Greg Ercolano wrote:
> Below is an interesting variation that just draws a black
> rectangle with a 2 pixel white border, but every second
> it alternates drawing the border:
>
> 1) using fl_line_style(FL_SOLID,2) + fl_rect()
>
> 2) using fl_line_style(FL_SOLID,0) + two fl_rect()s,
> one single pixel rect inside the other.
Yes, really interesting, nice demo.
> Theoretically, both should look the same (though I guess it depends
> which side of the dimensions the extra pixel thickness should go for
> a two pixel thick line), but on all platforms you can see there's
> flashing going on.
No, they shouldn't, because...
According to the docs, fl_rect() "Draws a 1-pixel border inside the
given bounding box." Emphasis on "1-pixel" and "inside".
As I read this, Jane's original code is wrong, and the correct fix is to
draw two fl_rect()'s, as she did already :-) .
However FLTK (maybe?) has a bug as well, because setting fl_line_style()
_changes_ the behavior of fl_rect() WRT line width.
Although it is documented "If you change this it is your responsibility
to set it back to the default using fl_line_style(0)", it is not
completely clear (for me) if this means that you must _reset_ the line
style before calling fl_rect(). If this is true, then everything else is
undefined behavior, and FLTK is right. ;-)
> On linux, with the line style set to 2, it looks like only the
> right and bottom edges are drawn with two pixel thickness,
> and the top and left edges are drawn with one.
>
> On windows, there's a pretty horrible thing going on at
> the lower right corner, apparently dropping pixels.
So far I could only test on Windows. My additional observation: Changing
the line style like below can fix the "dropping pixels" effect.
OTOH, with the default line settings (fl_line_style(0)), there is this
missing pixel in the lower right corner, and this might be a real bug.
BUT: Without _any_ fl_line_style() call, there is a correct bottom right
corner (no missing pixel). Now I'm really confused - fl_line_style(0)
seems to be different from setting no line style at all :-(
> So it seems like the only way to get pixel accurate
> results is to leave the line style alone, and draw two
> separate rectangles.
Right :-)
> Here's the program described above:
>
> #include <FL/Fl.H>
> #include <FL/Fl_Double_Window.H>
> #include <FL/fl_draw.H>
> #include <FL/Fl_Box.H>
Add this:
// #define LINE_STYLE FL_SOLID|FL_CAP_FLAT
// #define LINE_STYLE FL_SOLID|FL_CAP_ROUND
// #define LINE_STYLE FL_SOLID|FL_CAP_SQUARE
#define LINE_STYLE FL_SOLID
and experiment with different values, as shown below:
> class Foo: public Fl_Box {
> int count;
> void draw() {
> fl_color(FL_BLACK);
> fl_rectf(x(),y(),w(),h());
> fl_color(FL_WHITE);
> if ( count & 1 ) {
> // use 2 pixel thick lines
> fl_line_style(FL_SOLID, 1);
fl_line_style(LINE_STYLE, 2); // (2 pixel test)
> fl_rect(x(),y(),w(),h());
> fl_line_style(FL_SOLID, 0); // docs say we must leave it
> at zero
> } else {
> // use 1 pixel thick lines, but draw two rects, one inside the
> other
> fl_line_style(FL_SOLID, 0);
// fl_line_style(LINE_STYLE,0); // (also try this)
fl_line_style(0); // (default line style)
> fl_rect(x(),y(),w(),h());
> fl_rect(x()+1,y()+1,w()-2,h()-2);
> }
fl_line_style(0); // *always* reset line style
[rest of code snipped for brevity]
Albrecht
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