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Round and Round, In and Out

By Judy Hartley (Intel) (13 posts) on October 13, 2009 at 1:30 pm

If you have been following my blogs, you know that I am porting a DirectX-based demo application to OpenGL as the first phase in the project to play the app on a netbook with an Atom processor. If you’ve missed a few or would like to read all of my blogs pertaining to this project, I have provided links to the previous entries at the end of this blog.
As of the last blog, I have the objects rendering and the cannon moving in conjunction with the mouse movements. Writing this reminds me that there is one more bit of code I put into the project that I should mention. I don’t want the user being confused as to the slight difference between the cannon position and the mouse cursor, so I inserted the last line shown in the code snippet below:

As you can see, I create a pointer to a Graphics Window object and I set the “useCursor” option to false. This hides the cursor so the user can focus on where the cannon is positioned, rather than the cursor. Another bit of code I added sets the starting position of the cursor to the middle of the screen. Because I start the application with a window that is 800x600, the beginning position is (400, 300). I originally set this position to (0,0) since when you are setting angles, the center point is (0,0). However, it turns out that this is not correct for setting this function. Here’s the line that sets the cursor:

One thing I discovered is that you have to set this cursor position AFTER the window is realized, even though you hide the cursor previous to this.
Now that I have the cannon moving with the mouse and the camera following the cannon, I want to be able to move the cannon to the right and left as well as backwards and forwards. Although the cannon will move to the “right” or “left”, the actual movement I want to generate is a circular movement around the central object which is the castle. So the center of this circle is (0, 0, 0). The cannon is originally positioned at (2000, 9000, 180). At first I was thinking that the radius of the circle would be 9000. Remember, in OSG the ‘z’ axis is up, so the ‘y’ axis is the “distance” direction (for lack of a better description). The actual radius of my circle, then, is found by use of the Pythagorean Theory. The equation is below:

The radius then turns out to be approximately 9219.544. Using the radius as the distance from the Castle leaves me with the following code:

The change in location around the diameter of the circle with a radius of 9219.544 is determined by dividing 2PI by one less than an arbitrary number (g_scnNumCircleDivisions). The sin of this value is then multiplied by the radius found in the global “g_fDistanceFromCastle” . The height above ground is retained from the original position of the cannon. This vector is added to the circle center vector to obtain the correct position around the circle. I use “g_v3TerrainPos” as the center mark because it is centered on (0, 0, 0). To circle right, I add the delta amount to the total angle and to circle left I subtract it from the total.
The code for moving forward and backward is pretty much the same as above except that the changing value is the distance from the Castle (g_fDistanceFromCastle). This section also clamps the distance value so that the cannon cannot get too close or too far away from the castle. The code follows:

Now the cannon is pretty flexible so the time has come to animate the bugs in their various paths. That will be the subject of my next blog.

For those of you who missed the previous blogs, here are some links:

Blog 1: "Threaded Graphics to MID: A New Journey"
Blog 2: "From DirectX to OpenGL - Wheels Up"
Blog 3: "OpenGL Graphic Formats and Conversion"
Blog 4: "Scene Graphs and Instancing"
Blog 5: "Load, Follow, or Get Out of the Way"

Categories: Game Development, Graphics & Media, Mobility, Open Source, Software Tools, Uncategorized
Tags: graphics, OpenGL, OSG, programming, Visual Adrenaliine

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• Intel Software Network Blogs » One Step Forward, Two Steps Back
  December 8, 2009 10:25 AM PST

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