Zoom Image at specific location - centered - c++

Question

So I wanted to add a texture to a map, but the problem with it is that I can't quite get why it doesn't get to the correct place while zooming with different zoom sizes.
I'm normally trying to achieve setting the texture position on the background to my position, keeping myself centered into the frame: for example my texture size is 1500x1600 and I'm located at X140, Y590 in that picture ( yes, the coordinates are retrieved correctly as I've checked with the console ), zooming in with some value and scaling the texture and setting it's position to where I'm at.
The code is the following:
if (!areTexturesInit) {
InitiateTextures();
areTexturesInit = true;
}
wxBitmap bit(imageTest);
wxPaintDC dc(this);
double zoomSize = 0.9; // here I'm applying the zooming proportions ( 0.1 - bigger size of the texture, 0.9 - more zoomed in )
this->SetSize(wxSize(386, 386)); // size of the main frame
backgroundSize.x = GetSize().x; // get the size of the main frame
backgroundSize.y = GetSize().y;
middlePoint.x = (backgroundSize.x / 2); // calculate the middle point of the frame
middlePoint.y = (backgroundSize.y / 2);
mapSizeX = 25600 / -zoomSize; // scale vs zoom size
mapSizeY = 25600 / zoomSize;
Vector3 myPosition;
GetPlayerPosition(&myPosition); // gets my location
float TextureCoordinateX = middlePoint.x + (myPosition.x / mapSizeX) * backgroundSize.x;
float TextureCoordinateY = middlePoint.y - (myPosition.y / mapSizeY) * backgroundSize.y;
dc.DrawBitmap(bit, TextureCoordinateX, TextureCoordinateY);
Vector3 myPosOnMap = PositionToMapPosition(myPosition, myPosition); // calculates my position on the map vs mapSizeX and Y & rotates vector
dc.SetPen(wxPen(wxColor(255, 0, 0), 4));
dc.DrawRectangle(wxRect(myPosOnMap.x, myPosOnMap.y, 2, 2)); // draws me on the map with a red square
The problem is that I think I've messed up the zooming part somewhere.
I've attached some demos so you can see what I'm talking about:
"zoomSize" of 0.9:
"zoomSize" of 0.67 - which kind of works, but I need to change it to different zoomSizes, there being the problem:

