A code-walkthrough

1. A code-walkthrough Commentary on our ‘model3d.cpp’ demo-program – an initial ‘prototype’ for 3D wire-frame animations

2. Data structures typedef struct { float x, y, z; } float3_t; • Used for locating points in 3-space • Also used for representing vectors typedef struct { float3_t vrp, vpn, vup; } camera_t; - Used to specify crucial viewing directions

3. Graphics acronyms • VRP: View Reference Point • VPL: View-Plane Normal • VUP: Vector Upward Pointing

4. More structures… typedef int edge_t [ 2 ]; • Used to specify a line-segment (by means of its two end-points) • We will be using an array of such ‘edges’

5. Our ‘model’ type #define MAXVERT 50 #define MAXEDGE 50 typedef struct { int numverts; float3_t vert[ MAXVERT ]; int numedges; edge_t edge[ MAXEDGE ]; } model_t;

6. Some ‘helper’ functions int get_model_data( char *filename, model_t model ); double dot_product( float3_t p, float3_t q ); void normalize( float3_t &v ); void cross_product( float3_t u, float3_t v, float3_t &w );

7. The ‘draw_model()’ function void draw_model( model_t model, camera_t camera, float3_t eye ); - It uses these ‘helper’ functions: void draw_pixel( int x, int y, int color ), draw_line( int x1, int y1, int x2, int y2, int color ), fill_rectangle( int x, int y, int h, int v, int color );

8. Algorithm for ‘main()’ • Preliminary steps (to set up for the demo): 1) setup filename (for the wire-frame data) 2) read the data into our ‘model’ structure 3) setup i/o permissions, vram mapping, noncanonical input, and graphics mode 4) draw a screen border (as confirmation) 5) setup variables for ‘camera’ and ‘eye’

9. The demo’s main loop 6) Adjust the location of the viewer’s eye 7) Setup the camera and view parameters 8) Compute the 3D perspective projection 9) Erase old image and draw the new one 10) Then wait for the user to press a key Unless done (i.e., <ESCAPE>-key was hit), go back and repeat steps 6 through 10; Otherwise, restore text mode and then quit

10. The ‘draw_model()’ function • 1) setup axes for a new coordinate-system based on camera-angles and viewer’s eye • 2) transform the model’s vertices into their coordinates relative to these new axes • 3) perform a perspective projection of the points in 3-space onto the 2D view-plane • 4) for each edge, draw the line-segment that joins the images of its two end-points

11. Using the ‘split-screen’ • We can demonstrate the benefit of using a ‘page-flipping’ animation algorithm, taking advantage of the CRT Line-Compare and Start-Address parameters • Our demo-program is called ‘flipdemo.cpp’ • With a split-screen display, we can show two wire-frame animations on the same screen, one without page-flipping (flickers) and the other with page-flipping (smooth!)

12. Theory behind our demo Page 0 We draw (and erase) to page 0 CRT Start-Address offset (The viewer can watch this on lower screen) Flips between pages 1/2 Page 1 Here or Here Always shows page 0 Page 2 split-screen display CRT Line-Compare offset VRAM

13. In-class exercises • Try modifying the ‘model3d.cpp’ program (so as to allow for a greater degree of ‘user control’) • Specifically, let the user move the viewer’s eye higher or lower, by pressing up-arrow or down-arrow on the keyboard • Apply the ideas from our ‘animate2.cpp’ demo to add animation (by removing the keyboard input delay from the main loop), but let the user still be able to exert control (via asynchronous keyboard input notification and a signal-handler function)