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Adding Position Reporting Code
[Tutorial 3 - User Interaction 2]

Next we will learn an important lesson about coordinate space in RenderWare Graphics. The code will be extended to report the position of the picked atomic. As we drag the atomic in world space we will get feedback telling us where the atomic is.

• Add a static, file global variable called ShowPos. This will be a Boolean (type RwBool), so initialize it FALSE.

• Add another entry to the menu in InitializeMenu(). This should control the setting of the ShowPos variable.

• Compile the application.

  The application can currently report the frames-per-second rate. This is displayed by the DisplayOnScreenInfo() function. We will now change this function to also report the position of the picked atomic if the ShowPos variable is TRUE.

• Add the following code to the DisplayOnScreenInfo() function:

      static void 
      DisplayOnScreenInfo(RwCamera *camera)
      {
          RwChar caption[256];
          RtCharsetDesc charsetDesc;
          RwInt32 crw, crh;
  

          crw = RwRasterGetWidth(RwCameraGetRaster(camera));
          crh = RwRasterGetHeight(RwCameraGetRaster(camera));
  

          RtCharsetGetDesc(Charset, &charsetDesc);
  

          if( FPSOn )
          {
              RsSprintf(caption, RWSTRING("FPS: %03d"), FramesPerSecond);
              RsCharsetPrint( Charset, caption, 0, 0, rsPRINTPOSTOPRIGHT );
          }
  

          if (ShowPos)
          {
              if (PickedAtomic)
              {
                  RwFrame *f = RpAtomicGetFrame(PickedAtomic);
                  RwMatrix *m = RwFrameGetLTM(f);
                  RsSprintf(caption, RWSTRING("%4.2f %4.2f %4.2f"), 
                      m->pos.x, m->pos.y, m->pos.z);
                  RsCharsetPrint( Charset, caption, 0, 2, rsPRINTPOSTOPRIGHT );
              }
          }
          return;
  

      }
  

• Compile and run the application. Use the menu to show the position of the picked atomic. Drag the atomic around the world.
  

• Observe the following points about the coordinate system.

1. The camera is located at the origin of the world; the cube created by pressing the F1 key is located 5 units in front of the camera, i.e. at (0.0, 0.0, 5.0).
2. The z-axis for the world co-ordinate system increases in the direction the camera is looking (such that there is a correspondence between the z co-ordinate and depth). In our tutorial the unit cube is offset 5 units from the camera (at 0.0, 0.0, 5.0) and so is at a depth of 5.
3. RenderWare Graphics uses a right-handed co-ordinate system. Therefore with z increasing into the display screen, and y increasing upwards, the x co-ordinate system increases towards the left.

Note:

This co-ordinate system may be different from what you are used to. The following picture illustrates the RenderWare Graphics world co-ordinate system. Using RenderWare Graphics terminology this picture says that the default orientation of the camera aligns the camera's at vector with the world z-axis, and the camera's up axis with the world y-axis.

You might wonder why matrices in RenderWare Graphics use a vector called "right" which in this illustration points towards the left of the screen. This often is a cause of concern. To resolve this, imagine that you can fly into the computer monitor and look back towards the screen. In this situation you will have the axis labeled "x" in the diagram above pointing towards the right, and the "z" axis will be pointing "at" you. We have not modified the coordinate system, but by changing our position we have resolved the names of the vectors in the matrices that RenderWare Graphics uses.

Next...

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