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Frogger Tutorial part 2

Now that the sketch contains all the game graphics, the fun can begin: time to get the road and river traffic running.

The Score

The first step is to write the code that updates the score. This might seem like an odd thing to do first, but the score display will come in quite useful for debugging later.

Looking at the encoded background screen at http://gameduino.com/results/5f30d40b/ , as you mouse over the character image, you can see the code of each character. For example, the all-blue character is code 0, the number zero is code 12, the black square is code 34 and the frog ‘lives’ symbol is code 35. Adding these at the top of the sketch:

#define BG_BLUE   0
#define BG_ZERO   12
#define BG_BLACK  34
#define BG_LIFE   35

makes the code that draws on the screen a little clearer. Now add a couple of functions before setup:

static uint16_t atxy(byte x, byte y)
{
  return RAM_PIC + 64 * y + x;
}

static void draw_score(uint16_t dst, long n)
{
  GD.wr(dst + 0, BG_ZERO + (n / 10000) % 10);   // ten-thousands
  GD.wr(dst + 1, BG_ZERO + (n / 1000) % 10);    // thousands
  GD.wr(dst + 2, BG_ZERO + (n / 100) % 10);     // hundreds
  GD.wr(dst + 3, BG_ZERO + (n / 10) % 10);      // tens
  GD.wr(dst + 4, BG_ZERO + n % 10);             // ones
}

The atxy function converts from (x, y) screen coordinates to a video memory address.

The draw_score function writes a 5-digit number n onto the screen. It does this one digit at a time, doing the math to extract each digit. It draws each digit by adding BG_ZERO to the digit code, then writing that character to the screen with GD.wr.

So now you can add a line at the end of setup like:

draw_score(atxy(0,0), 1337);

and the number 01337 gets drawn in the top left of the screen.

Making things a little more fancy, this code in loop counts continuously:

static unsigned int t;

void loop()
{
  GD.waitvblank();
  draw_score(atxy(3, 1), t);
  t++;
}

it waits for the vertical blank, draws the variable t in the score field, then increments t. The result is a score count that increases by 72 every second. The t variable is going to be used as a time counter (hence “t”) to run most of the game’s animation.

The complete sketch is here: frogger_score.zip.

Animating Sprites: First Cars

Add some code like this to the bottom of loop

GD.__wstartspr(0);
draw_sprite(t % 100, 216, 3, 0);
GD.__end();

The first line, __wstartspr starts a block write to Gamduino sprite control RAM. This is a super-efficient way of updating all the sprites on the screen, and is the preferred method for getting smooth animation.

The second line draw_sprite draws a sprite on the screen. The draw_sprite function itself was automatically created by the sprite encoder tool. The arguments are:

  • X coordinate, here it’s the t variable modulo 100. This means that the X-coordinate will count from 0 to 99, then wrap back to 0.
  • Y coordinate is 216: this is the position of the first lane of cars on the road
  • animation frame, looking at the encoded sprite sheet the yellow car is animation frame 3
  • rotate, set to zero, meaning no sprite rotation

This is moving pretty nicely; it’s now time to make a couple of improvements. First, instead of wrapping the position every 100, by wrapping it at 256 the car will cross the whole screen before it reappears on the far side:

GD.__wstartspr(0);
draw_sprite(t % 256, 216, 3, 0);
GD.__end();

If you look closely, you’ll see that the car is moving backwards: we want it to drive from right-to-left instead. This is easy to fix: replace t with -t in the draw_sprite expression:

GD.__wstartspr(0);
draw_sprite(-t % 256, 216, 3, 0);
GD.__end();

Add a second car at a different position:

GD.__wstartspr(0);
draw_sprite(-t % 256, 216, 3, 0);
draw_sprite((-t + 128) % 256, 216, 3, 0);
GD.__end();

The code is looking a bit repetitive, this version is a bit cleaner:

static void sprite(unsigned int x, int y, byte anim, byte rot = 0)
{
  draw_sprite(x % 256, y, anim, rot);
}

void loop()
{
  GD.waitvblank();
  draw_score(atxy(3, 1), t);

  GD.__wstartspr(0);
  sprite(-t,       216, 3);
  sprite(-t + 128, 216, 3);
  GD.__end();

  t++;
}

The new function sprite factors out the repeated code, which makes the actual sprite drawing a little easier to read.

