libcaptcha

Libcaptcha

Highly customizable captcha making library.

The screenshot was made using example7.c

Table of contents

• Introduction
• Features
• How to install
• Principles
• How to use
• Advanced use (examples)
• License

Introduction

The main idea was to make a general purpose graphic library with no dependencies in C to draw some primitives with text in a web application. And then I've noticed that I don't have a good captcha to prevent robots to post their shit, so, now here it is.

Features

• TTF font support
• UTF-8 support, you can use any glyph
• An easy API (I suppose)
• Zero dependencies
• Entirely written in C
• Can be used as a single-header library (thanks to @acetone)

How to install

To create a single-header library just use:

./make-singleh.sh

If command succeed, you'll have the libcaptcha.h file that you can use as a single header library.

It is as simple as

./bootstrap
./configure
make
make install

Principles

I thought to name this section "limitations", because libcaptcha doesn't provide functionality to align fonts, work with alpha channels and so on. But captcha does not need those "features". Rather vice versa - it requires to be ugly and random.

The general idea is to provide a simplified API (I call it level zero or preset0) and more advanced primitives.

The simplified functionality consists of 3 simple steps:

• Create a font object (with text and other options)
• Render something (using pattern generators)
• Save file

The advanced functions are quite low level stuff, that requires some understaindig of every step. I've provided some examples which indended to help you with understanding those "low level" functions. By you I mean me in the future :)

How to use

First of all we need some text, and this text should be placed using some random values. Don't warry, preset functions make all those random stuff by itself.

Here is example how to use lc_preset_text() :

  lc_bmp * text = lc_preset_text( // returns single channel lc_bmp*
    "path_to_font.ttf",           // path to font file
    "HELLOWORLD",                 // our text
    38,                           // minimum font size
    50,                           // maximum font size (if zero, all glyphs will be 38)
    1,                            // 1 - rotate characters, 0 - off
    10,                           // horizontal random offset
    10);                          // vertical random offset

There's some simply functions that make noise and square captchas.

  lc_bmp * bmp = lc_preset_square(
    text,                         // is our font object with text
    1,                            // spacing
    2);                           // square size (2 is very small)

For circles:

  lc_bmp * bmp = lc_preset_circle(
    text,                         // is our font object with text
    0,                            // spacing
    14);                          // circle radius (it is not a real radius, but close to it)

Lets save our image into a png file:

 lc_save_png("helloworld.png", bmp);

Don't forget to free() memory:

  lc_free(bmp);
  lc_free(text);

Also you can make some noisy pattern instead of squares, its usage is simple:

  lc_bmp * noise = lc_preset_noise(text);

That's it.

Advanced Use

Check out examples

Get familiar with general concept. example1.c

How to make a random sized glyphs. example2.c

Apply random positions for glyphs. example3.c

Get familiar with patterns made from png images. example4.c

Patterns, patterns everywhere. example5.c

Some crazy stuff. example6.c

Don't forget to use lc_free() to free all the stuff libcaptcha returns to you.

I've noticed that it is very easy to get lost sometimes, just use valgrind.

License

GPLv2

  This program is free software; you can redistribute it and/or
  modify it under the terms of the GNU General Public License
  as published by the Free Software Foundation; either version 2
  of the License, or (at your option) any later version.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.
  Author: g0tsu
  Email:  g0tsu at dnmx.0rg