Question

My application is a vector drawing application. It works with OpenGL. I will be modifying it to instead use the Cairo 2D graphics library. The issue is with zooming. With openGL camera and scale factor sort of work like this:

 float scalediv = Current_Scene().camera.ScaleFactor / 2.0f;
 float cameraX = GetCameraX();
 float cameraY = GetCameraY();
 glMatrixMode(GL_PROJECTION);
 glLoadIdentity();

 float left = cameraX - ((float)controls.MainGlFrame.Dimensions.x) * scalediv;
 float right = cameraX + ((float)controls.MainGlFrame.Dimensions.x) * scalediv;
 float bottom = cameraY - ((float)controls.MainGlFrame.Dimensions.y) * scalediv;
 float top = cameraY + ((float)controls.MainGlFrame.Dimensions.y) * scalediv;

 glOrtho(left,
  right,
  bottom, 
  top,
  -0.01f,0.01f);

 // Set the model matrix as the current matrix
 glMatrixMode(GL_MODELVIEW);
 glLoadIdentity();

 hdc = BeginPaint(controls.MainGlContext.mhWnd,&ps);

Mouse position is obtained like this:

 POINT _mouse = controls.MainGlFrame.GetMousePos();
vector2f mouse = functions.ScreenToWorld(_mouse.x,_mouse.y,GetCameraX(),GetCameraY(),
             Current_Scene().camera.ScaleFactor,
            controls.MainGlFrame.Dimensions.x,
            controls.MainGlFrame.Dimensions.y );

vector2f CGlEngineFunctions::ScreenToWorld(int x, int y, float camx, float camy, float scale, int width, int height)
{
 // Move the given point to the origin, multiply by the zoom factor and
 // add the model coordinates of the center point (camera position)
 vector2f p;


 p.x =  (float)(x - width / 2.0f) * scale +
  camx;

 p.y = -(float)(y - height / 2.0f) * scale +
  camy;

 return p;
}

From there I draw the VBO’s of triangles. This allows me to pan and zoom in. Given that Cairo only can draw based on coordinates, how can I make it so that a vertex is properly scaled and panned without using transformations. Basically GlOrtho sets the viewport usually but I dont think I could do this with Cairo.

Well GlOrtho is able to change the viewport matrix instead of modifying the verticies but how could I instead modify the verticies to get the same result?

Thanks

*Given vertex P, which was obtained from ScreenToWorld, how could I modify it so that it is scaled and panned accordng to the camera and scale factor? Because usually OpenGL would essentially do this

Answer 1

I think Cairo can do what you want … see http://cairographics.org/matrix_transform/ . Does that solve your problem, and if not, why ?