Michener’s Algorithm

1. Michener’s Algorithm An efficient scheme for drawing circles (and filling circular disks) on a raster graphics display

2. Scan conversion • Geometric objects possess implicit parameters • Example: A circle has a ‘center’ and a ‘radius’ • Its equation is: (x – xc)2 + (y - yc)2 = R2 • x and y are from the continuum of real numbers • CRT display is a discrete 2D array of ‘pixels’ • So drawing a circle requires a conversion, from something continuous into something discrete • In computer graphics it’s called scan conversion • Imperfections: unavoidable, but to be minimized

3. Graphics Animations • Fast drawing is essential for animations • Some guidelines for speed: • eliminate all redundant computations • prefer integer arithmetic to floating-point • prefer add and subtract to multiply or divide • Famous example: ‘Michener Algorithm’ • We can use it later with our ‘pong’ game

4. Eight-fold symmetry (-x, y) (x, y) (y, x) (-y, x) (y, -x) (-y, -x) (-x, -y) (x, -y)

5. Compute only one octant

6. Subroutine: draw_octant_points Arguments: int x, y, xcent, ycent, color; draw_pixel( xcent + x, ycent + y, color ); draw_pixel( xcent + y, ycent + x, color ); draw_pixel( xcent - x, ycent + y, color ); draw_pixel( xcent - y, ycent + x, color ); draw_pixel( xcent + x, ycent - y, color ); draw_pixel( xcent + y, ycent - x, color ); draw_pixel( xcent - x, ycent - y, color ); draw_pixel( xcent - y, ycent - x, color );

7. The “best” pixel to draw? Blue pixel: too far from center ( E > 0 ) Red pixel: too near the center ( E < 0 ) Error-term: E = (x2 + y2) – R2

8. Decision at n-th stage Pn-1 An ? yn-1 Bn ? xn-1 xn Algorithm: compute sum = error( An ) + error( Bn ); If ( sum < 0 ) choose A n; otherwise choose B n.

9. Formula: sum of error-terms • Assume circle has radius R, center (0,0) • error( An) = (xn-1+1)2 + (yn-1)2 – R2 • error( Bn) = (xn-1+1)2 + (yn-1 – 1)2 – R2 • sumn = 2(xn-1+1)2 + 2(yn-1)2 –2(yn-1) + 1-2R2 • Now, how is sumn different from sumn+1: • If An is chosen at the n-th step? • If Bn is chosen at the n-th step?

10. Difference Equation • Observe that: sumn+1 – sumn = 4xn-1 + 6 + 2(yn2 – yn-12) – 2(yn – yn-1) • When An is selected at n-th stage: • we will have yn = yn-1 • thus: sumn+1 = sumn + 4xn-1 + 6 • When Bn is selected at n-th stage: • we will have yn = yn-1 - 1 • thus: sumn+1 = sumn + 4(xn-1 – yn-1) + 10

11. Algorithm initialization • We start with the point P0 = (x0,y0), where x0 = 0 and y0 = R • In this case: A1 = (1, R) and B1 = (1, R-1) • So the initial ‘sum-of-errors’ term will be: sum1 = error(A1) + error(B1) = [(12 + R2) – R2] + [(12 + R2–2R+1) – R2] = 3 – 2R

12. Michener’s Algorithm int x = 0, y = R, sum = 3 – 2*R; while ( x <= y ) { draw_octant_points( x, y, xc, yc, color ); if ( sum < 0 ) sum += 4*x + 6; else { sum += 4*(x - y) + 10; --y; } ++x; }

13. Reference • Francis S. Hill, jr., “Computer Graphics,” Macmillan (1990), pp. 433-435. • NOTE: Michener’s circle-drawing method owes its inspiration to a famous algorithm for efficient line-drawing, devised in 1965 by J. E. Bresenham (see the IBM Systems Journal, vol 4, pp. 305-311).

