2.1.1 Pixel Coordinates

• This is the section you should read it.
• He points out that pixels are not points.
  • Mathematically points have no width or height.
  • Pixels are 1/width and 1/height of the canvas.
  • Pixels are an approximation of the final picture.
• He introduces the idea of antialiasing
  • Ideally lines are one point thick.
  • But pixels are not points.
  • And also on integer boundaries.
  • Think of the line y=4x+2;  OK, but ...

    x	y	comment
    0	2
    $\frac{1}{4}$	3	Where/how are you going to plot this point?
    $\frac{1}{2}$	4	Where/how are you going to plot this point?
    $\frac{3}{4}$	5	Where/how are you going to plot this point?
    1	5

• Idea:
  • Draw a "virtual" pixel that shades each actual pixel with the percent that is covered.
• Note we could get into trouble if we wanted to plot a line that doesn't end on an integer pixel as well.
• There is a cool demo in the book that lets you explore this.
• PPI
  • Pixels per inch
  • DPI : Dots per inch.
  • Early screens had about 72 PPI
  • Modern screens have hundreds.
    • He states that at 100 PPI a one pixel line is probably visible
    • But at 400 PPI a one pixel line is probably not