RE: [ft-devel] Rotated text

[Frank W. Miller]

Thanks very much for the reply.  I have a few questions as you might guess.
Your mapping software looks really great btw.

FM


-----Original Message-----
From: Graham Asher [mailto:address@hidden 
Sent: Sunday, July 06, 2008 5:19 AM
To: 'Frank W. Miller'; address@hidden
Subject: RE: [ft-devel] Rotated text

Frank,

I'm sorry that I don't have time to look at your example, but I have
successfully drawn rotated text (and text along a path, for that matter)
using FreeType, and I can give you some pieces of advice.

1. Either (i) use an FT_Matrix to set the font transform (call
FT_Set_Transform), or (ii), create an unrotated outline glyph for each
character and then rotate it using FT_Glyph_Transform. I assume from the
fact that you say that your code 'spins' the text that you know how to set
the transform parameters.

FM: I have been using both FT_Matrix and FT_Set_Transform successfully as
you have guessed.  I've had a little trouble figuring out where 0 degrees
is.  In the sample code on the FreeType site, it seems to be the positive y
axis.  When I tweaked it, I was able to get that go be the positive x axis,
which seems more logical to me.



2. Use the advance vector after it has been transformed. This is
outline_glyph->root.advance (assuming that outline_glyph is your
FT_OutlineGlyph object), and is a 16.16 fixed-point number. You will need to
keep track of the current glyph origin in 16.16 format, to avoid the text
'looking like arse', because...

FM: The way the code works, there is a "pen" variable of type FT_Vector.
The pen x and y components are set to the origin prior to entering a for
loop.  Each iteration of the for loop does this: 1) FT_Set_Transform 2)
FT_Load_Char, 3) draw the bitmap, and 4) advance the "pen" using the
face->glyph->advance.  It seems like one of two things is happening, either
my rendering of the bitmap does not fall on the baseline correctly or the
pen advance is not following the baseline.  My guess is its my bitmap
rendering but I don't know for sure.



3. When you rasterize every glyph except the first one, it needs to be
offset to the sub-pixel position of its origin. For example: if your first
glyph has an unrotated baseline length of 8 pixels and you rotate the text
by 20 degrees, the rotated advance will be (8 * cos 20,8 * sin 20) = (7.518,
2.736). Thus, if your first glyph origin was at (0,0), your second glyph
origin needs to be at (7.518,2.736) to avoid a jiggling effect. Do this by
applying an offset of the fractional parts of the coordinates to each glyph
as you rasterize it, then drawing the glyph at the integer coordinates.
Thus, in this example, you need to offset the glyph by (0.518,0.736) and
draw at (7,2). FreeType works in 64ths of pixels when rasterizing, so you do
this by calling FT_Glyph_Transform with a delta argument of (0.518 *
64,0.736 * 64) = (33,47) before converting the outline glyph to a bitmap.

FM: Huh?  This makes me wonder even more what I'm doing in my previous
comment.  Is my baseline length my width or pitch from the previous glyph?
This is what's causing me trouble I think.  Maybe I'm too new to this but
there aren't that many things I can play with here, at least from what I see
in the documentation.  I do the rotation using the matrix.  I use width and
pitch and rows to figure out the bitmap size and I can take the advance.x >>
6 and advance.y >> 6 to get movements on the origin of each glyph.  I'm not
following where your application of the rotation angle to the baseline
length is coming from.  Can you be more specific about what fields in what
structures you are talking about?


4. Rotated text looks much better when it is anti-aliased. But you knew that
already ;-)

FM: I want to get it working before I worry about anti-aliasing, but I will
of course want to add that at some point.


Thanks again for the advice.  Due to my newbieness, its as clear as mud
right now.  :(

Thanks,
FM