How to use gimp_drawable_set_pixel()?

Question

I could like to set a pixel in GIMP, but I can't figure out how to specify the pixel value.

According to the GIMP plugin API it says:

gboolean            gimp_drawable_set_pixel             (gint32 drawable_ID,
                                                         gint x_coord,
                                                         gint y_coord,
                                                         gint num_channels,
                                                         const guint8 *pixel);

If I e.g. do:

  const guint8 *pixel;
  pixel = (guint8 *) 0;
  gboolean s;
  s = gimp_drawable_set_pixel (layer, 5, 5, 3, pixel);
  printf ("Was the pixel set?: %i", s);

The pixel is not set, and s is zero.

Can anyone figure this one out?

Lots of love, Louise

This is the code I am using with Makefile:

#include <libgimp/gimp.h>

#include <stdio.h>
#include <stdlib.h>
#include <string.h>


static void
query (void)
{
  static GimpParamDef args[] =
  {
    {
      GIMP_PDB_INT32,
      "run-mode",
      "Run mode"
    },
  };

  gimp_install_procedure (
    "plug-in-hello",
    "Hello, world!",
    "Displays \"Hello, world!\" in a dialog",
    "David Neary",
    "Copyright David Neary",
    "2004",
    "_Hello world...",
    NULL,
    GIMP_PLUGIN,
    G_N_ELEMENTS (args), 0,
    args, NULL);

  gimp_plugin_menu_register ("plug-in-hello",
                             "<Image>/Filters/Misc");
}

static void
run (const gchar      *name,
     gint              nparams,
     const GimpParam  *param,
     gint             *nreturn_vals,
     GimpParam       **return_vals)
{
  static GimpParam  values[1];
  GimpPDBStatusType status = GIMP_PDB_SUCCESS;
  GimpRunMode       run_mode;
  gint32            image;
  gint32            layer;
  gint32            display;

  /* Setting mandatory output values */
  *nreturn_vals = 1;
  *return_vals  = values;

  values[0].type = GIMP_PDB_STATUS;
  values[0].data.d_status = status;

  /* Getting run_mode - we won't display a dialog if 
   * we are in NONINTERACTIVE mode */
  run_mode = param[0].data.d_int32;

  image = gimp_image_new (800, 600, GIMP_RGB);
  layer = gimp_layer_new (image,
                          "foo",
                          800, 600,
                          GIMP_RGBA_IMAGE,
                          100.0,
                          GIMP_NORMAL_MODE);
  gimp_image_add_layer (image, layer, 0);


  gboolean s;
  guint8 pixel[] = { 0xff, 0, 0, 0xff };
  s = gimp_drawable_set_pixel (layer, 5, 5, 4, (guint8 *)pixel );
  printf ("Was the pixel set?: %i", s);

  display = gimp_display_new (image);

  if (run_mode != GIMP_RUN_NONINTERACTIVE)
    g_message("Hello, world!\n");
}

GimpPlugInInfo PLUG_IN_INFO =
{
  NULL,
  NULL,
  query,
  run
};

MAIN()

Makefile

CC = gcc
CFLAGS = -std=c99 -O2 -Wall \
        -I/usr/include/gimp-2.0 \
        -I/usr/include/glib-2.0 \
        -I/usr/lib64/glib-2.0/include \
        -I/usr/include/gtk-2.0 \
        -I/usr/lib64/gtk-2.0/include \
        -I/usr/include/atk-1.0 \
        -I/usr/include/cairo \
        -I/usr/include/pango-1.0 \
        -I/usr/include/pixman-1 \
        -I/usr/include/freetype2 \
        -I/usr/include/libpng12  

LDFLAGS = -lgimpui-2.0 -lgimpwidgets-2.0 -lgimpmodule-2.0 -lgimp-2.0 \
          -lgimpmath-2.0 -lgimpconfig-2.0 -lgimpcolor-2.0 \
          -lgimpbase-2.0 -lgtk-x11-2.0 -lgdk-x11-2.0 -latk-1.0 \
          -lgio-2.0 -lpangoft2-1.0 -lgdk_pixbuf-2.0 -lpangocairo-1.0 \
          -lcairo -lpango-1.0 -lfreetype -lfontconfig -lgobject-2.0 \
          -lgmodule-2.0 -lglib-2.0  

files = main.o

all: main

main: $(files)
        $(CC) $(CFLAGS) $(files) $(LDFLAGS) -o main

install:
        gimptool-2.0 --install main.c

%.o: %.c Makefile
        $(CC) -c $(CFLAGS) $<

clean:
        rm -f *.o
        rm -f *~
        rm -f main

.PHONY: all clean

Update: The code is now corrected according to the comments, so now it works. It draws a red pixel.

Answer 1

const guint8 *pixel;
pixel = (guint8 *) 0;

On the first line you declare a pointer to guint8, which doesn't allocate any memory and the pointer points to some garbage. On the second line you make the pointer point to NULL. You either need to malloc/free the pixel buffer, or better yet, use stack.

Malloc/Free

guint8 *pixel = malloc(sizeof(guint8) * num_channels);
/*    R             G             B             A */
pixel[0] = 0; pixel[1] = 0; pixel[2] = 0; pixel[3] = 0;
s = gimp_drawable_set_pixel (layer, 5, 5, 3, pixel);
free(pixel);

Stack:

guint8 pixels[] = {0, 0, 0, 0};
s = gimp_drawable_set_pixel (layer, 5, 5, 3, pixel);

Answer 2

The pixel value depends on the pixel format of the drawable you're drawing to. In this case, you've created the layer as RGBA, so the pixel parameter should be the address of an array containing four values. E.g. this value would give you an opaque red pixel:

guint8 pixel[] = { 0xff, 0, 0, 0xff };