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

1. proc zoominit {c {zfact {1.1}}}{
2. # save zoom state in a global variable with the same name as the canvas handle
3. upvar #0 $c data
4. set data(zdepth)1.0
5. }
7. proc zoom {c fact}{
8. upvar #0 $c data
9. # zoom at the current mouse position
10. set x [$c canvasx [expr {[winfo pointerx $c]-[winfo rootx $c]}]]
11. set y [$c canvasy [expr {[winfo pointery $c]-[winfo rooty $c]}]]
12. $c scale all $x $y $fact $fact
13. # save new zoom depth
14. set data(zdepth)[expr {$data(zdepth)* $fact}]
15. # zoom text since the "canvas scale all" command doesn't account for text items
16. zoomtext $c
17. puts "zoom fact:$data(zdepth)"
18. }
20. proc zoomtext {c}{
21. upvar #0 $c data
22. # adjust fonts
23. foreach{i}[$c find all]{
24. if{![string equal [$c type $i] text]}{continue}
25. set fontsize 0
26. # get original fontsize and text from tags if they were previously recorded
27. foreach{tag}[$c gettags $i]{
28. scan $tag {_f%f} fontsize
29. scan $tag "_t%\[^\0\]" text
30. }
31. # if not, then record current fontsize and text and use them
32. set font [$c itemcget $i -font]
33. if{!$fontsize}{
34. set text [$c itemcget $i -text]
35. set fontsize [font actual $font -size]
36. $c addtag _f$fontsize withtag $i
37. $c addtag _t$text withtag $i
38. }
39. # scale font
40. set newsize [expr {$fontsize * $data(zdepth)}]
41. set index [lsearch -exact $font -size]
42. incr index
43. set font [lreplace $font $index $index $newsize]
44. $c itemconfigure $i -font $font -text $text -anchor w
45. puts "font:$font"
46. }
48. # update canvas scrollregion
49. set bbox [$c bbox all]
50. if{[llength $bbox]}{
51. $c configure -scrollregion $bbox
52. }{
53. $c configure -scrollregion [list -4-4 \
54. [expr {[winfo width $c]-4}] \
55. [expr {[winfo height $c]-4}]]
56. }
57. # move srollbar to topright in order to easily observe whether the zoomed text
58. # goes outside of the rectangle
59. $c xview moveto [winfo width $c]
60. $c yview moveto 0
61. }
64. # test code
65. set c [canvas .c -width 600-height 500 \
66. -xscrollcommand ".shoriz set" \
67. -yscrollcommand ".svert set"]
68. # add x/y scrollbar for the canvas
69. scrollbar .svert -orient v -command "$c yview"
70. scrollbar .shoriz -orient h -command "$c xview"
71. grid .c -row 0-column 0-columnspan 3-sticky news
72. grid .svert -row 0-column 3-columnspan 1-sticky ns
73. grid .shoriz -row 1-column 0-columnspan 3-sticky ew
74. grid columnconfigure .0-weight 1
75. grid columnconfigure .1-weight 1
76. grid columnconfigure .2-weight 1
77. grid rowconfigure .0-weight 1
78. zoominit .c
79. button .zoomin -text "Zoom In"-command "zoom $c 1.25"
80. button .zoomout -text "Zoom Out"-command "zoom $c 0.8"
81. grid .zoomin .zoomout
82. set text "Hello, World!Here's a simple procedure called zoom that might help to get you started."
83. # tfont is a truetype font rendering by Windows GDI APIs (GDI font engine)
84. set tfont [font actual "-family Arial -size 8"]
85. # ofont is a outline font rendering by freetype font engine transplanted by ourself
86. set ofont [font actual "-family {OF Helvetica} -size 8"]
87. .c create text 450-text $text -font $tfont -anchor w -tag tfont
88. # create a rectangle with the same size of the bbox of the text. zoom in/out the canvas and we
89. # can observe whether the text is zoomed in the same proportion as the rectangle
90. .c create rect [.c bbox tfont]
91. .c create text 4100-text $text -font $ofont -anchor w -tag ofont
92. .c create rect [.c bbox ofont]

代码参考资源链接： 
Canvas zooming using mousewheel