https://adobe-type-tools.github.io/font-tech-notes/pdfs/T1_SPEC.pdf :
"Using charstring subroutines is not a requirement of a Type 1
font program."
And some versions of Computer Modern do in fact not contain a Subrs
array.
Together with #21473, makes Problemset.pdf from the pdffiles repro
render ok instead of crashing.
With this, all tables from the spec appendixes are in CFF.cpp.
This fixes a crash reading page 2 (and onward) of
2ThestructureoftheCIE1997ColourAppearanceModelCIECAM97s.pdf in
the pdffiles repo.
The encoding offset defaults to 0, i.e. the Standard Encoding.
That means reading the encoding only if the tag is present causes
us to not read it if a font uses the Standard Encoding.
Now, we always read an encoding, even if it's the (implicit) default
one.
The main encoding data maps glyph ID ("GID") to its codepoint.
If a glyph has several codepoints, then a secondary table mapping
codepoint to string ID ("SID") of the glyph's name is present.
(A separate table associates each glyph with its name already.)
I haven't seen this used in the wild, but the structure of the
supplemental data is also going to be needed for built-in encodings.
Two bugs:
1. We decoded a u32, not an i32 as the spec wants
2. (minor) Our fixed-point divisor was off by one
Fixes text rendering in Bakke2010a.pdf in pdffiles, and rendering of
other fonts with negative width adjustments from optcode 255.
That PDF was produced by "Apple pstopdf" and uses font SFBX1200,
which is apparently a variant of Computer Modern. So maybe this
helps with lots of PDFs produced from TeX files, but I haven't
checked that.
I haven't seen this being used in the wild (yet), but it's easy
to implement, and with this we support all charset formats.
So we can now mention if we see a format we don't know about.
From "10 String INDEX":
"Further space saving is obtained by allocating commonly occurring
strings to predefined SIDs. These strings, known as the standard
strings, describe all the names used in the ISOAdobe and Expert
character sets along with a few other strings common to Type 1 fonts. A
complete list of standard strings is given in Appendix A. The client
program will contain an array of standard strings with nStoStrings
elements. Thus, the standard strings take SIDs in the range 0 to
(nStaStrings-1)."
And "13 Charsets" says that charsets store SIDs.
Fixes all
"Couldn't find string for SID $n, going with space"
messages when going through the encoding pages (page 1010 and
thereabouts) in the PDF 1.7 spec.
Only really useful for reading SIDs in the Top DICT (copyright
text etc), which we currently don't do.
I haven't seen a difference from looking things up in the string
table. The only real effect from the commit that I need is that
it pulls a local resolve() labmda into a real function
resolve_sid(), which I want to call in a future commit.
But it makes things more spec-compliant, and if we ever want to
read SIDs in metadata in the future, now we can.
We'd unconditionally get the int from a Variant<int, float> here,
but PDFs often have a float for defaultWidthX and nominalWidthX.
Fixes crash opening Bakke2010a.pdf from pdffiles (but while the
file loads ok, it looks completely busted).
Type0 fonts can be either CFF-based or TrueType-based.
Create a subclass for each, put in some spec text, and
give each case a dedicated error code, so that `--debugging-stats`
can tell me which branch is more common.
LibGfx's ScaledFont doesn't do this, but in ScaledFont m_x_scale and
m_y_scale are immutable once the class is created, so it can get away
with not doing it.
In Type1Font, `width` changes in different calls to
Type1Font::draw_glyph(), so we need to make it part of the cache key.
Fixes rendering of the word "Version" on the first page of
pdf_reference_1-7.pdf.
If the font dictionary didn't specify custom glyph widths, we would fall
back to the specified "missing width" (or 0 in most cases!), which meant
that we would draw glyphs on top of each other in a lot of cases, namely
for TrueTypeFonts or standard Type1Fonts with an OpenType fallback.
What we actually want to do in this case is ask the OpenType font for
the correct width.
A limit of 1024 subroutines seemed like a sensible choice, but some
fonts actually do exceed it. We will now only assert that the specified
amount is positive.
These are not yet actually parsed, but detecting them means we at least
don't fail to understand the *actual* format value, which was causing
some CFF fonts to fail to load.
Type1 imposes a stack limit of 24 elements, but Type2 has a limit of 48.
We are better off relaxing the limit of the former in favour of properly
supporting the latter.
There were two issues with how we counted hints with Type2 CharString
commands: the first was that we assumed a single hint per command, even
though there are commands that accept multiple hints thanks to taking a
variable number of operands; and secondly, the hintmask/ctrlmask
commands can also take operands (i.e., hints) themselves in certain
situations.
