Tcl/tk缩放Truetype字体时的精度问题

最近有国内新客户抱怨我们产品显示的原理图太不专业了，在原理图上使用宋体GB2312设计好中文图表，经过几次缩放时，表格内的文字居然会跑到表格外边，更要命的是打印出来的文档也存在同样的问题。我研究了一下，原来又是Windows GDI Text APIs的一个大坑！

问题详细描述

用Tcl/tk script可以很容易地重现这个问题。如下图，第一行文字使用的Arial字体rendering by GDI APIs, 第二行文字使用的是OF Helvetica字体rendering by Freetype APIs，两行文字都用和字体无关的方式加了边框。

刚运行脚本时，两行文字都是8pt,虽然显示效果不一样，但看着是一样大。
放大几次后，第一行的GDI文字明显跑到框外了。

再看一下这时的log:
font:-family Arial -size 20.118148965022 -weight normal -slant roman -underline 0 -overstrike 0 -type truetype
font:-family {OF Helvetica} -size 19.53125 -weight normal -slant roman -underline 0 -overstrike 0 -type outline
zoom fact:2.44140625

缩放比例2.44140625，outline字体的大小是19.53125ot,正好等于 8 * 2.44140625，没有一点精度损失；而truetype字体的大小是20.118148965022pt,明显偏大了。

原因分析

Tcl tk在Windows上使用GDI font engine来渲染显示Truetype字体。GDI Text APIs 的font size必须是int类型，在代码调用过程中，需要把界面指定的phsicial font size(pt) 转换为 logical font size,从而导致了精度损失；作为对比，如果使用我们产品内置的Outline字体就没有类似的问题，因为Outline字体是我们过去为了支持跨平台，自己移植的Freetype字体，对应使用的是Freetype font engine, Freetype
APIs的font size都是float类型的，从phsicial font size 转换为 logical font size时不会产生精度损失。为了支持中文，用户只能使用基于GDI render 的truetype字体，因为Freetype字体库中还没有支持中文的字体，这次看来必须要对Tcl/tk的font engine做个大手术，用DirectWrite float-based APIs 更换老旧的GDI APIs了。

tcl/tk测试代码如下

proc zoominit {c {zfact {1.1}}}{
# save zoom state in a global variable with the same name as the canvas handle
upvar #0 $c data
set data(zdepth)1.0
}
proc zoom {c fact}{
upvar #0 $c data
# zoom at the current mouse position
set x [$c canvasx [expr {[winfo pointerx $c]-[winfo rootx $c]}]]
set y [$c canvasy [expr {[winfo pointery $c]-[winfo rooty $c]}]]
$c scale all $x $y $fact $fact
# save new zoom depth
set data(zdepth)[expr {$data(zdepth)* $fact}]
# zoom text since the "canvas scale all" command doesn't account for text items
zoomtext $c
puts "zoom fact:$data(zdepth)"
}
proc zoomtext {c}{
upvar #0 $c data
# adjust fonts
foreach{i}[$c find all]{
if{![string equal [$c type $i] text]}{continue}
set fontsize 0
# get original fontsize and text from tags if they were previously recorded
foreach{tag}[$c gettags $i]{
scan $tag {_f%f} fontsize
scan $tag "_t%\[^\0\]" text
}
# if not, then record current fontsize and text and use them
set font [$c itemcget $i -font]
if{!$fontsize}{
set text [$c itemcget $i -text]
set fontsize [font actual $font -size]
$c addtag _f$fontsize withtag $i
$c addtag _t$text withtag $i
}
# scale font
set newsize [expr {$fontsize * $data(zdepth)}]
set index [lsearch -exact $font -size]
incr index
set font [lreplace $font $index $index $newsize]
$c itemconfigure $i -font $font -text $text -anchor w
puts "font:$font"
}
# update canvas scrollregion
set bbox [$c bbox all]
if{[llength $bbox]}{
$c configure -scrollregion $bbox
}{
$c configure -scrollregion [list -4-4 \
[expr {[winfo width $c]-4}] \
[expr {[winfo height $c]-4}]]
}
# move srollbar to topright in order to easily observe whether the zoomed text
# goes outside of the rectangle
$c xview moveto [winfo width $c]
$c yview moveto 0
}
# test code
set c [canvas .c -width 600-height 500 \
-xscrollcommand ".shoriz set" \
-yscrollcommand ".svert set"]
# add x/y scrollbar for the canvas
scrollbar .svert -orient v -command "$c yview"
scrollbar .shoriz -orient h -command "$c xview"
grid .c -row 0-column 0-columnspan 3-sticky news
grid .svert -row 0-column 3-columnspan 1-sticky ns
grid .shoriz -row 1-column 0-columnspan 3-sticky ew
grid columnconfigure .0-weight 1
grid columnconfigure .1-weight 1
grid columnconfigure .2-weight 1
grid rowconfigure .0-weight 1
zoominit .c
button .zoomin -text "Zoom In"-command "zoom $c 1.25"
button .zoomout -text "Zoom Out"-command "zoom $c 0.8"
grid .zoomin .zoomout
set text "Hello, World!Here's a simple procedure called zoom that might help to get you started."
# tfont is a truetype font rendering by Windows GDI APIs (GDI font engine)
set tfont [font actual "-family Arial -size 8"]
# ofont is a outline font rendering by freetype font engine transplanted by ourself
set ofont [font actual "-family {OF Helvetica} -size 8"]
.c create text 450-text $text -font $tfont -anchor w -tag tfont
# create a rectangle with the same size of the bbox of the text. zoom in/out the canvas and we
# can observe whether the text is zoomed in the same proportion as the rectangle
.c create rect [.c bbox tfont]
.c create text 4100-text $text -font $ofont -anchor w -tag ofont
.c create rect [.c bbox ofont]

代码参考资源链接： 

Canvas zooming using mousewheel