Set the Viewport Clipping. Computer Graphics CSC470 1

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1 Set the Viewport Clipping Computer Graphics CSC470 1

2 Set the Viewport To draw an undistorted version of the data in a viewport, you need to ensure the viewport and the window have the same aspect ratio. i.e. W H window window = W H viewport viewport Computer Graphics CSC470 2

3 Set the Viewport Recall, if the aspect ratio of a rectangle is less than 0, the rectangle is taller than wide. E.g. W/H = 3/5 If the aspect ratio is greater than 0, the rectangle is wider than tall. E.g. W/H = 5/3 Computer Graphics CSC470 3

4 Set the Viewport a) R > W/H (where R is the aspect ratio of the world window) If the world window is flatter than the screen window, there will be unused space above and/or below. The width of the world window will be mapped to the entire width of the screen window. setviewport(0,w,0,w/r); Computer Graphics CSC470 4

5 Set the Viewport b) R < W/H (where R is the aspect ratio of the world window) If the world window is taller than the screen window, there will be unused space on the sides. The height of the world window will be mapped to the entire height of the screen window. setviewport(0,h*r,0,h); Computer Graphics CSC470 5

6 Resizing the Viewport In a windows environment, the user may resize the screen window and therefore affect the viewport while the program is running. When this occurs, you will want to be able to ensure redrawing of the image occurs within the same aspect ratio. Computer Graphics CSC470 6

7 Resizing the Viewport OpenGL provides a resize event that is called when the user resizes a window. This can be linked to your own resizing function. glutreshapefunc( myreshape); This works the same as for the glutmousefunc and glutkeyboardfunc If you don t run a glviewport after a window resize, the glviewport defaults to the window coordinates. Computer Graphics CSC470 7

8 Resizing the Viewport void myreshape(glsizei W, GLsizei H) { //R is the aspect ratio of the world window if(r > W/H) //use global window aspect ratio setviewport(0, W, 0, W/R); else setviewport(0, H*R, 0, H); } -example322.cpp Computer Graphics CSC470 8

9 Clipping is fundamental in Computer Graphics. It means to discard parts of the image that are not in the viewport. OpenGL takes care of clipping for you, but you still need to understand how it works. Computer Graphics CSC470 9

10 Cohen-Sutherland Clipping Algorithm 1. Trivially Accept or Reject if both ends of a line lie inside the window (e.g. AB) then the line can be trivially accepted as it doesn t need to be clipped. if both ends of a line lie outside the window (e.g. CD) then the line can be trivially rejected as all of it is clipped. Computer Graphics CSC470 10

11 Cohen-Sutherland Clipping Algorithm Testing for Trivial Acceptance or Rejection Divide the window and surrounding areas and assign codes. Left,Above,Right,Below Computer Graphics CSC470 11

12 Cohen-Sutherland Clipping Algorithm Testing for Trivial Acceptance or Rejection 1. Divide the window and surrounding areas and assign codes. 2. Assign the code to the end points of the line. P 1 = TTFF, P 2 = FFFF Trivially Accept if both points have the code FFFF Trivially Reject if both points have a T in the same position e.g. both points are left, above, right or below the window. P 1 P 2 Computer Graphics CSC470 12

13 Cohen-Sutherland Clipping Algorithm Clipping when there is neither trivial accept nor reject. A line that cannot be trivial accepted or rejected will have one end point inside the window and one outside, or; Will have one endpoint on one side and the other endpoint on the other so that the line crosses the window. These lines need to be chopped at the window border intersections. Computer Graphics CSC470 13

14 Cohen-Sutherland Clipping Algorithm We need to determine where A is. its x coordinate is window.right its y coordinate can be calculated using similar triangles.. d = dely e delx Computer Graphics CSC470 14

15 Cohen-Sutherland Clipping Algorithm We need to determine where A is. e = p1.x W.right delx = p2.x p1.x; dely = p2.y p1.y; d = e/delx * dely; p1.y += (W.right p1.x) * dely/delx Computer Graphics CSC470 15

16 Cohen-Sutherland Clipping Algorithm Computer Graphics CSC470 16

17 Let s try it for ourselves: Given the window: (50,50) (100,100) Perform the Cohen-Sutherland for the following lines: (15,15) (20,20) (60,90) (90,90) (55,65) (110,120) what lines are you left with? Computer Graphics CSC470 17

18 TTFF FTFF FTTF 110, , 90 90, TFFF FFFF FFTF 55, TFFT FFFT FFTT 20, 20 15, Computer Graphics CSC470 18

19 Let s try it for ourselves: Given the window: (50,50) (100,100) (15,15) TFFT (20,20) TFFT (reject) (60,90) FFFF (90,90) FFFF (accept) (55,65) FFFF (110,120) FTTF (keep and clip) Computer Graphics CSC470 19

20 Let s try it for ourselves: Given the window: (50,50) (100,100) (55,65) (110,120) (clip right point on top border of right) For top border y = 100, therefore find the point on the line where y = 100. Use y = mx + c and solve for y = 100; m = 1, c = 10. x = 90 The line is now (55,65)(90,100) 60, 90 90, 90 55, , 120 Computer Graphics CSC470 20

Set the Viewport. Set the Viewport Clipping. Set the Viewport. Set the Viewport. Set the Viewport. Resizing the Viewport W = H

Set the Viewport. Set the Viewport Clipping. Set the Viewport. Set the Viewport. Set the Viewport. Resizing the Viewport W = H To draw an undistorted version of the data in a viewport, you need to ensure the viewport and the window have the same aspect ratio. i.e. W H window window W = H viewport viewport Computer Graphics CSC470