[cairo] Locking font / pen space

[Bill Spitzak]

Behdad Esfahbod wrote:

> On 08/27/10 16:37, Bill Spitzak wrote:
>> If this is a "mode" it means a lot of other api can switch behavior and
>> this would avoid a lot of incompatibility problems or tricky emulation.
>> In this mode dash patterns are in "pen space", and font metrics returned
>> in "font space". Glyph positions would also be given in "font space",
>> relative to either 0,0 or the current point in the CTM.
> 
> What do you mean?  How is the font metrics and glyph positions currently
> different from what you are suggesting?

The most obvious reason is that now the CTM can be a degenerate one, but 
you will still be able to draw text and non-zero line thicknesses. This 
makes it impossible to measure or position glyphs unless they are in 
some space other than the CTM. I recommend that they be in the font 
space and pen space.

It is my opinion that the dash-pattern api for stroking lines will be 
immensely more useful and easier to implement if the dashes are defined 
in pen space. The topology for a given dash selection (ie whether the 
ends of the dashes intersect) will be the same in all cases. It also 
forces a definition for the square ends and line join types, which are 
otherwise quite vague in a space with a non-circular pen. It also avoids 
the need to add another api to read and set the dash pattern, because if 
the dash pattern is in another space then it will have to contain this 
matrix. Also it allows the dash pattern to be specified when the CTM is 
degenerate.