Don't use \fad on fades to black

Dear typesetters,

I have seen this countless times. Every time, it has bothered me. Please, don't do it anymore.

When the screen fades to black (or white, or any other solid colour, for that matter), DON'T use the \fad tag to fade the text along with it. When you use \fad, you're making the text translucent, and not darker. The result is that it will blend with the background - including all the usual associated bugs if you have borders and shadows - and get somewhat darker because the background itself is getting darker - it just won't get as dark as it SHOULD be, and the visual effect is that the text is getting brighter, relative to the background.

Original

With \fad

With \t(\c)

As you can see above, the CORRECT way to deal with this is by using \t to animate the colour (all relevant ones) to black (or whichever colour the screen is fading to). Also note that the background can be seen through the text in the \fad() case, which is not the correct behavior.

Keep in mind, though, \t(\c) is significantly slower than \fad() on VSFilter, so you might have softsub issues, depending on how complex is your text.

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Font hinting and you

... or why you should not use animation on the \fs override tag.

Do you know what font hinting is? If you haven't worked with digital typography you might not, but it's a technique used by (almost) all font rendering systems to make vector fonts (such as True Type, Open Type and Adobe Type 1) appear better on low-resolution mediums like computer monitors. (Actually CRT TV screens are even worse.)

Usually glyphs ("characters") in outline fonts have quite some detail in them, but if you only have 7x13 pixels to render a character in, you're going to have a hard time fitting all that detail in, even if you use sub-pixel rendering such as anti-aliasing and ClearType. That's where font hinting comes it. It's a technique for intelligently modifying the outlines of characters so they look better without completely losing the characteristics that makes the font face special. The basic idea in font hinting is to snap the outlines to the edges of pixels, such that stems and vertical lines take up a whole number of pixels instead of disappearing in quantisation or become a smudge of sub-pixel noise.

So what does that have to do with subtitles? Well, the amount of hinting applied to a glyph depends on the point size of it. The bigger the point size, the less strong the hinting needs to be. For example, here's some text in Verdana at different sizes:

Verdana 8 pixels

Verdana 12 pixels

Verdana 30 pixels

Depending on your font rendering system it might look different (eg. Windows and Macintosh OS X render quite differently) but at least if you're on Windows you should be able to see that the shapes of the letters actually change a bit. The smaller the text size, the more drastic the change.

It's this change of glyph shapes that's interesting in subtitle context. If you've ever needed to have some text change size on screen in ASS subtitles you might have considered whether you should use \t(\fs) or \t(\fscx\fscy). It's the latter that's correct. When you animate the \fs tag you're changing the actual font size requested of the font subsystem, and this also affects the hinting applied to the text.

This leads me to the image at the start of this post: Both of the top two lines with "Test" are rendered in what should have been Arial size 40. But the upper has been given its size with \fs1\fscx4000\fscy4000 while the lower has been given its size with \fs40\fscx100\fscy100. Because VSFilter internally works at 8 times resolution, the upper line is requested as Arial with a font-height of 8 pixels, so it's hinted to look best when rendered just 8 pixels tall, while the other line is requested as 320 pixels tall Arial. The red/blue at the bottom are the same two lines with the border removed, then laid over each other.

Do you now see why you shouldn't animate the font size, but rather the font scale?

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VSFilter hacks

There's lots of versions of Gabest's VSFilter subtitle renderer around, some people are even still using versions that are several years old and are missing features and bugfixes. There's cases of people still distributing these old versions around, sometimes as part of a package with other software such as VirtualDub.

Now it happens that some time in 2006, Gabest seems to have lost interest in working on VSFilter, so nothing has happened from his side, not even bugfixes. Fortunately VSFilter is open source, so we have picked it up and made a fork. We have included both some existing bugfixes and improvements that existed as patches, and also made our own bugfixes and further improvements.

I'm going to talk about some of the improvements we have made throughout this post.

First, I'm going to assume you're familiar with most regular ASS override tags, if not you can get a refresher in our manual. Second, not all of these improvements are available in the version of VSFilter we ship with the current release (2.1.2) of Aegisub, but they should all be in the next one (which will be 2.1.3 or 2.2.0, depending on circumstances.)

Sometimes you need sub-pixel precision when positioning text and drawings. Normally you've only been able to get this by setting the script resolution (PlayResX and PlayResY) to something larger than the video resolution, but the "float \pos" patch changes this. It allows you to use decimal/floating point numbers for positioning lines and gives you up to 1/8th pixel precision.

The image to the left shows four lines of text positioned at different X coordinates. You can see how they move ever so slightly, although not a full pixel.

The tags \fax and \fay allow you to do shearing operations on your text. This is a bit like rotations, except that it's not. They are especially useful for doing perspective correction when you are typesetting signs rotated in 3D.

Shortly before Gabest dropped development of VSFilter, he introduced the "ASS2" format. This isn't very well known, and it only has minor changes over the original ASS format. One of the things was a new karaoke timing tag, \kt. While Aegisub can read ASS2 files it can't write them, so if you edit them in Aegisub you will lose some information. It doesn't have support for timing with the \kt tag either, but you can of course still use that and use all the additional features of ASS2 if you edit the file with a text editor.

So what is \kt? It's "set absolute timing", it allows you to move both back and forth in time of highlight without highlighting other syllables. It is probably best illustrated with an example:

{\kf10}ABC {\kt20\kf10}DEF {\kt10\kf10}GHI

When you render this example, you will first see ABC highlight. Then GHI will follow, because its highlight is set to start at time 10 by \kt. Finally, DEF will highlight because its start time was set to 20 by \kt. You can also use \kt to make karaoke syllables overlap in timing.

Originally the \be tag (Blur Edges) only allowed blurring to be turned on or off, and was very "weak", the blur effect was hard to notice at all. We have updated it so it supports variable strength blur effects now, and you can also use it with \t to animate the blur strength.

Because \be doesn't look very high at high values we have also introduced the alternate \blur tag, which performs the blur effect using a two-pass gaussian blur algorithm. This gives a much nicer and wider blur, especially at high values.



I will discuss why \be and \blur are "blur edges" effects and not "blur everything" effects tomorrow.

If you've ever wanted the shadow to be positioned differently than just "X down and X right", the \xshad and \yshad tags are probably just what you want. These work like \shad except that they set only the X or Y displacement. You can even use negative values with them!

Finally there's the \xbord and \ybord tags. They can be useful for various things, but one of the intentions was to better support anamorphic video: The \xbord and \ybord tags allows you to control the border width in X and Y direction separately. You can even disable border in one direction entirely!

You can also combine this with strong \be or \blur and maybe some shadow for other interesting effects.

Again, remember that many of these tags require a very new version of VSFilter. For example, the \blur tag was only added yesterday..! The next release of Aegisub will ship with a VSFilter version that supports all of them, so if you want to safe you can wait until then. If you're impatient, you can follow development on the Aegisub forum.

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