This commit fixes these two issues by correctly counting hints in both
cases. This in turn fixes cases when there were more than 8 hints in
total, therefore a hintmask/ctrlmask command needed to read more than
one byte past the operator itself.
The PDFFont class hierarchy was very simple (a top-level PDFFont class,
followed by all the children classes that derived directly from it).
While this design was good enough for some things, it didn't correctly
model the actual organization of font types:
* PDF fonts are first divided between "simple" and "composite" fonts.
The latter is the Type0 font, while the rest are all simple.
* PDF fonts yield a glyph per "character code". Simple fonts char codes
are always 1 byte long, while Type0 char codes are of variable size.
To this effect, this commit changes the hierarchy of Font classes,
introducing a new SimpleFont class, deriving from PDFFont, and acting as
the parent of Type1Font and TrueTypeFont, while Type0 still derives from
PDFFont directly. This distinction allows us now to:
* Model string rendering differently from simple and composite fonts:
PDFFont now offers a generic draw_string method that takes a whole
string to be rendered instead of a single char code. SimpleFont
implements this as a loop over individual bytes of the string, with
T1 and TT implementing draw_glyph for drawing a single char code.
* Some common fields between T1 and TT fonts now live under SimpleFont
instead of under PDFfont, where they previously resided.
* Some other interfaces specific to SimpleFont have been cleaned up,
with u16/u32 not appearing on these classes (or in PDFFont) anymore.
* Type0Font's rendering still remains unimplemented.
As part of this exercise I also took the chance to perform the following
cleanups and restructurings:
* Refactored the creation and initialisation of fonts. They are all
centrally created at PDFFont::create, with a virtual "initialize"
method that allows them to initialise their inner members in the
correct order (parent first, child later) after creation.
* Removed duplicated code.
* Cleaned up some public interfaces: receive const refs, removed
unnecessary ctro/dtors, etc.
* Slightly changed how Type1 and TrueType fonts are implemented: if
there's an embedded font that takes priority, otherwise we always
look for a replacement.
* This means we don't do anything special for the standard fonts. The
only behavior previously associated to standard fonts was choosing an
encoding, and even that was under questioning.
The first iteration has enough SIDs to display simple documents, but
when trying more and more documents we started to need more of these
SIDs to be properly defined. This is a copy/paste exercise from the CFF
document, which is tedious, so it will continue in small drops.
This commit fills all the gaps until SID 228, which covers all the
ISOAdobe space, and should be enough for most use cases. Since this is a
continuous space starting at 0, we now use an Array instead of a Map to
store these names, which should be more performant. Also to simplify
things I've moved the Array out of the CFF class, making it a simpler
static variable, which allows us to use template type deduction.
When loading OpenType fonts, either as a replacement for the standard
14 fonts or an embedded one, we previously passed the font size as the
_point_ size to the loader class. The difference is quite subtle, being
that Gfx::ScaledFont uses the optional dpi parameter to convert the
input from inches to pixels.
This meant that our glyphs were exactly 1.333% too large, causing them
to overlap in places.
The mapping of standard font to replacement now looks like this:
Times New Roman -> Liberation Serif
Courier -> Liberation Mono
Helvetica, Arial -> Liberation Sans
The seac command provides the base and accented character that are
needed to create an accented character glyph. Storing these values is
all that was left to properly support these composed glyphs.
Type1 accented character glyphs are composed of two other glyphs in the
same font: a base glyph and an accent glyph, given as char codes in the
standard encoding. These two glyphs are then composed together to form
the accented character.
This commit adds the data structures to hold the information for
accented characters, and also the routine that composes the final glyph
path out of the two individual components. All glyphs must have been
loaded by the time this composition takes place, and thus a new
protected consolidate_glyphs() routine has been added to perform this
calculation.
Glyph was a simple structure, but even now it's become more complex that
it was initially. Turning it into a class hides some of that complexity,
and make sit easier to understand to external eyes.
While doing this I also decided to remove the float + bool combo for
keeping track of the glyph's width, and replaced it with an Optional
instead.
Storing glyphs indexed by char code in a Type1 Font Program binds a Font
Program instance to the particular Encoding that was used at Font
Program construction time. This makes it difficult to reuse Font Program
instances against different Encodings, which would be otherwise
possible.
This commit changes how we store the glyphs on Type1 Font Programs.
Instead of storing them on a map indexed by char code, the map is now
indexed by glyph name. In turn, when rendering a glyph we use the
Encoding object to turn the char code into a glyph name, which in turn
is used to index into the map of glyphs.
This is the first step towards reusability of Type1 Font Programs. It
also unlocks the ability to render glyphs that are described via the
"seac" command (standard encoding accented character), which requires
accessing the base and accent glyphs by name.
