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 28, 2007
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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.)
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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.
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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.
* magic_api * api
Pointer to a C structure containing pointers to Tux Paint
functions and other data that the plugin can (and sometimes
should) use. The contents of this struct are described below.
Note: The magic_api struct is defined in the C header file
"tp_magic_api.h", which you should include at the top of your
plugin's C source file:
#include "tp_magic_api.h"
* int which
An index the plugin should use to differentiate different 'Magic'
tools, if the plugin provides more than one. (If not, "which" will
always be 0.) See "Creating plugins with multiple effects", below.
* SDL_Surface * snapshot
A snapshot of the previous Tux Paint canvas, taken when the the
mouse was first clicked to activate the current magic tool. If you
don't continuously affect the image during one hold of the mouse
button, you should base your effects off the contents of this
canvas. (That is, read from "snapshot" and write to "canvas",
below.)
* SDL_Surface * canvas
The current Tux Paint drawing canvas. Your magical effects should
end up here!
* SDL_Rect * update_rect
A pointer to an SDL 'rectangle' structure that you use to tell
Tux Paint what part of the canvas has been updated. If your effect
affects a 32x32 area centered around the mouse pointer, you would
fill the SDL_Rect as follows:
update_rect->x = x - 16;
update_rect->y = y - 16;
update_rect->w = 32;
update_rect->h = 32;
Or, if your effect changes the entire canvas (e.g., flips it
upside-down), you'd fill it as follows:
update_rect->x = 0;
update_rect->y = 0;
update_rect->w = canvas->w;
update_rect->h = canvas->h;
Note: "update_rect" is a C pointer (an "SDL_Rect *" rather than
just an "SDL_Rect") because you need to fill in its contents.
Because it is a pointer, you access its elements via "->" (arrow)
rather than "." (dot).
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
plguin's Magic tools that accept colors becomes active, or 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) values in
'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 call the Tux Paint 'Magic' tool plugin "line()" helper
function. (See below).
The plugin should report back what part of the canvas was
affected, by filling in the (x,y) and (w,h) values in
'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) values in
'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 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.)
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);
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.
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 logically 'or'ed ("|")
together and sent to this function.
* SPECIAL_FLIP -- The contents of the canvas has been
flipped.
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.
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.
* min(x, y)
The minimum of 'x' and 'y'. (That is, if 'x' is less than or equal
to 'y', then the value of 'x' will be used. If 'y' is less than
'x', it will be used.)
* max(x, y)
The maximum of 'x' and 'y'. The opposite of min().
* clamp(lo, value, hi)
A value, clamped to be no smaller than 'lo', and no higher than
'hi'. (That is, if 'value' is less than 'lo', then 'lo' will be
used; if 'value' is greater than 'hi', then 'hi' will be used;
otherwise, 'value' will be used.)
Example: red = clamp(0, n, 255);
Tries to set 'red' to be the value of 'n', but without allowing it
to become less than 0 or greater than 255.
Note: This macro is simply a #define of:
"(min(max(value,lo),hi))".
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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 $@ $<
You may then install it globally into: /usr/lib/tuxpaint/plugins/ or
/usr/local/lib/tuxpaint/plugins/ (depending on how Tux Paint was
installed).
Or install it locally (for the current user only) into:
~/.tuxpaint/magic/
(FIXME: As of 2007-07-27, Tux Paint does not look here yet!)
Windows
TBD
Mac OS X
TBD
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Creating plugins with multiple effects
TBD
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Example Code
TBD
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Summary and contact info TBD.
