[Lazarus] Printer printable width&height

[Michael Van Canneyt]

On Thu, 9 Aug 2018, Jesus Reyes A. via Lazarus wrote:

> En Sat, 28 Jul 2018 08:05:31 -0500, Michael Van Canneyt via Lazarus 
> <lazarus at lists.lazarus-ide.org> escribió:
>
>>
>> Hi,
>>
>> The selectprinter demo prints a kind of test page.
>>
>> However, on linux the rectangle it draws on the page seems to be off. The
>> left margin seems to be about right, but the bottom and top margins fall 
>> off
>> the page.
>>
>> If I try to code something myself, I would code it like this:
>>
>> procedure TMainForm.GetPageRect(Out PageRect : TRect);
>>
>>    Function ToPixels(aUnit,ares : Integer) : Integer;
>>
>>    begin
>>      if PSDemo.Units in [pmDefault,pmMillimeters] then
>>        // 1/1000 of a mm
>>        Result:=Round(((aUnit div 1000) / 25.4) * Ares)
>>      else
>>        Result:=aUnit * Ares
>>    end;
>>
>> Var
>>    LM,RM,TM,BM,H,W : Integer;
>>
>> begin
>>    H:=Printer.PaperSize.Height;
>>    W:=Printer.PaperSize.Width;
>>    LM:=ToPixels(PSDemo.MarginLeft,Printer.XDPI);
>>    RM:=ToPixels(PSDemo.MarginRight,Printer.XDPI);
>>    TM:=ToPixels(PSDemo.MarginLeft,Printer.YDPI);
>>    BM:=ToPixels(PSDemo.MarginBottom,Printer.YDPI);
>>    PageRect:=Rect(LM,TM,W-RM,H-BM);
>> end;
>>
>> PSDemo is a TPageSetupDialog. The values displayed by the lazarus test
>> program match more or less what my code gets. Similarly, if I draw a 
>> rectangle with the rectangle defined as above, the bottom-right falls 
>> off the page.
>>
>> So the question is: what is the origin of the printer canvas ? Top-left 
>> corner of the paper (What it should be IMO ?)
>> Or is it the Top-left corner of the printable area of the page ?
>>
>
> The top-left of the paper canvas is 0,0 and coincide with the area of the 
> whole paper. Any drawing outside of this area would be clipped. Some 
> printers also do not allow drawing outside of the working area.

This does not correspond to my findings.

I did many tests (using a lot of paper in the process) and found 
that (0,0) is the origin of the printable area, NOT the paper area.

>> How to obtain the origin of the printable area ?
>> Is the PageWidth/Pageheight relative to this origin ?
>>
>>
>
> The printable area should be Printer.PaperSize.PaperRect.WorkRect

Yes.

I printed a rectangle with 2 cm sides, at (0,0), and it is printed exactly 
where workrect top,left says it should be, so my conclusion is that (0,0) 
is the origin of the printable area.

(which, in my opinion, also makes more sense)

>> Or is there something else wrong in the printer class or the demo code ?
>> (using GTK2, 64-bit)
>>
>
> Should not matter in this case but it seems TM is using the left margin 
> instead of the top margin.

I know, I already corrected that before carrying out tests.

> As the TPageSetupDialog works with the current printer just need to check 
> that selected printer is not changed after TPageSetupDialog execution.

Little chance of that.
There is only 1 printer, and 1 paper type on my system.

> About why the selectprinter demo is wrong have to check, last time I did 
> it worked fine in windows, linux at least, IIRC.

Well, definitely not today on a HP laserjet P2015n, linux mint, 64bit, GTK2.
I can send a picture of the produced output if you want :-)

Michael.