Text Effects - Debugging

Having implemented the Accessor class getting the base class and the first text effect to compile was reasonably easy. The first text effect is a glyph substitution effect where glyphs in text are replaced with random glyphs in a simple animation.

After fixing a couple of crashes the output appeared to more or less work. To test the effect more thoroughly I overlaid the text with the effect applied (a single word) on top of the same text without the effect — this allows me to check things like positioning and character spacing. Doing this and publishing revealed that the text was positioned wrongly and had a vertical offset — so there is a bug.

After quite a few years of chasing bugs a method that often really helps me is to stop and think about it and see if you can figure out how it is misbehaving. Are there different — maybe better test cases that will tell you more about the problem.

So on to this offset — is it a one off offset (say the position of the text box) that is causing the problem or is it something that is incorrect with the calculation of the baseline. Knowing this will allow me to more tightly narrow my focus. There are a lot of numbers flying about in Cello and being able to look in the right area is going to be much less painful.

To test this I made two lines of test (two paragraphs with one word) in the PC version of Cello and published. If you look at the results below:



So from this it looks like the offset increases with successive lines. My guess is that the problem is somewhere in the calculation of the baseline or bounding rectangle of the text — both of which have logic to flip from CG style coordinates to QD style coordinates.