Set Dashed Line's Dash And Space Lengths In Pixels Instead Of In Line Width Units

Frogs-Mulligan

As far as I can tell there is no way in Qt to set a dashed line's dash and space lengths in any unit other than multiples of line widths. That is a show stopper for me as I need to set them in pixels in order to draw a dashed line with dashes shorter than the line is wide. Is there any way to do that?

I did try to set the dash and space lengths as fractions of the line width, but it did not work.

Thanks.

SGaist

Hi and welcome to devnet,

You should explain what you are using to draw these lines and on what you are drawing them.

Frogs-Mulligan

Thanks for the reminder.

It' just QPainter and QPen in a QPaintEvent on a simple MainWindow. I did this as a test. I have very little Qt experience so I wanted to see if what I want to do is even possible.

SGaist

Can you show an example of drawing you want to achieve ?

Frogs-Mulligan

Sorry for the delay in getting back.

The example is a single 130 pixel wide spaced line drawn using the Cairo graphics library, where cr is a cairo context. This works with any Cairo line, not just straight lines.

I was hoping that Qt had a similar capabilty.

 double dashes[2] = {7.5, 30.0};
 cairo_set_source_rgba(cr, 1.0, 0.0, 0.0, 0.5);     
 cairo_set_line_width(cr, 130);                      
 cairo_set_antialias(cr,  CAIRO_ANTIALIAS_FAST);
 cairo_set_dash(cr, dashes, 2, 5);
 cairo_move_to(cr, 200, 200);
 cairo_line_to(cr, 1000, 200);

SGaist

Doesn't QPen's width and dash pattern do what you want ?

Frogs-Mulligan

@SGaist

No, I'm afraid not. From the documentation: The dash pattern is specified in units of the pens width; e.g. a dash of length 5 in width 10 is 50 pixels long.

And fractional line width units simply don't work. I tried that.

Frankly I am surprised that the developers did this. I can see line width units for dashes and spaces as a convenience, but not at the expense of not being able to specify them in pixels.

Unless I can find a way to use a Cairo context in Qt, I am afraid I am not going to be able to use Qt. So far, despite searching, I have found no information on that possibility.

SGaist

Based on this file from the Cairo sources I would say you should be good to go.