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The method named setCoordinateFrame
This method sets the origin to a point near the upper-left corner of the off-screen image (see Figure 1 ) and draws orthogonal axes on the off-screen image intersecting at the origin.
| Listing 6 . The method named setCoordinateFrame. |
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private void setCoordinateFrame(Graphics2D g2D){
//Translate the origin.g2D.translate(0.2*osiWidth,0.2*osiHeight);//Draw new X and Y-axes in default BLACK
g2D.drawLine(-(int)(0.2*osiWidth),0,(int)(0.8*osiWidth),0);g2D.drawLine(0,-(int)(0.2*osiHeight),
0,(int)(0.8*osiHeight));}//end setCoordinateFrame method |
There is no intention to perform mathematical operations on the axes, so they are drawn independently of the classes and methods in the game-mathlibrary using the simplest method available for drawing a line.
The name of that simple method is drawLine , and it is a method of the standard Java Graphics class. The translate method is also a method of the Graphics class. Given that information, the code in Listing 6 is straightforward and should not require further explanation.
Adding two vectors
Returning to the method named drawOffScreen , Listing 7 begins by instantiating two objects of the GM2D04.Vector class.
| Listing 7 . Adding two vectors. |
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GM2D04.Vector vecA = new GM2D04.Vector(
new GM2D04.ColMatrix(50,100));GM2D04.Vector vecB = new GM2D04.Vector(
new GM2D04.ColMatrix(75,25));GM2D04.Vector sumOf2 = vecA.add(vecB); |
Then Listing 7 calls the new add method (see Listing 1 ) on one of the vectors, passing the other vector as a parameter to the method.The add method returns a third vector that is the sum of the other two vectors. The new vector is referred to as sumOf2 in Listing 7 .
Draw vecA in RED with its tail at the origin
Recall that a vector has only two properties: length and direction. It does not have a position property. Therefore, if you decide to draw a vector, you candraw it anywhere in space, and one position is equally as valid as any other position.
Listing 8 sets the drawing color to RED and calls the draw method on vecA producing the red visual manifestation of the Vector object shown in Figure 1 .
(Note that there is also a magenta vector in Figure 1 , and it may be difficult to distinguish from the red vector, depending on the quality of thecolor on your monitor. The magenta vector is longer than the red vector.)
| Listing 8 . Draw vecA in RED with its tail at the origin. |
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g2D.setColor(Color.RED);
vecA.draw(g2D,new GM2D04.Point(new GM2D04.ColMatrix(0,0))); |
Note that I elected to draw the vector with its tail at the origin, but that was an arbitrary decision that I will discuss in more detail later.
Draw vecB in GREEN head-to-tail with vecA
While it's legal to draw a vector anywhere in space, certain positions may have advantages over other positions in some cases. You will see what I meanshortly. In the meantime, Listing 9 creates a visual manifestation of the vecB object with its tail at the head of the vecA object as shown by the green vector in Figure 1 .
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