The complete sketch is here: frogger_onecar.zip.

The Road

The rest of the road sprites follow a similar pattern.

  // Trucks
  sprite(-t/2,       152, 5);
  sprite(-t/2 + 16,  152, 6);
  sprite(-t/2 + 100, 152, 5);
  sprite(-t/2 + 116, 152, 6);

  // Green and white racecars
  sprite(2 * t,    168, 8);

  // Purple cars
  sprite(-t,       184, 7);
  sprite(-t + 75,  184, 7);
  sprite(-t + 150, 184, 7);

  // Dozers
  sprite(t,        200, 4);
  sprite(t + 50,   200, 4);
  sprite(t + 150,  200, 4);

  // Yellow cars
  sprite(-t,       216, 3);
  sprite(-t + 128, 216, 3);

The dozers on the second lane (16 pixels above the first) are moving to the right, so their X-coordinate is just t. The purple cars move to the left, so they use -t, just like the yellow cars. The racecar moves to the right, but faster, so an expression of 2 * t makes it move across the screen at double the regular speed. The trucks move right-to-left at half speed, so their position is -t / 2. If you look at the original sprite sheet you will see that the trucks are actually made from two sprites: frames 5 and 6. To draw the whole truck, the code draws frame 5, then frame 6 is 16 pixels to the right.

The complete sketch is here: frogger_road.zip.

The River

The river sprites - turtles and logs - move much like the road traffic. But to keep the code easy to read, it’s a good idea to again make functions to draw groups of sprites. Because the turtles and logs are different lengths in the different lanes, the drawing functions have a length parameter:

static void turtle(byte length, byte x, byte y)
{
  byte anim = 50 + ((t / 32) % 3);
  while (length--) {
    sprite(x, y, anim);
    x += 16;
  }
}

void log(byte length, byte x, byte y)
{
  sprite(x, y,      86);
  while (length--) {
    x += 16;
    sprite(x, y, 87);
  }
  sprite(x + 16, y, 88);
}

static int riverat(byte y, uint16_t tt)
{
  switch (y) {
  case 120: return -tt;
  case 104: return tt;
  case 88:  return 5 * tt / 4;
  case 72:  return -tt / 2;
  case 56:  return tt / 2;
  }
}

The riverat function computes the sprite X-coordinate given a lane Y-coordinate and a time tt. The reason for splitting out the river math like this will become clear later.

One small trick here is the animation in the turtle function. The turtles use sprite frames 50,51 and 52. To animate them, the game needs to display these sprites in order. The function uses the time variable t and divides it by 32. This gives a timer that counts 32 times more slowly than t:

\texttt{t / 32} = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 ...

because there are three repeating sprite frames, the next step is to use the modulo 3 operator to make it into a repeating cycle:

\texttt{(t / 32) \% 3} = 0, 1, 2, 0, 1, 2, 0, 1, 2, 0, 1, 2 ...

and because the sprite frames are 50, 51, and 52, adding 50 gives the sprite frames:

\texttt{50 + ((t / 32) \% 3)} = 50, 51, 52, 50, 51, 52, 50, 51, 52, 50, 51, 52, ...

With these functions written, the river drawing itself looks like:

  // Top logs
  for (int i = 0; i < 210; i += 70)
    log(2, riverat(56, t) + i, 56);

  // Turtles again, but slower
  for (int i = 0; i < 250; i += 50)
    turtle(2, riverat(72, t) + i, 72);

  // Long logs
  for (int i = 0; i < 256; i += 128)
    log(5, riverat(88, t) + i, 88);

  // Short logs
  for (int i = 0; i < 240; i += 80)
    log(1, riverat(104, t) + i, 104);

  // Turtles
  for (int i = 0; i < 256; i += 64)
    turtle(3, riverat(120, t) + i, 120);

Because the river elements repeat so much, it makes more sense to use for() loops here to draw all the objects in each lane.

The complete sketch is here: frogger_river.zip.

In Frogger Tutorial part 3 rest of the game code will be added.