Re: [ROOT] bug in gPad->PixeltoY()?

From: Dirk Dodt (
Date: Wed Jun 23 2004 - 18:33:17 MEST

Ok Here it is. I'm using just a single Pad on the Canvas, The Output 
generated when moving the mouse over the pad is:
* Pixel Coordinates,
* User Coordinates passed back from Pixelto?() [already "corrected" by 
calling -can->PixeltoY(-y)!]
* and the User Coordinates calculated the way which I understood from 
the root manual they should work.

The "corrected" coordinates seem to be right, but when you click the 
left mouse button the User Coordinate Range is changed and from there on 
the ouput from my defines is different from the -PixeltoY(-y) result. 
(which one is correct now :)?)

kind regards

Rene Brun wrote:
> Dirk,
> Could you send a simple script showing your problem?
> It could be that you are using PixeltoX where you should use AbsPixeltoX.
> There is a difference if you have a canvas divided into subpads.
> Rene brun
> Dirk Dodt wrote:
>>I have the feeling the TPad::PixeltoX Method Or Rather the underlying
>>method whichs sets the needed Coefficients might be buggy.
>>The mistake (if any) can sometimes(!) be corrected by using
>>Maybe I have not fully understand everything concerning the different
>>coordinate systems, but if someone else is having similar problems it
>>might be worth looking into it. (For my own case I already have found a
>>For Completeness:
>>I'm using a TRootEmbeddedCanvas in Root 3.10_03 (Suse 7.3/=Desy linux 4)
>>inside a QT Gui to Display some Detector Information.
>>In order to do a Zoom into the Detector, I Connected the
>>ProcessedEvent(Int_t,Int_t,Int_t,TObject*) Slot with my code, that uses
>>PixeltoX and PixeltoY to transform the x,y[pixel] provided by the signal
>>into user coordinates and set a new ...->Range(x1,y1,x2,y2) for the Pad.

TCanvas *can;
void PixeltoY()
can = new TCanvas("test","test",500,500);
can->Connect("ProcessedEvent(Int_t,Int_t,Int_t,TObject*)", 0, "", "DisplayGetCoordinates(Int_t, Int_t, Int_t, TObject *)");
#define PIXELTOX(x) ((x2-x1)*x/can->GetWw()+x1)
#define PIXELTOY(y) ((y2-y1)*(-y)/can->GetWh()+y2)

void DisplayGetCoordinates(Int_t event, Int_t x, Int_t y, TObject *selected)	
	Double_t x1,x2,y1,y2;
	printf("Event %d: at (%3d, %3d)=(%4.1f, %4.1f)=(%3.1f, %3.1f)\n", event,x,y, can->PixeltoX(x), -can->PixeltoY(-y), PIXELTOX(x), PIXELTOY(y));//=(%3.1f, %3.1f)	
	switch (event) {
		case 1://ButtonDown

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