14. Circle ‘fill’ also exploits symmetry (-x, y) (x, y) (y, x) (-y, x) (y, -x) (-y, -x) (-x, -y) (x, -y)

15. Subroutine: draw_segments Arguments: int x, y, xc, yc, color; draw_horiz( xc – x, xc + x, yc + y, color ); draw_horiz( xc – x, xc + x, yc - y, color ); draw_horiz( xc – y, xc + y, yc + x, color ); draw_horiz( xc – y, xc + y, yc - x, color );

16. draw_horiz( int xlo, xhi, y, color ); Clipping to screen boundaries: If (( y < ymin )||( y > ymax )) return; if ( xlo < xmin ) xlo = xmin; if ( xhi > xmax ) xhi = xmax; Drawing the horizontal segment: for (x = xlo; x <= xhi; x++) draw_pixel( x, y, color );

17. Demo-program • Try the ‘michener.cpp’ demo • It uses VESA graphics-mode 0x0103 • Screen resolution is 800x600 • Color depth is 8 bits-per-pixel (8bpp) • SVGA’s Linear Frame Buffer is enabled by calling VESA’s Set Display Mode service with bit #14 of the mode ID-number set

18. In-Class Exercise #1 • Modify the ‘michener.cpp’ demo: • Use the standard ‘rand()’ function • Draw lots of color-filled circles • Stop if user hits <ESCAPE> key • NOTE: For this latter feature, we need to recall how to set up the terminal keyboard so it uses a ‘non-canonical’ input-mode

19. Animation with ‘image tearing’ • We used Michner’s Algorithm in a graphics animation demo (named ‘bouncing.cpp’) • But you will see undesirable ‘tearing’ of the bouncing ball’s image – because we drew a large number of pixels, and computed a large number of pixel-locations – twice for each displayed frame (erase, then redraw) • Not enough drawing time during ‘retrace’!

20. What can we do about it? • One idea is to draw a smaller-sized ball • Another idea is to draw only once – i.e. include enough extra pixels around the new ball’s perimeter to hide the old one • Also we could ‘pre-compute’ coordinates • An important general technique is to use off-screen display memory to ‘prepare’ a new screen-image, and then do a ‘bitblit’

21. SuperVGA’s hardware • The graphics processor has the capability to switch the CRTC’s ‘start_address’, so we see a new full-screen image displayed almost instantly (i.e., just one or two CPU instructions), instead of a copying a screen • But doing this for higher-resolution SVGA display-modes requires accessing one or more of the nonstandard VGA extensions

22. VESA’s BIOS-Extensions • Video Electronics Standards Association defines a ROM-BIOS service that can be invoked to reprogram CRT Start_Address in a standard way for compliant systems • For a particular vendor’s graphics card, we can discover how to directly reprogram the CRT’s Start-Address, by ‘trapping’ the I/O instructions used in the ROM-BIOS routine

23. Using ‘off-screen’ video memory CRT Controller’s Start-Address (default=0) Visible screen The currently displayed image is defined in this region of VRAM while an alternative image is prepared for display in this region of VRAM Alternate screen When the ‘alternate’ memory region is ready to be displayed, the CRT Start-Address register is rewritten, instantaneously changing what the user sees on the display monitor The technique is called ‘page flipping’ VRAM

24. VBE service 0x07 • This ROM-BIOS service can be called to obtain or modify the CRT Start-Address • It isn’t instantaneous – because of all the parameter-passing and changing of CPU execution-mode and transitions back and forth from user-space to kernel-space • But it might be fast enough to ‘cure’ the image-tearing we saw in ‘bouncing’ demo

25. In-class exercise #2 • Look at documentation for VESA service 7 • See if you can modify the main animation loop in our ‘bouncing.cpp’ program so the image-frames are alternately drawn in two different ‘pages’ of the video memory (i.e., implement the ‘page-flipping’ idea): while one frame is being viewed by the user, the next frame is being readied out-of-sight