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CAPTCHA library - anti-bot image generator

Small portable CAPTCHA images generator library without dependencies.

~~Please, note that this toy captcha is easily beaten by OCR~~. Perhaps, it is improved somewhat in recent version by making letters hollow.

It generates 200x70 images in style of reCAPTCHA ones with five random lower case letters. Instead of 'g' and 'e' letters 'a' substituted, because they are too similar to 'q' and 'c'.

Library includes GIF generation function.

Program included writes GIF image to stdout and letters generated to stderr.

Version: 2012-02-20 (git:bbbaaa3)
Download: libcaptcha.c (32k)
Github:   http://github.com/ITikhonov/captcha
Build:    gcc -DCAPTCHA -o captcha libcaptcha.c
Usage:    ./captcha > captcha.gif 2> letters.txt
Example:  ~/captcha$ ./captcha > captcha.gif

If you want to use it as a library compile libcaptcha.o and add it to linker:

   gcc -c libcaptcha.c
   gcc -o myprog myprog.o libcaptcha.o

Functions and constants exported:

const int gifsize; — makegif function will generate GIF of that size.

void captcha(unsigned char im[70*200], unsigned char l[6]); — fills buffer: im with greyscale 1-byte pixels for 200x70 image; l with five letters generated and trailing zero byte.

void makegif(unsigned char im[70*200], unsigned char gif[gifsize]); - write GIF image of pixels from im to gif buffer. Image is always exactly that gifsize (17646) bytes long.

Here is a complete example of usage:


const int gifsize;
void captcha(unsigned char im[70*200], unsigned char l[6]);
void makegif(unsigned char im[70*200], unsigned char gif[gifsize]);

int main() {
        char l[6];
        unsigned char im[70*200];
        unsigned char gif[gifsize];

        captcha(im,l);
        makegif(im,gif);

        write(1,gif,gifsize);
        write(2,l,5);

        return 0;
}

Compile it with:

gcc -o test test.c libcaptcha.c

Letters are generated from a sample image of free HPLHS OldStyle font taken from dafont.com

Thanks to Martin Townsend who fixed segfaults on ARM platform due to some chars not being int8_t's.

Software

Proof-of-concept

Random

CAPTCHA library - anti-bot image generator