What is the difference between TrueType and OpenType?

Discussions about the development of TrueType and OpenType fonts.
walterspublishing
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What is the difference between TrueType and OpenType?

Postby walterspublishing » Mon Dec 18, 2006 5:06 pm

I was told we should be using OpenType fonts here at work. How do I tell if it is OpenType? When I look into my Windows Fonts folder some have a TT icon and some have a O icon but they all have a .ttf extension. At first I thought any thing with a .ttf extension and a TT icon was a TrueType font. Searching the internet I found out that TrueType fonts could not have more than 256 characters. Many of my .ttf fonts have more than 256 characters and with FontCreator I can create more. In FontCreator I couldn't find anywhere to tell what type of font it is.
One thing I did find out on the Internet was that "OpenType fonts also contain a "digital signature" that allows operating systems and browsing applications to identify the source of fonts". I noticed my Times New Roman font had an "O" icon. When I opened it up in FontCreator and went to Format – Tables. I could see it had a digital signature. When I deleted this signature and saved the file. It's icon changed from the O to a TT. It still had over 1600 characters. It seems to me that most fonts have characteristics of TrueType fonts and characteristics of OpenType fonts. How can I tell what type of font it really is? ? ? ?

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Postby Erwin Denissen » Mon Dec 18, 2006 10:45 pm

Both TrueType fonts and OpenType fonts can hold up to 65535 glyphs. When on average there is one mapping from a character to a glyph, this results in at most 65535 characters in a single font.

The DSIG table makes Windows show the OpenType icon instead of the TrueType icon in Windows Explorer. This is true for all TrueType based OpenType fonts. PostScript based OpenType fonts always show the OpenType icon, however FontCreator always converts PostScript based OpenType fonts to TrueType based OpenType fonts.

The information below is an exact copy of the Font Types topic of the MainType user manual.

Font Types
Over the years support for several different font types has been developed and evolved:

Raster Fonts
In a raster font each glyph is defined as a little bitmap designed for a specific resolution. They are not scalable like vector based fonts. Nowadays raster fonts are mostly used for MS-DOS-based applications and onscreen display in user interface elements.

Vector Fonts
Vector fonts contain glyphs that are defined as a set of points that define line segments. Although they can be scaled to any size, both quality and performance are poor compared to modern font types like mentioned below.

PostScript Type 1 Fonts
Adobe launched PostScript Type 1 together with PostScript Type 3 in 1984. This scalable font technology became very popular among desktop publishers, but is now phasing out as OpenType fonts are the future.

TrueType Fonts
TrueType is a scalable font technology designed by Apple Computer, available since 1991 (Apple's Macintosh System 7). Apple traded the technology with Microsoft and became available on Microsoft Windows 3.1 in 1992.

TrueType Font Collections
A TrueType Font Collection file is one or more TrueType fonts combined into one file.

OpenType Fonts
The OpenType font format is an extension of the TrueType font format, allowing support for PostScript font data. Technically there are two OpenType Font flavours; TrueType based (.ttf) and PostScript based (.otf). OpenType was developed jointly by Microsoft and Adobe to produce a hybrid between Type 1 and TrueType fonts, with additional features that work on Macintosh and Windows computers. OpenType fonts can include the OpenType Layout tables, which allow font creators to design better international and high-end typographic fonts.
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walterspublishing
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Postby walterspublishing » Tue Dec 19, 2006 3:50 pm

The part I am still not clear about is this:
Most of the fonts in my fonts folder are .ttf and they have the TT icon. Are they OpenType or just TrueType.
Everything I find says there are TrueType fonts and some of the OpenType fonts are TrueType based.

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Postby Erwin Denissen » Tue Dec 19, 2006 5:25 pm

First I want to make sure, we both agree PostScript based OpenType fonts are always OpenType fonts. Right?

Now back to your question; what about TrueType based OpenType fonts?

I guess it is correct to say:
TrueType fonts are TrueType based OpenType fonts without DSIG table and without OpenType Layout tables. So TrueType fonts are in fact OpenType fonts. However when a TrueType based OpenType font contains a DSIG table and/ or one or more OpenType Layout tables, you can't call it a TrueType font.

So when you are told you should be using OpenType fonts at work, you can use both TrueType and OpenType fonts.

I'm not sure if this is what you want to hear, but it's the whole truth about the definition of OpenType fonts.

Note: In my opinion the way Windows makes a distinction between plain TrueType fonts and TrueType based OpenType fonts (e.g. based on the existence of a DSIG table) is pretty much worthless, as that table is not required for OpenType fonts.
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walterspublishing
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Postby walterspublishing » Tue Dec 19, 2006 6:03 pm

You say
I'm not sure if this is what you want to hear, but it's the whole truth about the definition of OpenType fonts.


This is not the whole truth. Here is what I want to hear: Is GARA.TTF an OpenType font? Yes or No. You say that "TrueType fonts are in fact OpenType fonts." Is that true of EVERY TrueType font? I thought only some of the TrueType fonts were not OpenType.
If someone says to me "don’t use GARA.TTF use an OpenType font." Should I say "Every single TrueType font is an OpenType font!" Or if it is correct to say some TrueType fonts might not be OpenType then I would need to find out if GARA.TTF is an OpenType font. How do I find that out? It isn’t mentioned when I get the properties of the file. Is there somewhere I can find out with FontCreator or some other program.

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Postby Erwin Denissen » Tue Dec 19, 2006 8:19 pm

If someone says to me "don’t use GARA.TTF use an OpenType font." Should I say "Every single TrueType font is an OpenType font!"

