Creating Tux Paint Magic Tool Plugins

Copyright 2007-2007 by Bill Kendrick and others
New Breed Software

bill@newbreedsoftware.com
http://www.tuxpaint.org/

July 5, 2007 - July 31, 2007

Overview

Beginning with version 0.9.18, Tux Paint's 'Magic' tools were converted from routines that lived within the application itself, to a set of 'plugins' that are loaded when Tux Paint starts up.

This division allows more rapid development of 'Magic' tools, and allows programmers to create and test new tools without needing to integrate them within the main Tux Paint source code. (Users of more professional graphics tools, such as The GIMP, should be familiar with this plugin concept.)

Table of Contents

Prerequisites

Tux Paint is written in the C programming language, and uses the Simple DirectMedia Layer library ('libSDL', or simply 'SDL'; available from http://www.libsdl.org/). Therefore, for the moment at least, one must understand the C language and how to compile C-based programs. Familiarity with the SDL API is highly recommended, but some basic SDL concepts will be covered in this document.

Interfaces

Those who create 'Magic' tool plugins for Tux Paint must provide some interfaces (C functions) that Tux Paint may invoke.

Tux Paint utilizes SDL's "SDL_LoadObject()" and "SDL_LoadFunction()" routines to load plugins (shared objects files; e.g., ".so" files on Linux or ".dll" files on Windows) and find the functions within.

In turn, Tux Paint provides a number of helper functions that the plugin may (or sometimes is required to) use. This is exposed as a C structure (or "struct") which contains pointers to functions and other data inside Tux Paint. A pointer to this structure gets passed along to the plugin's functions as an argument when Tux Paint invokes them.

Plugins should #include the C header file "tp_magic_api.h", which exposes the 'Magic' tool plugin API. Also, when you run the C compiler to build a plugin, you should use the command-line tool "tp-magic-config" to get the appropriate compiler flags (such as where the compiler can find the Tux Paint plugin header file, as well as SDL's header files) for building a plugin.

The C header file and command-line tool mentioned above are included with Tux Paint — or in some cases, as part of a "Tux Paint 'Magic' Tool Plugin Development package".

'Magic' tool plugin functions

'Magic' tool plugins must contain the functions listed below. Note: To avoid 'namespace' collisions, each function's name must start with the shared object's filename (e.g., "blur.so" or "blur.dll" would have functions whose names begin with "blur_"). This includes private functions (ones not used by Tux Paint directly), unless you declare those as 'static'.

Common arguments to plugin functions:

Here is a description of arguments that many of your plugin's functions will need to accept.

Required Plugin Functions:

Your plugin is required to contain, at the least, all of the following functions.

Note: Remember, your plugin's function names must be preceded by your plugin's filename. That is, if your plugin is called "zoom.so" (on Linux) or "zoom.dll" (on Windows), then the names of your functions must begin with "zoom_" (e.g., "zoom_get_name(...)").

Plugin "housekeeping" functions:
  • Uint32 api_version(void)
    The plugin should return an integer value representing the version of the Tux Paint 'Magic' tool plugin API the plugin was built against. The safest thing to do is return the value of TP_MAGIC_API_VERSION, which is defined in "tp_magic_api.h". If Tux Paint deems your plugin to be compatible, it will go ahead and use it.

    Note: Called once by Tux Paint, at startup. It is called first.

  • int init(magic_api * api)
    The plugin should do any initialization here. Return '1' if initialization was successful, or '0' if not (and Tux Paint will not present any 'Magic' tools from the plugin).

    Note: Called once by Tux Paint, at startup. It is called first. It is called after "api_version()", if Tux Paint believes your plugin to be compatible.

  • int get_tool_count(magic_api * api)
    This should return the number of Magic tools this plugin provides to Tux Paint.

    Note: Called once by Tux Paint, at startup. It is called after your "init()", if it succeeded.

  • char * get_name(magic_api * api, int which)
    This should return a string containing the name of a magic tool. This will appear on the button in the 'Magic' selector within Tux Paint.

    Tux Paint will free() the string upon exit, so you should wrap it in a C strdup() call.

    Note: Called once for each Magic tool your plugin claims to contain (by your "get_tool_count()").

  • SDL_Surface * get_icon(magic_api * api, int which)
    This should return an SDL_Surface containing the icon representing the tool. (A greyscale image with alpha, no larger than 40x40.) This will appear on the button in the 'Magic' selector within Tux Paint.

    Tux Paint will free ("SDL_FreeSurface()") the surface upon exit.

    Note: Called once for each Magic tool your plugin claims to contain (by your "get_tool_count()").

  • char * get_description(magic_api * api, int which)
    This should return a string containing the description of a magic tool. This will appear as a help tip, explained by Tux the Penguin, within Tux Paint.

