Canvas - Drawing in mm
lazarus / 39-Graphics

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celso · 2012-11-22 19:13

I´m trying to draw a grid on the screen, but using mm as a unit of
measure.

I´d read that it depends on the screen resolution, but I didn´t find
any example on how to do it. My code to draw the grid is:

Code: [Select]    Bitmap.Height := Height;
    Bitmap.Width  := Width;

    // Drawing the background
    Bitmap.Canvas.Pen.Color := clWhite;
    Bitmap.Canvas.Rectangle(0, 0, Width, Height);

    // Drawing the grid
    Bitmap.Canvas.Pen.Color := RGBToColor(243,243,243);
    for x:= 1 to (Width div 10) do
        for y:= 1 to (Height div 10) do
            Bitmap.Canvas.Rectangle( round((x-1)*10),round((y-1)*10),
                                     round(x*10),round(y*10));   

the grid draw lines with 10 of distance, I want to draw with 10mm of
distance.

Thanks!

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Blaazen · 2012-11-22 19:26

Forms have the property PixelsPerInch (96 in my case). One inch is
25,4 mm.
So you should calculate: Distance := 10 * ppi / 25,4 (which is here ~
38 px / 10mm)

But my laptop LCD (15.6") is 332 mm wide and has horizontally 1680
pixels. It results to ~50 px / 10mm.

It is > 20% difference. 

If you need exact value you will have to tune your program for each
LCD / monitor.

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celso · 2012-11-22 20:03

I used your suggestion, but I got a PPI, of the screen:

Code: [Select]xmm := 10 * ScreenInfo.PixelsPerInchX / 25.4;
ymm := 10 * ScreenInfo.PixelsPerInchY / 25.4;  

then I used these values as increment in x and y. Looks Ok, I´ll do
some other tests.

But, thanks for the answer!