libschrift/schrift.3

.Dd January 30, 2021
.Dt SCHRIFT 3
.Os
.Sh NAME
.Nm schrift ,
.Nm libschrift
.Nd Lightweight TrueType font rendering library
.Sh SYNOPSIS
.Lb libschrift
.In schrift.h
.Ft const char *
.Fn sft_version "void"
.Ft SFT_Font *
.Fn sft_loadmem "const void *mem" "unsigned long size"
.Ft SFT_Font *
.Fn sft_loadfile "const char *filename"
.Ft void
.Fn sft_freefont "SFT_Font *font"
.Ft int
.Fn sft_lmetrics "const SFT *sft" "SFT_LMetrics *metrics"
.Ft int
.Fn sft_lookup "const SFT *sft" "SFT_UChar codepoint" "SFT_Glyph *glyph"
.Ft int
.Fn sft_gmetrics "const SFT *sft" "SFT_Glyph glyph" "SFT_GMetrics *metrics"
.Ft int
.Fn sft_kerning "const SFT *sft" "SFT_Glyph leftGlyph" "SFT_Glyph rightGlyph" "SFT_Kerning *kerning"
.Ft int
.Fn sft_render "const SFT *sft" "SFT_Glyph glyph" "SFT_Image image"
.Sh DESCRIPTION
The function
.Fn sft_version
may be called to determine the version of
.Nm .
Since
.Nm
uses semantic versioning, the returned string is of the form \(dqMAJOR.MINOR.PATCH\(dq.
.Ss Loading fonts
.Fn sft_loadfile
will load a font from a given filename (by mapping it into memory),
whereas
.Fn sft_loadmem
can be given an arbitrary memory address and size (in bytes).
This allows loading fonts from ZIP file streams etc.
.sp
Once a particular font is no longer needed, its memory should be freed again by calling
.Fn sft_freefont .
If the font has been loaded with
.Fn sft_loadmem ,
the underlying memory region will have to be freed separately.
.Ss the SFT structure
Most functions take a
.Vt SFT
as their primary argument.
This is a structure to be filled out by the caller.
It bundles multiple commonly needed parameters.
The fields are as follows:
.Bl -tag -width 8
.It Va font
The font to render with
.It Va xScale
The width of one em-square in pixels
.It Va yScale
The height of one em-square in pixels
.It Va xOffset
The horizontal offset to be applied before rendering to an image
(Useful for subpixel-accurate positioning)
.It Va yOffset
The vertical offset to be applied before rendering to an image
(Useful for subpixel-accurate positioning)
.It Va flags
A bitfield of binary flags
.El
.sp
If the
.Dv SFT_DOWNWARD_Y
flag is set, the Y axis is interpreted to point downwards.
Per default, it points upwards.
.Ss Glyph ids
.Nm
operates in terms of glyph ids (of type
.Vt SFT_Glyph ) ,
which are font-specific identifiers assigned to renderable symbols (glyphs).
The way to obtain a glyph id is to call
.Fn sft_lookup .
This function takes a Unicode codepoint in
.Va codepoint
and writes its corresponding glyph id into the variable pointed to by
.Va glyph .
.Ss Querying metrics
.Fn sft_lmetrics
calculates the typographic metrics neccessary for laying out multiple lines of text.
.sp
This function writes its output into the structure pointed to by
.Va metrics .
The fields are as follows:
.Bl -tag -width 8
.It Va ascender
The distance from the baseline to the visual top of the text
.It Va descender
The distance from the baseline to the visual bottom of the text
.It Va lineGap
The default amount of horizontal padding between consecutive lines
.El
.sp
When displaying multiple glyphs in a line, you have to keep track of the pen position.
.Fn sft_gmetrics
tells you where to draw the next glyph with respect to the pen position,
and how to update it after rendering the glyph.
.sp
This function writes its output into the structure pointed to by
.Va metrics .
The fields are as follows:
.Bl -tag -width 8
.It Va advanceWidth
How much the pen position should advance to the right after rendering the glyph
.It Va leftSideBearing
The offset that should be applied along the X axis from the pen position to the edge of the glyph image
.It Va yOffset
An offset along the Y axis relative to the pen position that should be applied when
displaying the glyph
.It Va minWidth
The minimum width that an image needs such that the glyph can be properly rendered into it
.It Va minHeight
The minimum height that an image needs such that the glyph can be properly rendered into it
.El
.sp
Some sequences of glyphs may look awkward if they're layed out naively.
For example, the gap between the two characters \(dqVA\(dq might look disproportionally large.
Kerning is a way to combat this artifact, by slightly moving the second character closer or further
away by a small amount.
.Fn sft_kerning
may be used to retrieve kerning information for a given pair of glyph ids.
.sp
This function writes its output into the structure pointed to by
.Va kerning .
The fields are as follows:
.Bl -tag -width 8
.It Va xShift
An amount that should be added to the pen's X position in-between the two glyphs
.It Va yShift
An amount that should be added to the pen's Y position in-between the two glyphs
.El
.Ss Rendering glyphs
To actually render a glyph into a easily-displayable raster image, use
.Fn sft_render .
.sp
Other than most functions,
.Fn sft_render
takes a structure in
.Va image
that is to be filled out by the caller.
The fields are as follows:
.Bl -tag -width 8
.It Va pixels
A pointer to an array of bytes, width * height in size
.It Va width
The number of pixels in a row of the image
.It Va height
The number of pixels in a column of the image
.El
.sp
The image will be rendered into the memory provided in
.Va pixels .
Each byte corresponds to one pixel,
with rows of the image being directly adjacent in memory without padding between them.
Glyphs are rendered \(dqwhite on black\(dq,
meaning the background has a pixel value of 0,
and the foreground goes up to a value of 255.
Pixel values follow a linear color ramp, unlike conventional computer screens.
That is to say, they are
.Em not gamma-corrected .
These properties make it very easy to use images rendered with
.Fn sft_render
as alpha masks.
.Sh RETURN VALUES
.Fn sft_loadmem
and
.Fn sft_loadfile
return
.Dv NULL
on error.
.sp
.Fn sft_lmetrics ,
.Fn sft_lookup ,
.Fn sft_hmetrics ,
.Fn sft_kerning ,
.Fn sft_extents ,
and
.Fn sft_render
all return 0 on success and -1 on error.
.Sh EXAMPLES
See the source code of the
.Sy demo
for a detailed example of real-world usage of
.Nm .
.Sh AUTHORS
.An Thomas Oltmann Aq Mt thomas.oltmann.hhg@gmail.com
.Sh CAVEATS
The only text encoding that
.Nm
understands is Unicode.
.sp
Similarly, the only kind of font file supported right now
are TrueType (.ttf) fonts (Some OpenType fonts might work too,
as OpenType is effectively a superset of TrueType).
.sp
As of v0.10.2, there is no support for right-to-left scripts
.Em yet .
.sp
.Nm
currently does not implement font hinting and probably never will.