Basic Tutorials

Basic Game Tutorial #8 - Sprites and animation

Introduction

The easiest way to approach an animation is to think of it as a series of images that are shown one after the other. This tutorial will show you one way of achieving this.

Compile and run tutorial08. The star will spin on the screen until the window is closed or Escape is pressed.

An in-depth look

We will start this tutorial by looking at the animation file, data/anim/star.dat. The animation data file contains two lines. The first line is the number of frames in the animation and the rest are the names of the individual frames to load. This file is read by the loadAnimation function in animation.c which we will look at later. For the moment we will start with structs.h.

We define a structure for our animation as follows:

typedef struct Animation
{
    int frameCount, frameIndex, counter;
    SDL_Surface **frame;
} Animation;

The animation.c file contains 4 functions. As always, we will look at each one in turn.

void loadAnimation(char *name, Animation *anim)
{
    /* Load up the data file that describes the animation */

    int i;
    FILE *fp = fopen(name, "rb");
    char frameName[20];

    if (fp == NULL)
    {
        printf("Failed to load animation file %s\n", name);

        exit(1);
    }

    /* Read the frame count */

    fscanf(fp, "%d", &anim->frameCount);

    /* Allocate space for the animation */

    anim->frame = (SDL_Surface **)malloc(anim->frameCount * sizeof(SDL_Surface *));

    if (anim->frame == NULL)
    {
        printf("Ran out of memory when creating the animation for %s\n", name);

        exit(1);
    }

    /* Now load up each frame */

    for (i=0;iframeCount;i++)
    {
        fscanf(fp, "%s", frameName);

        anim->frame[i] = loadImage(frameName);

        if (anim->frame[i] == NULL)
        {
            printf("Failed to load animation frame %s\n", frameName);

            exit(1);
        }
    }

    /* Set the initial frame index to 0 */

    anim->frameIndex = 0;

    /* Set the animation counter */

    anim->counter = ANIM_SPEED;
}

void freeAnimation(Animation *anim)
{
    int i;

    /* Loop through each of the frames and free it */

    if (anim->frame != NULL)
    {
        for (i=0;i<anim->frameCount;i++)
        {
            if (anim->frame[i] != NULL)
            {
                SDL_FreeSurface(anim->frame[i]);
            }
        }

        free(anim->frame);
    }
}

Performing the animation is relatively straight forward:

void doAnimation(Animation *anim)
{
    anim->counter--;

    if (anim->counter <= 0)
    {
        anim->frameIndex++;

        if (anim->frameIndex == anim->frameCount)
        {
            anim->frameIndex = 0;
        }

        anim->counter = ANIM_SPEED;
    }
}

The final function in this file draws the current animation frame:

void drawAnimation(Animation *anim, int x, int y)
{
    drawImage(anim->frame[anim->frameIndex], x, y);
}

In graphics.c, we use the updateScreen function to draw our animation to the screen. In graphics.c, we call the drawAnimation function as follows:

drawAnimation(&starAnim, 288, 216);

Finally, in main.c, we load up the animation and update and draw it in our standard loop:

/* Load up the star animation */

loadAnimation("data/anim/star.dat", &starAnim);

/* Loop indefinitely for messages */

while (go == 1)
{
    getInput();

    /* Update the animation frame */

    doAnimation(&starAnim);

    /* Draw the animation to the screen */

    updateScreen();

    /* Sleep briefly to stop sucking up all the CPU time */

    delay(frameLimit);

    frameLimit = SDL_GetTicks() + 16;
}

Conclusion

The disadvantage to the way that we have done animation is that any game objects using this method will all animate in synchronization. This might not be a bad thing though, depending upon what you want to do. In another tutorial we will look at making game objects animate independently.

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