Why might the scaling of SetWindowExtEx be just wrong? - windows

Question

I am trying to scale images/text etc using MM_ANISTROPIC and what I've done is the following (by the way if the syntax is a little weird, it's originally from delphi so treat the following as pseudocode)
I would expect the following code to produce a rectangle that is 70% of the width of the PaintBox and 30% of the height, yet it doesn't, instead it it noticeably too small.
SetMapMode(hdc,MM_ANISOTROPIC);
SetWindowExtEx(hdc,100,100,0);
SetViewportExtEx(hdc,70,30,0);
Rectangle(hdc, 0,0,PaintBox.width-1,PaintBox.Height-1);
if, on the other hand I change the code so that the SetWindowExtEx has 91 instead of 100 as its parameters (as shown below) then it works, which makes no sense to me at all...
SetMapMode(hdc,MM_ANISOTROPIC);
SetWindowExtEx(hdc,91,91,0);
SetViewportExtEx(hdc,70,30,0);
Rectangle(hdc, 0,0,PaintBox.width-1,PaintBox.Height-1);
My sanity test case was to add the following pseudocode
SetMapMode(hdc,MM_TEXT);
DrawLine(hdc,Round(PaintBox.width*0.7),0,Round(PaintBox.width*0.7),PaintBox.Height-1);
DrawLine(hdc,0,Round(PaintBox.height*0.3),PaintBox.width-1,Round(PaintBox.height*0.3));
I would have expected this to overwrite the lower / bottom edges of my original Rectangle but it does not unless I uses that 91,91 SetWindowExtEx.
Can anyone duplicate this?
FURTHER EDIT: Here is my exact original code I had given pseudo code before to make the question more accessible to non-delphi users but one of my commenters wanted full code to see if my contention that it was a delphi quirk was true or not.
The entire project consisted of a VCL form with a rectangular paintbox dropped on it centered so there was space all around it, and its onPaint event was set to the code below resulting in this image::
procedure TForm11.PaintBox2Paint(Sender: TObject);
var
hdc:THandle;
res:TPoint;
procedure SetupMapMode;
begin
SetMapMode(hdc,MM_ANISOTROPIC);
SetWindowExtEx(hdc,100,100,0);
SetViewportExtEx(hdc,70,30,0);
// These lines are required when we're painting to a TPaintBox but can't be used
// if we're paiting to a TPanel and they were NOT in my original question but only
// got added as part of the answer
// SetViewportOrgEx(hdc,PaintBox2.Left,PaintBox2.Top,#ZeroPoint);
// SetWindowOrgEx(hdc,0,0,#ZeroPoint);
end;
begin
//draw a rectangle to frame the Paintbox Surface
PaintBox2.Canvas.Pen.Style:=psSolid;
PaintBox2.Canvas.Pen.width:=2;
PaintBox2.Canvas.Pen.Color:=clGreen;
PaintBox2.Canvas.Brush.Style:=bsClear;
PaintBox2.Canvas.Rectangle(0,0,PaintBox2.Width-1,PaintBox2.Height-1);
PaintBox2.Canvas.Brush.Style:=bsSolid;
//initialize convenience variable
hdc:=PaintBox2.Canvas.Handle;
SetTextAlign(hdc,TA_LEFT);
//as doing things to the PaintBox2.Canvas via Delphi's interface tends to reset
//everything, I'm ensuring the map mode gets set **immediately** before
//each drawing call
SetupMapMode;
/// Draw Text at the bottom of the PaintBox2.Canvas (though as it's mapped it
/// SHOULD be 1/3 of the way down and much smaller instead)
TextOut(hdc,200,PaintBox2.Height-PaintBox2.Canvas.TextHeight('Ap'),'Hello, World!',13);
PaintBox2.Canvas.Pen.Color:=clblue;
PaintBox2.Canvas.Brush.Style:=bsClear;
//ensure it's set before doing the rectangle
SetupMapMode;
// Redraw the same rectangle as before but in the mapped mode
Rectangle(hdc, 0,0,PaintBox2.Width-1,PaintBox2.Height-1);
PaintBox2.Canvas.Brush.Style:=bsSolid;
//reset the map mode to normal
SetMapMode(hdc,MM_Text);
//draw text at the "same" position as before but unmapped...
TextOut(hdc,200,PaintBox2.Height-PaintBox2.Canvas.TextHeight('Ap'),'Goodbye, World!',15);
//Draw lines exactly at 70% of the way across and 30% of the way down
//if this works as expected they should overwrite the right and bottom
//borders of the rectangle drawn in the mapped mode
PaintBox2.Canvas.Pen.Color:=RGB(0,255,255);
PaintBox2.Canvas.MoveTo(Round(PaintBox2.Width*0.7),0);
PaintBox2.Canvas.LineTo(Round(PaintBox2.Width*0.7),PaintBox2.Height);
PaintBox2.Canvas.MoveTo(0,Round(PaintBox2.Height*0.3));
PaintBox2.Canvas.LineTo(PaintBox2.Width,Round(PaintBox2.Height*0.3));
end;

Answer 1

Okay, I don't know WHY the following is necessary -- it may be a Delphi quirk, the fact that I'm using a TPaintBox with is a TGraphicControl rather than a Component, or if I'm missing out on some fundamental concept on how this whole mapping mode works, BUT if I add the following code:
ZeroPoint:=TPoint.Zero;
SetViewportOrgEx(hdc,PaintBox1.Left,PaintBox1.Top,#ZeroPoint);
SetWindowOrgEx(hdc,0,0,#ZeroPoint);
Then it all displays as expected. Anyone have any explanations as to why this is necessary?
EDIT: Okay, I've got a PARTIAL explanation. It has to do with the control I was painting on being a TPaintBox, which is a TGraphic control rather than a TWinControl. To wit:
TGraphicControl is the base class for all lightweight controls.
TGraphicControl supports simple lightweight controls that do not need the ability to accept keyboard input or contain other controls. Since lightweight controls do not wrap Windows screen objects, they are faster and user fewer resources than controls based on TWinControl.
As such, although they APPEAR to have a separate canvas, I have this sneaking feeling that they are really sharing the form's canvas which is why, when I switched to a TWinControl descendant, which DOES own its own Windows DC, then the display worked as expected without setting the ViewpointOrg.
So it was a Delphi quirk after all...!