| TechNote #147:
MathType: GIFs do not retain their DPI properties
The information in this document applies to:
- MathType 6.9 (Windows)
- MathType 6.7 (Macintosh)
When you save a MathType equation as a "high resolution" GIF image and
subsequently open the image in a graphics editor such as Windows Paint, the GIF's
resolution is reported as 72 pixels per inch (ppi) or 96ppi.
The GIF image format spec does not include bit density information (pixels
per inch). GIFs are
saved with particular image dimensions -- a number of pixels of width and
height. Software (MathType included) that allows a user to specify a GIF's ppi or dpi (dots per inch) artificially create such a resolution by increasing
the image's physical size -- and hence the number of pixels it contains.
Compare the two equations below, both created by MathType. We created the one
on the left using all the factory default settings. For the one on the right, we
changed only the Web and GIF Preferences, and set the Bitmap
resolution to Other: 300.
image #1: 46x41 pixels
image #2: 145x129 pixels
Clearly the physical size of the 2 equations is different. The one on the
left has dimensions of 46x41 pixels; the one on the right is 145x129. In fact,
the difference shouldn't be surprising. You're most likely reading this page on
a computer with a screen bit density of either 72 ppi (default for Mac) or 96
ppi (default for Windows). Mobile devices will likely have a different bit
density, but the relative size should still be obvious. The one on the right is
roughly 3 times the size of the one on the left.
Here's where the potential for confusion really starts to build: if we open
both of these equations in Microsoft Paint (or any other bitmap editor), they
both report a bit density of 96ppi. Wait a minute -- didn't we set the "bitmap
resolution" to "300dpi" in MathType? Yes, we did, but here's the true
meaning of that...
Since the GIF format doesn't contain -- and cannot contain --
information about bit density (IOW, the number of pixels or dots per inch), the
only way to "create" a high-resolution GIF is to simulate that process by
creating one with larger physical dimensions. So what would happen if you were
to reduce the size of image #2 to be 46x41 (i.e., the same size as image #1)?
- If you're creating content for the web: You may be looking at those and thinking there's no way you'd use the
one on the right. (Actually, depending on your browser, the one on the right may
look pretty good. It looks pretty good on an iPad and it looks pretty good on
Chrome for Windows. Read on though...) IF your target audience will
likely be printing the page rather than reading it on a screen, then the one on
the right will look much better than the one on the left. (Go ahead; try
it. Print this page and compare the two.) In fact, it's even possible to create
2 images, include them both in the web page, and let JS/CSS choose which one
to display, depending on whether the page will be displayed or printed. The
"how-to" for that is beyond the scope of this article, but if that's what
you need to do, you should be able to find an article or tutorial on the web
describing how to do that.
- If you're creating content for print or presentation: The
software you're using for that (like Microsoft Word and Microsoft
PowerPoint) will likely convert the GIF equation into a "vector" image when
you insert it, so create the high-ppi version and scale it down. It will
look great in print or on the screen. Tip: To make it easier
to scale the image such that it's exactly the size you need in your
destination software, we recommend a setting of 384 ppi if you're using Windows, or
360 ppi if you're using Mac. The reason for that is that you can scale it in
Word (or whatever you're using) to 25% of the original if you're on
Windows, or 20% of the original if you're using a Mac. No guesswork,
but this assumes you've changed MathType's Size/Define
settings so that you're using the same font size in MathType that you're
using in the other software.
As always, please let us know if you
have questions about this, or if you have additional techniques that work. We'd
love to hear from you.