GIF encoder

This is a small C library that can be used to create GIF animations.

Features

• user-defined palette of any depth from 1 up to 8
• each frame has its own (user-specified) delay time
• flexible looping options: no loop, N repetitions, infinite loop
• GIF size optimization: only stores frame differences
• memory efficient: saves frames to file as soon as possible
• small and portable: less than 300 lines of C99
• public domain

Limitations

• no frame-local palettes (incompatible with size optimization)
• no interlacing (bad for compression, useless for animations)

Documentation

There are only three functions declared in "gifenc.h": ge_new_gif(), ge_add_frame() and ge_close_gif().

The ge_new_gif() function receives GIF global options and returns a ge_GIF handler:

ge_GIF *ge_new_gif(
    const char *fname,                  /* GIF file name */
    uint16_t width, uint16_t height,    /* frame size */
    uint8_t *palette, int depth,        /* color table */
    int loop                            /* looping information */
);

The palette parameter must point to an array of color data. Each entry is a 24-bits RGB color, stored as three contiguous bytes: the first is the red value (0-255), then green, then blue. Entries are stored in a contiguous byte array.

The depth parameter specifies how many colors are present in the given palette. The number of color entries must be 2 ^ depth, where 1 <= depth <= 8.

Example palette and depth values:

uint8_t palette[] = {
    0x00, 0x00, 0x00,   /* entry 0: black */
    0xFF, 0xFF, 0xFF,   /* entry 1: white */
    0xFF, 0x00, 0x00,   /* entry 2: red */
    0x00, 0x00, 0xFF,   /* entry 3: blue */
};
int depth = 2;          /* palette has 1 << 2 (i.e. 4) entries */

If palette is NULL, entries are taken from a default table of 256 colors. If depth < 8, the default table will be truncated to the appropriate size. The default table is composed of the 16 standard VGA colors, plus the 216 web-safe colors (all combinations of RGB with only 6 valid values per channel), plus 24 grey colors equally spaced between black and white, excluding both.

If the loop parameter is zero, the resulting GIF will loop forever. If it is a positive number, the animation will be played that number of times. If loop is negative, no looping information is stored in the GIF file (for most GIF viewers, this is equivalent to loop == 1, i.e., "play once").

The ge_add_frame() function reads pixel data from a buffer and saves the resulting frame to the file associated with the given ge_GIF handler:

void ge_add_frame(ge_GIF *gif, uint16_t delay);

The delay parameter specifies how long the frame will be shown, in hundreths of a second. For example, delay == 100 means "show this frame for one second" and delay == 25 means "show this frame for a quarter of a second". Note that short delays may not be supported by some GIF viewers: it's recommended to keep a minimum of delay == 6. If delay == 0, no delay information will be stored for the frame. This can be used when creating still (single-frame) GIF images.

Pixel data is read from gif->frame, which points to a memory block like this:

uint8_t _frame_[gif->width * gif->height];

Note that the address of gif->frame changes between calls to ge_add_frame() (*). For this reason, each frame must be written in its entirety to the current address, even if one only wants to change a few pixels from the last frame. The encoder will automatically detect the difference between two consecutive frames in order to minimize the size of the output.

Each byte in the frame buffer represents a pixel. The value of each pixel is an index to a palette entry. For instance, given the example palette above, we can create a frame displaying a red-on-black "F" letter like this:

uint8_t pixels[] = {
    2, 2, 2, 2,
    2, 0, 0, 0,
    2, 0, 0, 0,
    2, 2, 2, 0,
    2, 0, 0, 0,
    2, 0, 0, 0,
    2, 0, 0, 0
};
ge_GIF *gif = ge_new_gif("F.gif", 4, 7, palette, depth, -1);
memcpy(gif->frame, pixels, sizeof(pixels));
ge_add_frame(gif, 0);
ge_close_gif(gif);

The function ge_close_gif() finishes writting GIF data to the file associated with the given ge_GIF handler and does memory clean-up. This function must be called once after all desired frames have been added, in order to correctly save the GIF file. After calling this function, the ge_GIF handler cannot be used anymore.

void ge_close_gif(ge_GIF* gif);

(*) The encoder keeps two frame buffers internally, in order to implement the size optimization. The address of gif->frame alternates between those two buffers after each call to ge_add_frame().

Example

See the file "example.c". It can be tested like this:

$ cc -o example gifenc.c example.c $ ./example

That should create an animated GIF named "example.gif".

Copying

All of the source code and documentation for gifenc is released into the public domain and provided without warranty of any kind.