Taking inspiration from the classic video game series "Monkey Ball", we wrote this game for the Tiva Launch Pad (ARM Cortex-M).
Note: This application was written as part of an open ended final project for ECE353 Introduction to Microprocessor Systems, taught by Eric Hoffman at University of Wisconsin Madison in 2018.
In this game, users roll a ball around the grass, trying to avoid hitting the fire or falling in the lava. Sometimes fire blocks the way, luckily touching the fire puts it out. Once the user reaches the checkered end area, they win! The player with the fastest time gets their initials displayed on the homescreen for all to see.
It was very important for us to make sure that the game controls felt real. One of our main focuses was on simulating inertia, to give the ball some weight. I came up with this little "hack" while writing another program, and thought it would work perfectly here.
The main idea is to have user input only update a target speed, rather than the true speed. In this case, it was the raw accelerometer data that set our target speeds. Then each frame, we increment the real speed towards the target speed by some fraction of the difference. This results in the real speed smoothly chasing a more hectic input speed.
ball->y_speed_target = board_tilt.y / SPEED_FACTOR; // Scale down the input data
ball->x_speed_target = -1 * (board_tilt.x / SPEED_FACTOR);
ball->x_speed += (ball->x_speed_target - ball->x_speed) / EASING; // Update "real" speed
ball->y_speed += (ball->y_speed_target - ball->y_speed) / EASING;Below is the structure we used to track the ball's state
typedef struct{
uint16_t x; // Current position in pixel coordinates
uint16_t y;
uint16_t x_hres; // Keep position at a higher resolution, project down to pixel coordinates when rendering
uint16_t y_hres;
int16_t x_speed; // Current speed (at higher resolution) in tilt values
int16_t y_speed;
int16_t x_speed_target; // Target speed to increment to
int16_t y_speed_target;
}ball_t; In order to allow for some variety in game play, we wanted to come up with an easy way to write new levels. Our solution was to simply make a two-dimensional array of integers, each integer associated with a different level component (grass, lava, obstacle, winzone).
Each array index corresponds to a 16x16px square on the screen, when rendering we iterate through the array, and render each level component bitmap. This also is how we do collision detection, take the screen coordinates, divide by 16, and index into the 2d-array to get back the level component.
int main()
{
// Each level object is used for both rendering and logic
// Level Data Structure
// Each level is a 2d array of uint8_t
// 0: PATH
// 1: LAVA
// 2: Obstacle
// 3: WinZone
//
// BOARD IS INDEXED "BACKWARDS"! lvl[y][x]
//
// Helpful elements to build levels
// {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1},
// {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
uint8_t lvl_2[LVL_H][LVL_W] = {
{0,0,0,0,0,0,0,1,1,1,1,1,1,1,1},
{0,0,0,0,0,0,0,1,1,1,1,1,1,1,1},
{0,0,0,0,0,0,0,1,1,1,1,1,1,1,1},
{0,0,0,0,0,0,0,1,1,1,1,1,1,1,1}
// ...
// ...etc
};
}In order to figure out what board components correspond to actual pixels, we wrote two helper functions. Touch events and ball position are stored in screen coordinates, so all we need to do is convert these coordinates to board coordinates and index into the level array.
// Convert board coordinates (16x16 blocks) to screen coordinates
uint16_t b2s_x(uint16_t x){
return x*IMG_WIDTH + IMG_WIDTH/2;
}
uint16_t b2s_y(uint16_t y){
return y*IMG_HEIGHT + IMG_HEIGHT/2;
}
// Convert screen coordinates to board coordinates (16x16 blocks)
uint16_t s2b_x(uint16_t x){
uint8_t coord = x / IMG_WIDTH;
// Bounds Check
coord = (int)coord < 0 ? 0 : coord;
coord = (int)coord >= LVL_W ? LVL_W - 1 : coord;
return coord;
}
uint16_t s2b_y(uint16_t y){
uint8_t coord = y / IMG_HEIGHT;
// Bounds Check
coord = (int)coord < 0 ? 0 : coord;
coord = (int)coord >= LVL_H ? LVL_H - 1 : coord;
return coord;
}Redistribution and use in source or binary form, with or without modification, are permitted provided that the following conditions are met:
- Redistributions in source form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
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