Re: [ft-devel] GSoC - Image comparison in test framework

[Arvinder Bhathal]

Hello Werner,

Thanks for the warm welcome :)

I've been reading through the docs and some of the source code (the
glyph conventions and design document were particularly helpful), and
I had some more questions about the project.

> Neither `fast' nor `simple' is important IMHO.  The first step is to
> check whether there are differences at all.  Currently, FreeType
> already emits MD5 checksums for rendered glyphs if asked for, i.e.,
> using the `bitmap' group and debug tracing level 3 or higher in the
> `FT2_DEBUG' environment variable, for example
>
>   FT2_DEBUG=bitmap:3 ftview ...
>

The MD5 checksum will be good for checking for any errors up until and
including rendering the glyph. However, what about when adjusting the
kerning for successive glyphs when printing a string of text? If a bug
results in improper kerning for a given pair of gylphs, is it true the
MD5 checksum will report no differences and an additional check for
differences would be required? Does this hold for all operations
concerning the placement of successive glyphs?

>   . In a string of glyphs, the gap between glyphs changes.

For example, would an additional check besides the MD5 sums be
required for the above differences?

>   . A glyph isn't rendered at all due to an internal problem of
>     handling the glyph's data.
>
>   . A stem gets positioned higher or lower; this affects a series of
>     consecutive horizontal or vertical pixels.  This is the most
>     frequent – and most important case.
>
>   . In B/W mode, a rendered structure has one pixel more or less.

If I'm correct, I believe the MD5 sums should detect these.

Concerning the image comparison,

> Neither `fast' nor `simple' is important IMHO.

I think I was going about the image comparisons the wrong way with my
initial thoughts on them. I'm going to research better ways of doing
the comparison and displaying the results.

Unrelated to the project but I've had to fiddle with freetype in the
past when trying to improve my font rendering and it's interesting
(and humbling) to see just how much is involved in rendering text :-p

Regards,

Arvinder