Yes, technically you are right, but it might be wise to find out the reason why someone asks for an OpenType font. Is it because the font will be used on both Windows and Macintosh computers, or because it will be used within a sophisticated DTP application to produce international documents that require advanced typographic features, like glyph substitution and baseline positioning?

To discover what font tables are stored in a font file, select the font in MainType and look at the File Structure within the Font Information pane:

Image

As a reminder:

TrueType Based OpenType Font Tables
cvt, fpgm, glyf, loca and prep

PostScript Based OpenType Font Tables
CFF, VORG

OpenType Layout Tables
BASE, GDEF, GPOS, GSUB and JSTF

DSIG Table
The DSIG table contains the digital signature of the OpenType font and makes Windows show the OpenType icon instead of the TrueType icon in Windows Explorer. This is true for all TrueType based OpenType fonts. PostScript based OpenType fonts always show the OpenType icon.
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Dick Pape
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Are Tables The Only Way? And Even Then!!

Postby Dick Pape » Sat Dec 23, 2006 5:03 pm

Thanks for this discussion. It's helpful for understanding. But I've still got questions, of course...

1. Nicely the Type 1 OTF extension is very obvious however, many fonts are pushed through so many font programs it is hard to tell what they contain.

Is there anyway to externally tell if a TTF font is opentype or not? Are there Naming indicators showing OT proclivities?

Is there something I can look for without having to review the tables?

2. I have OTF fonts that don't have any of the tables you referenced. Only: OS/2, cmap, glyf, head, hhea, hmtx, loca, maxp, name, and post.

In comparison, the same font labeled TTF also has: cvt, fpgm, and prep tables.

What have I got? (Font-wise).

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Postby Erwin Denissen » Tue Dec 26, 2006 1:40 pm

Dick wrote:Is there something I can look for without having to review the tables?


The last update of the TrueType Specification is revision 1.66 and is over a decade old (it was published in August 1995). Microsoft released the TrueType Open Specification in July 1995. It is based on the TrueType Specification and contains additional tables for advanced typography. There was hardly any interest in this new technology. In April 1997 Microsoft and Adobe released the OpenType Specification (also known as TrueType Open v2). That added support for CFF (PostScript) outlines. Over the years many changes (both enhancements and corrections) have further improved the specifications.

So when you have a font that only contains data as described in the TrueType Specification, your font is a TrueType font. But how should you call it when you add a License Description to the font? That name id was added to version 1.01 of the OpenType Specification, released in October 1997. So strictly the font is now an OpenType font!

Dick wrote:2. I have OTF fonts that don't have any of the tables you referenced. Only: OS/2, cmap, glyf, head, hhea, hmtx, loca, maxp, name, and post.

In comparison, the same font labeled TTF also has: cvt, fpgm, and prep tables.

What have I got? (Font-wise).


Again, TrueType fonts are in fact OpenType fonts, so you should either call this a TrueType font file with an invalid file extension or an OpenType font!

I wouldn't be amused if I bought this font expecting to get a rich featured OpenType font.
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Fred Goldman
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Postby Fred Goldman » Fri Aug 03, 2007 6:19 pm

Excellent thread! Very informal, thank you Erwin.


You're right it really makes no sense that the DSIG table should be the deciding factor on what is shown as an OT font. You could basically have a font with all kinds of positioning and ligature changes and it still would show as a TT font, whereas you could have nothing bot a DSIG table (which has nothing to do with the way the actual font works) and it will be considered a OT font.

Come to think of it, in order to get a DSIG don't you need to have either a GSUB or a GPOS table in the font?
Last edited by Fred Goldman on Fri Aug 03, 2007 10:12 pm, edited 1 time in total.
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Postby Erwin Denissen » Fri Aug 03, 2007 9:27 pm

Hi Fred,
Fred Goldman wrote:You're right it really makes no sense that the DSIG table should be the deciding factor on what is shown as an OT font.

Yes, I wonder when Microsoft decides to change this odd behavior.

Fred Goldman wrote:Come to think of it, in order to get a DSIG don't you need to have either a GSUB or a GPOS table in the font?

No, the Microsoft font signing tool can sign both TrueType and OpenType fonts. If you sign a font, a DSIG table is added to the font file. Since the DSIG table is part of the OpenType font format (and not part of the TrueType font format), a signed TrueType font is automatically upgraded to be an OpenType font.

More about font signing and the DSIG table:
DSIG - Digital Signature Table
Digital signatures for font files
OpenType font signing tool
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Postby Fred Goldman » Fri Aug 03, 2007 10:14 pm

I fixed the misspelling of your name in my post. I meant Erwin.

Thanks for the links.

Then you're right, it is definitely weird why Microsoft uses that table to designate a font as OT. Do ou now how Macintosh decides whether a font is OT or TT?
Fred

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Postby Erwin Denissen » Sat Aug 04, 2007 12:26 pm

No, I don't know how this is managed on a Mac.
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Re: What is the difference between TrueType and OpenType?

Postby Palendrome » Wed Feb 04, 2015 7:30 pm

Hello,
This screen shot above of MainType 2.0 has much more information in the Font information panel than my Maintype 6.00 Professional ?
I dont get:

FIle Structure or any information about opentype tables =

where do I find out that information?

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Re: What is the difference between TrueType and OpenType?

Postby Erwin Denissen » Wed Feb 04, 2015 7:33 pm

That information is no longer shown. You have to open a font file in a font editor. A hex editor will do as well if you only want to verify what font tables reside in a file.
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