Writing a Sequence Plugin

Relevant to Blender v2.31

In this Section we will write a basic sequence plugin and then go through the steps use a sequence plugin. The basics behind a sequence plugin are you are given some inputs; 1-3 input image buffers as well as some other information and you output a resulting image buffer.

All the files necessary to develop plugins as well as a few sample plugins can be found in the blender/plugins directory. You can alternately get a bunch of plugins from http://www.cs.umn.edu/~mein/blender/plugins


Relevant to Blender v2.31

  • #include <plugin.h>

    Every Blender plugin should include this header file, which contains all of the structures and defines needed to properly work with Blender.

  • char name[]="Blur";

    A character string containing the plugin name, this value will be displayed for the texture's title in the Texture Buttons window.

  • VarStruct varstr[]= {...};

    The varstr contains all of the information Blender needs to display buttons for a plugin. Buttons for plugins can be numerical for input data, or text for comments and other information. Plugins are limited to a maximum of 32 variables.

    Each VarStruct entry consists of a type, name, range information, and a tool tip.

    The type defines the data type for each button entry, and the way to display the button. For number buttons this value should be a combination (ORed) of INT or FLO for the number format, and NUM, NUMSLI, or TOG, for the button type. Text buttons should have a type of LABEL.

    The name is what will be displayed on (or beside) the button. This is limited to 15 characters.

    The range information consists of three floats that define the default, minimum, and maximum values for the button. For TOG buttons the minimum is set in the pressed state, and the maximum is set in the depressed state.

    The tip is a string that will be displayed when the mouse is over this button (if the user has tool tips on). This has a limit of 80 characters, and should be set to the NULL string ("") if unused.

  • typedef struct Cast {...};

    The cast structure is used in calling the doit function, and serves as a way to simply access each plugin's data values.

    The cast should contain, in order, an integer or float for every button defined in the varstr, including text buttons. Typically these should have the same name as the button for simple reference.

  • float cfra

    The cfra value is set by Blender to the current from before every render pass. This value is an the frame number +/- .5 depending on the field settings.

  • plugin_seq_doit prototype

    The plugin_seq_doit function should be prototyped for use by the getinfo function. You do not need to change this line.

  • plugin_seq_getversion

    This function must be in each plugin for it to be loaded correctly. You should not change this function.

  • plugin_but_changed

    This function is used to pass information about what buttons the user changes in the interface. Most plugins should not need to use this function, only when the interface allows the user to alter some variable that forces the plugin to do recalculation (a random hash table for example).

  • plugin_init

    If needed plugins may use this function to initialize internal data. NOTE: This init function can be called multiple times if the same plugin sequence is copied. Do not init global data specific to a single instance of a plugin in this function.

  • plugin_getinfo

    This function is used to communicate information to Blender. You should never need to change it.

  • plugin_seq_doit

    The sequence doit function is responsible for applying the plugin's effect and copying the final data into the out buffer.

    The Arguments

    • Cast *cast

      The Cast structure which contains the plugin data, see the Cast entry above.

    • float facf0

      The value of the plugin's IPO curve for the first field offset. If the user hasn't made an IPO curve this ranges between 0 and 1 for the duration of the plugin.

    • float facf1

      The value of the plugin's IPO curve for the second field offset. If the user hasn't made an IPO curve this ranges between 0 and 1 for the duration of the plugin.

    • int x int y

      The width and height of the image buffers, respectively.

    • Imbuf *ibuf1

      A pointer to the first image buffer the plugin is linked to. This will always be a valid image buffer.

    • Imbuf *ibuf2

      A pointer to the second image buffer the plugin is linked to. Plugins using this buffer should check for a NULL buffer, as the user may not have attached the plugin to two buffers.

    • Imbuf *out

      The image buffer for the plugin's output.

    • Imbuf *use

      A pointer to the third image buffer the plugin is linked to. Plugins using this buffer should check for a NULL buffer, as the user may not have attached the plugin to three buffers.

ImBuf image structure

The ImBuf structure always contains 32 bits RGBA pixel data.

ImBuf structs are always equal in size, indicated by the passed x and y value.

User Interaction

There is no way for Blender to know how many inputs a plugin expects, so it is possible for a user to attach only one input to a plugin that expects two. For this reason it is important to always check the buffers your plugin uses to make sure they are all valid. Sequence plugins should also include a text label describing the number of inputs required in the buttons interface.