    Tux Paint will free() the string upon exit, so you should wrap it in a C strdup() call.

    Note: Called once for each Magic tool your plugin claims to contain (by your "get_tool_count()").

  • int requires_colors(magic_api * api, int which)
    Return a '1' if the 'Magic' tool accepts colors (the 'Colors' palette in Tux Paint will be available), or '0' if not.

    Note: Called once for each Magic tool your plugin claims to contain (by your "get_tool_count()").

  • void shutdown(magic_api * api)
    The plugin should do any cleanup here. If you allocated any memory or used SDL_Mixer to load any sounds during init(), for example, you should free() the allocated memory and Mix_FreeChunk() the sounds here.

    Note: This function is called once, when Tux Paint exits.

Plugin event functions:
  • void set_color(magic_api * api, Uint8 r, Uint8 g, Uint8 g)
    Tux Paint will call this function to inform the plugin of the RGB values of the currently-selected color in Tux Paint's 'Colors' palette. (It will be called whenever one of the plugin's Magic tools that accept colors becomes active, and whenever the user picks a new color while such a tool is currently active.)

  • void click(magic_api * api, int which, SDL_Surface * snapshot, SDL_Surface * canvas, int x, int y, SDL_Rect * update_rect)
    The plugin should apply the appropriate 'Magic' tool on the 'canvas' surface. The (x,y) coordinates are where the mouse was (within the canvas) when the mouse button was clicked.

    The plugin should report back what part of the canvas was affected, by filling in the (x,y) and (w,h) elements of 'update_rect'.

    The contents of the drawing canvas immediately prior to the mouse button click is stored within the 'snapshot' canvas.

  • void drag(magic_api * api, int which, SDL_Surface * snapshot, SDL_Surface * canvas, int ox, int oy, int x, int y, SDL_Rect * update_rect)
    The plugin should apply the appropriate 'Magic' tool on the 'canvas' surface. The (ox,oy) and (x,y) coordinates are the location of the mouse at the beginning and end of the stroke.

    Typically, plugins that let the user "draw" effects onto the canvas utilize Tux Paint's "line()" 'Magic' tool plugin helper function to calculate the points of the line between (ox,oy) and (x,y), and call another function within the plugin to apply the effect at each point. (See "Tux Paint Functions and Data," below).

    The plugin should report back what part of the canvas was affected, by filling in the (x,y) and (w,h) elements of 'update_rect'.

    Note: The contents of the drawing canvas immediately prior to the mouse button click remains as it was (when the plugin's "click()" function was called), and is still available in the 'snapshot' canvas.

  • void release(magic_api * api, int which, SDL_Surface * snapshot, SDL_Surface * canvas, int x, int y, SDL_Rect * update_rect)
    The plugin should apply the appropriate 'Magic' tool on the 'canvas' surface. The (x,y) coordinates are where the mouse was (within the canvas) when the mouse button was released.

    The plugin should report back what part of the canvas was affected, by filling in the (x,y) and (w,h) elements of 'update_rect'.

    Note: The contents of the drawing canvas immediately prior to the mouse button click remains as it was (when the plugin's "click()" function was called), and is still available in the 'snapshot' canvas.

Tux Paint Functions and Data

Tux Paint provides a number of helper functions that plugins may access via the "magic_api" structure, sent to all of the plugin's functions. (See "Required Plugin Functions," above.)

Pixel Manipulations

  • Uint32 getpixel(SDL_Surface * surf, int x, int y)
    Retreives the pixel value from the (x,y) coordinates of an SDL_Surface. (You can use SDL's "SDL_GetRGB()" function to convert the Uint32 'pixel' to a set of Uint8 RGB values.)

  • void putpixel(SDL_Surface * surf, int x, int y, Uint32 pixel)
    Sets the pixel value at position (x,y) of an SDL_Surface. (You can use SDL's "SDL_MapRGB()" function to convert a set of Uint8 RGB values to a Uint32 'pixel' value appropriate to the destination surface.)

  • SDL_Surface * scale(SDL_Surface * surf, int w, int h, int keep_aspect)
    This accepts an existing SDL surface and creates a new one scaled to an arbitrary size. (The original surface remains untouched.)

    The "keep_aspect" flag can be set to '1' to force the new surface to stay the same shape (aspect ratio) as the original, meaning it may not be the same width and height you requested. (Check the "->w" and "->h" elements of the output "SDL_Surface *" to determine the actual size.)

Helper Functions

  • int in_circle(int x, int y, int radius)
    Returns '1' if the (x,y) location is within a circle of a particular radius (centered around the origin: (0,0)). Returns '0' otherwise. Useful to create 'Magic' tools that affect the canvas with a circular brush shape.

