[Gambas-user] Drawing Area

[Doriano Blengino]

tobiasboe at ...221... ha scritto:
>    Dear Mailing-Liste,
>    my name is Tobias and I have the following problem:
>    The function y=f(x) shall be displayed with the values x_min <= x <= x_max
>    in a drawing area (dwg.ClientW = const = 801, dwg.ClientH = const = 601)
>    using the methods Draw.Scale and Draw.Translate.
>    My questions:
>    *What do these methods do?
>    *How do I use them to fix my problem?
>    Every tip to use these methods or a source to read is welcome - especially
>    with examples.
>   
The scale() function multiplies the X and Y coordinates for the values 
you specify, respectively in the first and second argument. So, if you 
issue a line(0,0,100,100), the normal result would be a diagonal line 
"\" starting in the upper-left corner of the drawing area, and going to 
the right and down. If a scale(2,2) is in effect, then the line will 
have the same appearance, but it will extend to (200;200) because every 
single coordinate is first multiplied by the scale() values. If these 
values are equal, then the aspect ratio is mantained; if these values 
are different an anamorphic transformation takes effect; you can use 
this to correct some monitor problem...
Note that the computer graphic conventions say that the Y coordinate 
gets bigger going down to the screen, which is exactly the contrary in 
the normal world. So you can use draw.scale(..., -1) to flip the 
graphics in the correct way.

To fit your function in you drawing area, you should use 
ScaleX=801/(x_max-x_min), and OffsetX=-x_min:

    draw.begin(...)
    draw.scale(801/(x_max-x_min), ???)
    draw.translate(-x_min, 0?)
    blahblahblah (draw.point(), draw.line()...)
    draw.end()

Note that draw.translate() takes coordinates which then get scaled by 
scale(): this is why you can use "-x_min" in the draw.translate. If you 
invert the order of scale() and translate(), then the coordinates passed 
to translate() are no more "world" coordinates, but "device" coordinates.

Anyway, I attach a project: play with the controls to see their effect. 
Note that in the drawing routine the order is "scale, translate". Try to 
invert this order to see the effect.

Last thing to say: drawingareas can work in two modes - cached and not 
cached. In cached mode, you draw on them when you want, and they keep 
the data, much like normal paper (so you must erase them to change the 
contents). The mode I used is not cached, which is more correct in this 
case; when the graphic needs to be displayed, it "asks" your code to 
paint the area (using an event) - no image is kept in memory and, 
besides, it is possible to optimize a few things. To update the image, 
you refresh() the drawingarea, which in turn raises the event; this also 
happens when you cover the graphic with another window and later you 
unveil it.

Hope this introduction can help: ask for more if you are in need.

Regards,
Doriano


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