Generic Sequence Plugin:

#include "plugin.h"
char name[24]= "";

/* structure for buttons,
 * butcode name default min max 0

VarStruct varstr[]= {
 { LABEL, "In: X strips", 0.0, 0.0, 0.0, ""},

/* The cast struct is for input in the main doit function
 Varstr and Cast must have the same variables in the same order */

typedef struct Cast {
 int dummy; /* because of the 'label' button */
} Cast;

/* cfra: the current frame */

float cfra;

void plugin_seq_doit(Cast *, float, float, int, int,
 ImBuf *, ImBuf *, ImBuf *, ImBuf *);

int plugin_seq_getversion(void) {

void plugin_but_changed(int but) {

void plugin_init() {

void plugin_getinfo(PluginInfo *info) {
 info->name= name;
 info->nvars= sizeof(varstr)/sizeof(VarStruct);
 info->cfra= &cfra;

 info->varstr= varstr;

 info->init= plugin_init;
 info->seq_doit= (SeqDoit) plugin_seq_doit;
 info->callback= plugin_but_changed;

void plugin_seq_doit(Cast *cast, float facf0, float facf1, int xo, int yo,
 ImBuf *ibuf1, ImBuf *ibuf2, ImBuf *outbuf, ImBuf *use) {
 char *in1= (char *)ibuf1->rect;
 char *out=(char *)outbuf->rect;


Our Modifications:

The first step is to come up with a game plan. What is this plugin going to do, how are the users going to interact with it. For this example we will create a simple filter that will have a slider for intensity from 0-255. If any of the R,G, or B components of a pixel in the source image are less then our chosen intensity, it will return black and alpha, otherwise it will return whatever is in the image. Now we'll copy our generic plugin to simpfilt.c and will fill in the gaps.

Its always a good idea to add some comments. First off tell users what the plugin does, where they can get a copy, who they should contact for bugs/improvements, and any licensing restrictions on the code. When using comments make sure you use /* */ style comments. The plugins are in c and some c compilers do not accept // style comments.

Description: This plugin is a sample sequence plugin that filters out lower
intensity pixels.  I works on one strip as input.
Author: Kent Mein (mein@cs.umn.edu)
Website: http://www.cs.umn.edu/~mein/blender/plugins
Licensing: Public Domain
Last Modified: Sun Sep  7 23:41:35 CDT 2003

Next we need to fill in the Name, you should really keep this the same as your .c file. Preferably descriptive, less than 23 chars, no spaces, and all lowercase.

char name[24]= "simpfilt.c";

The Cast and varstr need to be in sync. We want one slider so we'll do the following:

varStruct varstr[]= {
   { LABEL,     "In: 1 strips", 0.0, 0.0, 0.0, ""},
   { NUM|INT,     "Intensity", 10.0, 0.0, 255.0, "Our threshold value"},

typedef struct Cast {
        int dummy;                      /* because of the 'label' button */
	int intensity;
} Cast;

Now we need to fill in plugin_seq_doit. We basically want to loop through each pixel and if RGB are all less than intensity set the output pixel to: 0,0,0,255 else set it to the input values for that position.

   int x,y;

   for(y=0;y cast->intensity) &&
             (in1[1] > cast->intensity) &&
	     (in1[2] > cast->intensity)) {
            out[0] = out[1] = out[2] = 0;
            out[3] = 255;
         } else {
            out[0] = in1[0];
            out[1] = in1[1];
            out[2] = in1[2];
            out[3] = in1[3];


So we wind up with simpfilt.c


bmake is a simple utility (shell script) to aid in the compilation and development of plugins, and can be found in the plugins/ sub-directory of the Blender installation directory. It is invoked by: bmake (plugin_name.c) and will attempt to link the proper libraries and compile the specified C file properly for your system. If you are trying to develop plugins on a windows machine, bmake may not work for you. In that case you should look into using lcc. You can use the following to compile a plugin with lcc: Assuming you have your plugins in c:\blender\plugins. Here is an example of how you would compile the sequence plugin sweep.c Open a dos prompt and do the following: (Note: You'll want to make sure the lcc\bin directory is in your path).

 cd c:\blender\plugins\sequence\sweep
 lcc -Ic:\blender\plugins\include sweep.c
 lcclnk -DLL sweep.obj c:\blender\plugins\include\seq.def
 implib sweep.dll