[cairo] Query regarding Cairo.

[Bryce W. Harrington]

Hi Gaurav,

If you use all the cairo_move_to() calls then you end up with a rectangle made up of four discontinuous line segments.

http://www.bryceharrington.org/files/g.pruthi.before.png

Instead just use the first cairo_move_to() and then only cairo_line_to() after that.  See:

http://www.bryceharrington.org/files/g.pruthi.c
http://www.bryceharrington.org/files/g.pruthi.after.png

Btw, the final cairo_line_to() call (when i==3) is unnecessary in this case; the cairo_close_path() call will complete the last line of the rect.

http://www.bryceharrington.org/files/g.pruthi.c

I couldn't find the tutorial that this sample code comes from.  If you could please point me at it; if it's a cairographics.org tutorial it may need to be improved.

--
Bryce Harrington
Senior Open Source Developer  -  b.harrington at samsung.com
Open Source Group             -  Samsung Research America

________________________________
From: Gaurav Pruthi [g.pruthi at samsung.com]
Sent: Thursday, July 04, 2013 1:07 AM
To: Bryce W. Harrington
Subject: Query regarding Cairo.


Hi Bryce,

I found you on cairo mailing list. Just have one query if you have time to look into. I have just started using cairo library... I have seen examples to fill a

polygon which are repetitive calls to move_to() and line_to() routines. When I tried following snippet of code which is mentioned in most of the tutorials, but

it didn't worked.

void drawRectangle(cairo_t* cr)
{
    cairo_path_t path;
    cairo_path_data_t pathData[4];
    pathData[0].point.x = 10 ;
    pathData[0].point.y = 20;

    pathData[1].point.x = 210;
    pathData[1].point.y = 20;

    pathData[2].point.x = 210;
    pathData[2].point.y = 320;

    pathData[3].point.x = 10;
    pathData[3].point.y =  320;
    //Set path data in path

    cairo_move_to(cr, pathData[0].point.x, pathData[0].point.y);
    for(int i =0 ; i<= 3; ++i)
    {
        cairo_move_to(cr, pathData[i].point.x, pathData[i].point.y);
        if(i == 3){
            cairo_line_to(cr, pathData[0].point.x, pathData[0].point.y);
        }
        else
            cairo_line_to(cr, pathData[i+1].point.x, pathData[i+1].point.y);
    }

    cairo_close_path(cr);

    cairo_set_source_rgb(cr,1.0,0.0,0.0);
    cairo_fill_preserve(cr);

    cairo_set_source_rgba(cr,0.0,1.0,0.0,0.5);
    cairo_set_line_width(cr,10.0);
    cairo_stroke(cr);

}


But this one worked where I am moving to the first point and just making line_to() calls is working .. below one:

void drawRectangle(cairo_t* cr)
{
    cairo_path_t path;
    cairo_path_data_t pathData[4];
    pathData[0].point.x = 10 ;
    pathData[0].point.y = 20;

    pathData[1].point.x = 210;
    pathData[1].point.y = 20;

    pathData[2].point.x = 210;
    pathData[2].point.y = 320;

    pathData[3].point.x = 10;
    pathData[3].point.y =  320;

    cairo_move_to(cr, pathData[0].point.x, pathData[0].point.y);
    for(int i =0 ; i< =3 ; ++i)
    {

        cairo_line_to(cr, pathData[i].point.x, pathData[i].point.y);
    }

    cairo_close_path(cr);

    cairo_set_source_rgb(cr,1.0,0.0,0.0);
    cairo_fill_preserve(cr);

    cairo_set_source_rgba(cr,0.0,1.0,0.0,0.5);
    cairo_set_line_width(cr,10.0);
    cairo_stroke(cr);

}



First sample is based on  the examples available on internet. Can you pls tell whats the problem with the first one.



Thanks a lot for your time.

Gaurav Pruthi

Display Platform

Samsung R&D India - Delhi Lab



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