Answer 1

Panning and zooming a dc is surprisingly complicated. It requires working with 3 separate coordinate systems and it's really easy to accidentally work in the wrong coordinate system.
Here's an example I wrote a while ago that shows how to do the calculations that allow a dc to be pan and zoomed.
It sounds like you're not interested in the pan part, so you can ignore all the stuff that allows a user to set their own pan. However, it's still necessary to to use a pan vector just for the zoom in order to center the zoom at the correct location.
// For compilers that support precompilation, includes "wx/wx.h".
#include "wx/wxprec.h"
#ifdef __BORLANDC__
#pragma hdrstop
#endif
// for all others, include the necessary headers (this file is usually all you
// need because it includes almost all "standard" wxWidgets headers)
#ifndef WX_PRECOMP
#include "wx/wx.h"
#endif
#include <wx/graphics.h>
#include <wx/dcbuffer.h>
class PanAndZoomCanvas:public wxWindow
{
public:
PanAndZoomCanvas(wxWindow *parent,
wxWindowID id = wxID_ANY,
const wxPoint &pos=wxDefaultPosition,
const wxSize &size=wxDefaultSize,
long style=0,
const wxString &name="PanAndZoomCanvas");
wxRect2DDouble GetUntransformedRect() const;
protected:
void DoDrawCanvas(wxGraphicsContext*);
private:
void OnPaint(wxPaintEvent&);
void OnMouseWheel(wxMouseEvent&);
void OnLeftDown(wxMouseEvent&);
void OnMotion(wxMouseEvent&);
void OnLeftUp(wxMouseEvent&);
void OnCaptureLost(wxMouseCaptureLostEvent&);
void ProcessPan(const wxPoint&,bool);
void FinishPan(bool);
int m_zoomFactor;
wxPoint2DDouble m_panVector;
wxPoint2DDouble m_inProgressPanVector;
wxPoint m_inProgressPanStartPoint;
bool m_panInProgress;
};
PanAndZoomCanvas::PanAndZoomCanvas(wxWindow *parent, wxWindowID id,
const wxPoint &pos, const wxSize &size,
long style, const wxString &name)
:wxWindow(parent, id, pos, size, style, name)
{
Bind(wxEVT_PAINT,&PanAndZoomCanvas::OnPaint,this);
Bind(wxEVT_MOUSEWHEEL,&PanAndZoomCanvas::OnMouseWheel,this);
Bind(wxEVT_LEFT_DOWN,&PanAndZoomCanvas::OnLeftDown,this);
SetBackgroundStyle(wxBG_STYLE_PAINT);
m_zoomFactor = 100;
m_panVector = wxPoint2DDouble(0,0);
m_inProgressPanStartPoint = wxPoint(0,0);
m_inProgressPanVector = wxPoint2DDouble(0,0);
m_panInProgress = false;
}
void PanAndZoomCanvas::DoDrawCanvas(wxGraphicsContext* gc)
{
gc->SetPen(*wxBLACK_PEN);
wxGraphicsPath path = gc->CreatePath();
path.MoveToPoint(100,100);
path.AddLineToPoint(300,100);
path.AddLineToPoint(300,300);
path.CloseSubpath();
gc->StrokePath(path);
}
void PanAndZoomCanvas::OnPaint(wxPaintEvent& WXUNUSED(event))
{
wxAutoBufferedPaintDC dc(this);
dc.Clear();
wxGraphicsContext* gc = wxGraphicsContext::Create(dc);
if ( gc )
{
double a = m_zoomFactor / 100.0;
wxPoint2DDouble totalPan = m_panVector + m_inProgressPanVector;
gc->Translate(-totalPan.m_x, -totalPan.m_y);
gc->Scale(a, a);
DoDrawCanvas(gc);
delete gc;
}
}
void PanAndZoomCanvas::OnMouseWheel(wxMouseEvent& event)
{
if ( m_panInProgress )
{
FinishPan(false);
}
int rot = event.GetWheelRotation();
int delta = event.GetWheelDelta();
int oldZoom = m_zoomFactor;
m_zoomFactor += 10*(rot/delta);
if ( m_zoomFactor<10 )
{
m_zoomFactor = 10;
}
if ( m_zoomFactor>800)
{
m_zoomFactor = 800;
}
double a = oldZoom / 100.0;
double b = m_zoomFactor / 100.0;
// Set the panVector so that the point below the cursor in the new
// scaled/panned cooresponds to the same point that is currently below it.
wxPoint2DDouble uvPoint = event.GetPosition();
wxPoint2DDouble stPoint = uvPoint + m_panVector;
wxPoint2DDouble xypoint = stPoint/a;
wxPoint2DDouble newSTPoint = b * xypoint;
m_panVector = newSTPoint - uvPoint;
Refresh();
}
void PanAndZoomCanvas::ProcessPan(const wxPoint& pt, bool refresh)
{
m_inProgressPanVector = m_inProgressPanStartPoint - pt;
if ( refresh )
{
Refresh();
}
}
void PanAndZoomCanvas::FinishPan(bool refresh)
{
if ( m_panInProgress )
{
SetCursor(wxNullCursor);
if ( HasCapture() )
{
ReleaseMouse();
}
Unbind(wxEVT_LEFT_UP, &PanAndZoomCanvas::OnLeftUp, this);
Unbind(wxEVT_MOTION, &PanAndZoomCanvas::OnMotion, this);
Unbind(wxEVT_MOUSE_CAPTURE_LOST, &PanAndZoomCanvas::OnCaptureLost, this);
m_panVector += m_inProgressPanVector;
m_inProgressPanVector = wxPoint2DDouble(0,0);
m_panInProgress = false;
if ( refresh )
{
Refresh();
}
}
}
wxRect2DDouble PanAndZoomCanvas::GetUntransformedRect() const
{
double a = m_zoomFactor / 100.0;
wxSize sz = GetSize();
wxPoint2DDouble zero = m_panVector/a;
return wxRect2DDouble(zero.m_x, zero.m_y, sz.GetWidth()/a, sz.GetHeight()/a);
}
void PanAndZoomCanvas::OnLeftDown(wxMouseEvent& event)
{
wxCursor cursor(wxCURSOR_HAND);
SetCursor(cursor);
m_inProgressPanStartPoint = event.GetPosition();
m_inProgressPanVector = wxPoint2DDouble(0,0);
m_panInProgress = true;
Bind(wxEVT_LEFT_UP, &PanAndZoomCanvas::OnLeftUp, this);
Bind(wxEVT_MOTION, &PanAndZoomCanvas::OnMotion, this);
Bind(wxEVT_MOUSE_CAPTURE_LOST, &PanAndZoomCanvas::OnCaptureLost, this);
CaptureMouse();
}
void PanAndZoomCanvas::OnMotion(wxMouseEvent& event)
{
ProcessPan(event.GetPosition(), true);
}
void PanAndZoomCanvas::OnLeftUp(wxMouseEvent& event)
{
ProcessPan(event.GetPosition(), false);
FinishPan(true);
}
void PanAndZoomCanvas::OnCaptureLost(wxMouseCaptureLostEvent&)
{
FinishPan(true);
}
class MyFrame : public wxFrame
{
public:
MyFrame(wxWindow* parent, int id = wxID_ANY, wxString title = "Demo",
wxPoint pos = wxDefaultPosition, wxSize size = wxDefaultSize,
int style = wxDEFAULT_FRAME_STYLE );
};
MyFrame::MyFrame( wxWindow* parent, int id, wxString title, wxPoint pos
, wxSize size, int style )
:wxFrame( parent, id, title, pos, size, style )
{
PanAndZoomCanvas* canvas = new PanAndZoomCanvas(this);
}
class myApp : public wxApp
{
public:
virtual bool OnInit()
{
MyFrame* frame = new MyFrame(NULL);
frame->Show();
return true;
}
};
wxIMPLEMENT_APP(myApp);
On windows, this looks like this: