Can somebody suggest what I might be doing wrong to cause the
cumulative
text extent to not scale linearly with font size * DPI? Nothing is
actually being
rendered, in this application what must be calculated is how long the
string would
be if it were rendered (which it will be, much later).
Consider the Arial text string: "First Second Third"
(not including the double quotes). Using FontConfig and
FreeType the extent of the string was calculated for different font
sizes and dpi settings. The extent was the sum of the advances in X and
also
included the kerning corrections, but in this string there was only one
such correction,
affecting the second space. These were the results:
Font Dpi Total Advance
16 75 8621
32 75 16985
16 150 16985
32 150 34995
16 300 34995
32 300 69478
As expected, let C = Font*Dpi, then total advance for any line
with the same C is the same. However, and this is the problem, the
extent
should be simply proportional to C. Doubling C should double the
extent.
It doesn't, not quite. Look at the ratios for the successive advances,
which
should be 2.0 plus a small amount of rounding error:
16985/8621 = 1.970189
34995/16985 = 2.060347
69478/34995 = 1.985369
Yes it is near 2, but not very near 2. Where is the error coming from?
In every case the file selected was:
/usr/share/fonts/truetype/msttcorefonts/Arial.ttf
so it wasn't using one font for one size, and another for another.
This was done on Linux and key parts of the code were:
...
if(FT_New_Face( library, file, 0, &face )){
...
// fd is font size in points
if(FT_Set_Char_Size(
fsp->face, /* handle to face object */
0, /* char_width in 1/64th of points */
fd*64, /* char_height in 1/64th of points */
300, /* horizontal device resolution, DPI */
300 ) /* vebrical device resolution, DPI */
){ return(9); }
...
glyph_index = FT_Get_Char_Index( fsp->face, wc);
if (!FT_Load_Glyph( face, glyph_index, FT_LOAD_TARGET_NORMAL )){
if ( !FT_Get_Glyph( face->glyph, &glyph ) ) {
advance += face->glyph->advance.x;
if(pc){
prev_glyph_index = FT_Get_Char_Index( fsp->face, pc);
if(!FT_Get_Kerning( face,
prev_glyph_index,
glyph_index,
FT_KERNING_UNFITTED,
&akerning )){
advance += akerning.x;
}
}
}
}
...
Tried FT_KERNING_DEFAULT instead of FT_KERNING_UNFITTED and that
sequence of extents was
16960,35008,69504, with ratios: 2.064151 and 1.985375. Those ratios
are very close
to the ratios for the similar sizes with the different Kerning option.
In fact,
I wish the extents were as close as these ratios:
2.064151/2.060347 = 1.001846
1.985375/1.985369 = 1.000003
Thank you,
David Mathog
mathog@xxxxxxxxxxx
Manager, Sequence Analysis Facility, Biology Division, Caltech
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