  • void line(int which, SDL_Surface * canvas, SDL_Surface * snapshot, int x1, int y1, int x2, int y2, int step, FUNC callback)
    This function calculates all points on a line between the coordinates (x1,y1) and (x2,y2). Every 'step' iterations, it calls the 'callback' function.

    It sends the 'callback' function the (x,y) coordinates on the line, Tux Paint's "magic_api" struct (as a "void *" pointer), a 'which' value, represening which of the plugin's 'Magic' tool is being used, and the current and snapshot canvases.

    Example prototype of a callback function that may be sent to Tux Paint's "line()" 'Magic' tool plugin helper function:
    void exampleCallBack(void * ptr_to_api, int which_tool, SDL_Surface * canvas, SDL_Surface * snapshot, int x, int y);

  • Uint8 touched(int x, int y)
    This function allows you to avoid re-processing the same pixels multiple times when the user drags the mouse across an area of the canvas, thus increasing Tux Paint's response time, especially with math-heavy effects.

    If your effect's "click()", "drag()" and/or "release()" functions take the contents of the source surface ("snapshot") and always create the same results in the desintation surface ("canvas"), you should wrap the effect in a call to "api->touched()".

    This function simply returns whether or not it had already been called for the same (x,y) coordinates, since the user first clicked the mouse. In other words, the first time you call it for a particular (x,y) coordinate, it returns '0'. Future calls will return '1' until the user releases the mouse button.

    Note: Magic effects that continuously affect the destination surface ("canvas") (ignoring the "snapshot surface) have no reason to use this function. The "Blur" and "Smudge" tools that ship with Tux Paint are examples of such effects.

Informational

  • char * tp_version
    A string containing the version of Tux Paint that's running (e.g., "0.9.18").

  • int canvas_w Returns the width of the drawing canvas.

  • int canvas_h Returns the height of the drawing canvas.

  • int button_down(void)
    A '1' is returned if the mouse button is down; '0' otherwise.

  • char * data_directory
    This string contains the directory where Tux Paint's data files are stored. For example, on Linux, this may be "/usr/share/tuxpaint/".

    Magic tools should include an icon (see "get_icon()", above) and are encouraged to include sound effects, it's useful for plugins to know where such things are located.

    When compiling and installing a plugin, the "tp-magic-config" command-line tool should be used to determine where such data should be placed for the installed version of Tux Paint to find them. (See "Installing," below.)

Tux Paint System Calls

  • void show_progress_bar(void)
    Asks Tux Paint to animate and draw one frame of its progress bar (at the bottom of the screen). Useful for routines that may take a long time, to provide feedback to the user that Tux Paint has not crashed or frozen.

  • void playsound(Mix_Chunk * snd, int pan, int dist)
    This function plays a sound (one loaded by the SDL helper library "SDL_mixer"). It uses SDL_mixer's "Mix_SetPanning()" to set the volume of the sound on the left and right speakers, based on the 'pan' and 'dist' values sent to it.

    A 'pan' of 128 causes the sound to be played at equal volume on the left and right speakers. A 'pan' of 0 causes it to be played completely on the left, and 255 completely on the right.

    The 'dist' value affects overall volume. 255 is loudest, and 0 is silent.

    The 'pan' and 'dist' values can be used to simulate location and distance of the 'Magic' tool effect.

  • void special_notify(int flag)
    This function notifies Tux Paint of special events. Various values defined in "tp_magic_api.h" can be 'or'ed together (using C's boolean 'or': "|") and sent to this function.
    • SPECIAL_FLIP — The contents of the canvas has been flipped vertically.

      If a 'Starter' image was used as the basis of this image, it should be flipped too, and a record of the flip should be stored as part of Tux Paint's undo buffer stack. Additionally, the fact that the starter has been flipped (or unflipped) should be recorded on disk when the current drawing is saved.

    • SPECIAL_MIRROR — Similar to SPECIAL_FLIP, but for magic tools that mirror the contents of the canvas horizontally.

Color Conversions

  • float sRGB_to_linear(Uint8 srbg)
    Converts an 8-bit sRGB value (one between 0 and 255) to a linear floating point value (between 0.0 and 1.0).

    See also: sRGB article at Wikipedia.

  • uint8 linear_to_sRGB(float linear)
    Converts a linear floating point value (one between 0.0 and 1.0) to an 8-bit sRGB value (between 0 and 255).

  • void rgbtohsv(Uint8 r, Uint8 g, Uint8 b, float * h, float * s, float * v)
    Converts 8-bit sRGB values (between 0 and 255) to floating-point HSV (Hue, Saturation and Value) values (Hue between 0.0 and 360.0, and Saturation and Value between 0.0 and 1.0).

    See also: HSV Color Space article at Wikipedia.

  • void hsvtorgb(float h, float s, float v, Uint8 * r, Uint8 * g, Uint8 * b)
    Converts floating-point HSV (Hue, Saturation and Value) values (Hue between 0.0 and 360.0, and Saturation and Value between 0.0 and 1.0) to 8-bit sRGB values (between 0 and 255).

