will likely use these as U,V and for vertices. Not baseline-relative.
    • advance: The number of pixels to move forward to the next glyph, not including kerning (which depends on the previous glyph).
    • left: The left of the glyph, relative to its x. You will want to subtract this from x when placing the glyph.
    • top: The top of a glyph, relative to its y. You will want to subtract this from max-ascender when placing the glyph.
  • glyph-kerning: This is an alist of ((A . B) . K), where A and B are characters, and K is the kerning value. When placing characters:
    • Keep a running tally of the character offset, cx, incrementing this by advance + K after every glyph.
    • You may compute the horizontal position of character B by cx + left + K, when B follows A.
    • If there is no (A . B), or for the initial character, K=0.
    • Note that the kerning for two characters "XY" may differ from "YX".

This may all seem rather complicated, but generally, it is not:

  • Write out characters using the cx + left + K formula.
  • The vertical position of a glyph should be max-ascender - top.
  • For multiple lines, also add line * (height + some-value).
  • You should round values for pixel alignment.

See sdl2-manual.lisp for an example of doing this by hand. This may be desirable for complex font placement situations.

However, you can now use the DO-TEXATL-STRING macro to simplify this process for simple 2D static string placement. See below for details.


(make-font-atlas-files PNG-FILENAME METRICS-FILENAME
                       WIDTH HEIGHT FONT-NAME POINT-SIZE
                       &key (DPI 72) (STRING *default-characters*))

This function has the same conventions as MAKE-FONT-ATLAS, but you may specify filenames for a PNG and metrics file. The surface and metrics are not returned, but instead written to these files.

The metrics file contains the same values as returned by MAKE-FONT-ATLAS, written in the CONSPACK format. You may read these in sequence, and obtain the same values in the same format.


(do-texatl-string (STRING X0 Y0 X1 Y1 U0 V0 U1 V1
                   &key (tex-width 1) (tex-height 1)) TEXATL-FONT
    &body body)

This macro is provided for simplifying the placement of simple 2D strings. It provides no complex layout, line wrapping, etc, but it does provide a way to place simple strings without a lot of work. See sdl2-simple.lisp for an example.


Sprites are much simpler, though they have similar functions:

(make-sprite-atlas WIDTH HEIGHT FILE-LIST)


Surface is a cairo image surface of the specified size.

Texatl-spritesheet is a class which you can access with the following functions:


=> #(X0 Y0 X1 Y1)

This returns the texture coordinates (in pixels) for the sprite named NAME, for frame FRAME. See Sprite Naming below for details on how sprite names work.

For easy binding, there is WITH-SPRITE:

(with-sprite (x0 y0 x1 y1) NAME FRAME SPRITESHEET

This binds the specified variables to texture coordinates, given NAME, FRAME, and SPRITESHEET (all evaluated).

Additionally, you can find how many frames a given sprite has:

(frame-count SPRITESHEET NAME)

=> frame-count


Much like MAKE-FONT-ATLAS-FILES, this will write a PNG file and a CONSPACK file containing the metrics for you:


Sprite Naming

Sprites have names which are a list of keywords. For instance, you may have a sprite named (:HERO :FRONT :SHOOTING). In the example spritesheet at the top, sprites have names like (:WIZ :SIDE :WALK) or (:KNIGHT :FRONT :STAB).

Sprites also have one or more frames, and given a name and a frame, you can retrieve the specific texture coordinates with TEXATL.CL:SPRITE.

Sprite filenames are important, because they determine sprite names and frames. For instance:

  • wiz:side:walk:1.png => (:WIZ :SIDE :WALK) frame 1
  • knight:front:stab:0.png => (:KNIGHT :FRONT :STAB) frame 0

Note that frames are numbered from zero. Sprite name components are separated by colons, and the frame is specified as a single integer at the end.

In practice, it is relatively easy to create a sprite using a number of named layers in, e.g., GIMP, and export each layer based on its name using a script. That was done in the above example.


Texture atlas generation, including fonts






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