Detererming a Point's Position with respect to a line

1
Detererming a Point's Position with respect to a
line

• Generating PPM files for signal flags

2
Signal Flag

• Suppose we want to produce the following signal
  flag

3
A Point's Position with respect to a line

• We need to be able to tell whether a pixel is
  above or below a non-vertical line
• Two points, ,
  determine a line.
• The slope of the line is

4
A Point's Position with respect to a line

• The coordinates of any arbitrary point (x, y)
  that lies on the line is
• Thus, given a test point (x, y), we compute y'
  that corresponds to x and lies on the line
• If y' gt y, then (x, y) lies below the line
  otherwise, (x, y) lies above the line.

5
Generating the signal flag

• / signal.c /
• include ltstdio.hgt
• include ltstdlib.hgt
• int isAbove (int x, int y, int x1, int y1, int
  x2, int y2)
• void makeHeader( int width, int height)
• int main( )
• int width
• int height
• int x
• int y 0

6
Generating the signal flag

• fscanf(stdin, "d", width)
• fscanf(stdin, "d", height)
• makeHeader(width, height)
• while(y lt height)
• x 0
• while(x lt width)
• if(isAbove(x, y, 0, 0, width,
  height))
• printf("ccc", 255, 255, 0)
• else
• printf("ccc", 255, 0, 0)
• x x 1
• y y 1

7
Generating the signal flag

• /
  
• This function determines if a given point is
  above/below the line containing (x1,y1) and (x2,
  y2).
• It returns 1 if the point is above the line
  otherwise it returns 0.
• 
  /
• int isAbove(int x, int y, int x1, int y1, int x2,
  int y2)
• float slope
• float yValue // corresponds to y
  in the equation on slide 4
• int out
• // INSERT CODE TO COMPUTE THE SLOPE OF THE
  LINE
• // INSERT CODE TO COMPUTE yValue USING THE
  EQUATION ON SLIDE 4
• if (yValue gt y)
• out 1
• else
• out 0

8
Generating the signal flag

• /
  
• This function writes the header to standard
  output
• 
  /
• void makeHeader(int w, int h)
• fprintf(stdout, "P6\n")
• fprintf(stdout, "d d d\n", w, h, 255)

9
Another signal flag

• Suppose we want to produce the following signal
  flag?