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

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

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

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’

Our ‘model’ type #define MAXVERT 50 #define MAXEDGE 50 typedef struct { intnumverts; float3_t vert[ MAXVERT ]; intnumedges; edge_t edge[ MAXEDGE ]; } model_t;

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 );

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 );

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’

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., -key was hit), go back and repeat steps 6 through 10; Otherwise, restore text mode and then quit

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

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!)

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

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)