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

[Behdad Esfahbod]

On 02/05/11 05:20, Andrea Canciani wrote:
> A first step towards the handling of degenerate matrices can be the handling of
> degenerate font scale matrices.

So amazing to see you tackle this problem.  Removes a lot from my "on day I'll
do..." list :).


> This should be relatively easy, because cairo already supports 0-size fonts and
> because this drawing operation always degenerates in either a noop or a clear
> operation (depending on the mask-boundedness of the operator).

Right.  We do need to check the get_path() and extents() paths however.


> 0002 makes it possible to create scaled fonts with 1 rank matrices. This
> needs to be tested (as in "tests for it should be added"). It currently adds
> a failure in invalid-matrix, but this is expected, because the "invalid" matrix
> is now considered ok.

Looks good.  Maybe add a cairo_matrix_is_degenerate() instead of computing
determinants directly...

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.


> 0003 is the "real" thing.

Doing any kind of real work other than gstate-tracking in the gstate layer is
wrong.  Imagine replaying a recording surface against a surface with a
degenerate matrix, etc.  The check definitely belongs to the scaled-font
layer.  Should be as easy to add it there.

I agree that some of the stuff already in that function do not belong there
either.


> 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.


behdad