Helper Macros in "tp_magic_api.h":

Along with the "magic_api" C structure containing functions and data described above, the tp_magic_api.h C header file also contains some helper macros that you may use.

Constant Defintions in "tp_magic_api.h":

The following is a summary of constant values that are set (via "#define") within the 'Magic' tool API header file.

Compiling

Linux and other Unix-like Platforms

Use the C compiler's "-shared" command-line option to generate a shared object file (".so") based on your 'Magic' tool plugin's C source code.

Additionally, use the "tp-magic-config --cflags" command, supplied as part of Tux Paint, to provide additional command-line flags to your C compiler that will help it build your plugin.

As a stand-alone command, using the GNU C Compiler and BASH shell, for example:

$ gcc -shared `tp-magic-config --cflags` my_plugin.c -o my_plugin.so

Note: The characters around the "tp-magic-config" command are a grave/backtick/backquote ("`"), and not an apostrophe/single-quote ("'"). They tell the shell to execute the command within (in this case, "tp-magic-config ..."), and use its output as an argument to the command being executed (in this case, "gcc ...").

A snippet from a more generalized Makefile might look like this:

CFLAGS=-Wall -O2 $(shell tp-magic-config --cflags)

my_plugin.so: my_plugin.c
   $(CC) -shared $(CFLAGS) -o $@ $<

Windows

TBD

Mac OS X

TBD

Installing

Linux and other Unix-like Platforms

Use the "tp-magic-config --pluginprefix" command, supplied as part of Tux Paint, to determine where the plugin shared object (".so") files should be installed. The value returned by this command will be the global location where the installed version of Tux Paint looks for plugins (e.g., "").

As stand-alone commands, using the BASH shell, for example:

# cp my_plugin.so `tp-magic-config --pluginprefix`
# chmod 644 `tp-magic-config --pluginprefix`/my_plugin.so

Additionally, use the "tp-magic-config --dataprefix" command, supplied as part of Tux Paint, to determine where data files (PNG icon, Ogg Vorbis sound effects, etc.) should be installed. The value returned by this command will be the same as the value of the "data_directory" string stored within the "magic_api" structure that your plugin's functions receive.

Note: Tux Paint's default Magic tool plugins install their data within "magic" subdirectories of Tux Paint's "images" and "sounds" data directories (e.g., "/usr/share/tuxpaint/images/magic/"). You are encouraged to do the same.

As stand-alone commands, using the BASH shell, for example:

# cp my_plugin_icon.png `tp-magic-config --dataprefix`/images/magic/
# chmod 644 `tp-magic-config --dataprefix`/images/magic/my_plugin_icon.png

Putting it Together in a Makefile

A snippet from a more generalized Makefile might look like this:

PLUGINPREFIX=$(shell tp-magic-config --pluginprefix)
DATAPREFIX=$(shell tp-magic-config --dataprefix)

install:
   mkdir -p $(PLUGINPREFIX)
   cp *.so $(PLUGINPREFIX)/
   chmod 644 $(PLUGINPREFIX)/*.so
   mkdir -p $(DATAPREFIX)/images/magic
   cp *.png $(DATAPREFIX)/images/magic/
   chmod 644 $(DATAPREFIX)/images/magic/*.png

The first two lines set up Makefile variables that contain the paths returned by the "tp-magic-config" command-line tool.

Below that is an "install" target in the Makefile. (Invoked by, for example, "$ sudo make install" or "# make install".)

The "install" target uses "mkdir -p" to make sure that the plugin directory exists, then uses "cp" to copy all plugin (".so") files into it, and invokes "chmod" to make sure they are readable.

It then does a similar series of commands to install icon files (".png" images) into a subdirectory within Tux Paint's data directory.

Windows

TBD

Mac OS X

TBD

Creating plugins with multiple effects

Plugins for Tux Paint may contain more than one effect. If you have multiple effects that are similar, it may make sense to place them in one plugin file, to reduce overhead and share code.

These following suggestions can help you create plugins that contain multiple effects:

Note: Even if your plugin currently contains only one effect, it may be useful to follow the steps above so that you can add a new variation of an effect with little effort. ("NUM_TOOLS" will simply be '1', your arrays will be of length '1', etc.)

Example Code

The C source file "tp_magic_example.c" contains a complete example of a plugin with multiple simple effects.

Getting Help

For more information, check the Tux Paint website: http://www.tuxpaint.org/, and the Simple DirectMedia Layer library website: http://www.libsdl.org/.

Additionally, other Tux Paint developers and users can be found on the "tuxpaint-devel" and "tuxpaint-users" mailing lists: http://www.tuxpaint.org/lists/.

Glossary