[cairo] [PATCH] Improve handling of degenerate font matrices

[Behdad Esfahbod]

On 02/07/11 04:51, Andrea Canciani wrote:

>> I took a quick look to see if we can completely remove scale_inverse.  It may
>> be possible, but I check the current uses and they are mostly harmless, so its
>> not a big deal.
> 
> I had a commit performing that. It should be quite trivial as soon as
> I manage to get rid of it in cairo-user-font.c

I have a feeling that we are using the wrong matrix there.  I have a feeling
that we should be using ctm_inverse, not scale_inverse there, since our target
is to have user-space extents.


>>> I don't completely understand what
>>> _cairo_matrix_compute_basis_scale_factors is supposed to do.
>>> It is only used in font code and its documentation seems to be font-oriented
>>> and incorrect. Was it supposed to be an SVD?
>>
>> The Return value docs are wrong for sure, but the rest is correct.  No, it's
>> not SVD.  It's mostly used to compute det(M) / |M*(1,0)'|, which is the line
>> height of the font in user space when applied on the font matrix.
> 
> Ok, the geometrical insight was very useful (it also explains why 0,0 should
> be a correct answer when det(M)==0).

For the div-by-zero case you're right.  For the other case it doesn't make
sense.  The det(M)==0 path needs to be removed.  If we want to get correct
glyph advance with 1rank matrices we need non-zero x_scale out of that
function I guess.


> I'll try to fix the doc and maybe inline
> a couple of comments expaining the geometrical meaning of the
> computations.

Feel free to rename it, and remove the case of x_basis==FALSE as the output
looks inverted and wrong for that case, and we don't currently use that case
anyway (may do in the future for vertical text).

behdad


> Andrea
>