gimp_create_mainimage (Gimp     *gimp,
                       gboolean  with_unusual_stuff,
                       gboolean  compat_paths,
                       gboolean  use_gimp_2_8_features)
{
  GimpImage     *image             = NULL;
  GimpLayer     *layer             = NULL;
  GimpParasite  *parasite          = NULL;
  GimpGrid      *grid              = NULL;
  GimpChannel   *channel           = NULL;
  GimpRGB        channel_color     = GIMP_MAINIMAGE_CHANNEL1_COLOR;
  GimpChannel   *selection         = NULL;
  GimpVectors   *vectors           = NULL;
  GimpCoords     vectors1_coords[] = GIMP_MAINIMAGE_VECTORS1_COORDS;
  GimpCoords     vectors2_coords[] = GIMP_MAINIMAGE_VECTORS2_COORDS;
  GimpStroke    *stroke            = NULL;
  GimpLayerMask *layer_mask        = NULL;

  /* Image size and type */
  image = gimp_image_new (gimp,
                          GIMP_MAINIMAGE_WIDTH,
                          GIMP_MAINIMAGE_HEIGHT,
                          GIMP_MAINIMAGE_TYPE,
                          GIMP_MAINIMAGE_PRECISION);

  /* Layers */
  layer = gimp_layer_new (image,
                          GIMP_MAINIMAGE_LAYER1_WIDTH,
                          GIMP_MAINIMAGE_LAYER1_HEIGHT,
                          GIMP_MAINIMAGE_LAYER1_FORMAT,
                          GIMP_MAINIMAGE_LAYER1_NAME,
                          GIMP_MAINIMAGE_LAYER1_OPACITY,
                          GIMP_MAINIMAGE_LAYER1_MODE);
  gimp_image_add_layer (image,
                        layer,
                        NULL,
                        0,
                        FALSE/*push_undo*/);
  layer = gimp_layer_new (image,
                          GIMP_MAINIMAGE_LAYER2_WIDTH,
                          GIMP_MAINIMAGE_LAYER2_HEIGHT,
                          GIMP_MAINIMAGE_LAYER2_FORMAT,
                          GIMP_MAINIMAGE_LAYER2_NAME,
                          GIMP_MAINIMAGE_LAYER2_OPACITY,
                          GIMP_MAINIMAGE_LAYER2_MODE);
  gimp_image_add_layer (image,
                        layer,
                        NULL,
                        0,
                        FALSE /*push_undo*/);

  /* Layer mask */
  layer_mask = gimp_layer_create_mask (layer,
                                       GIMP_ADD_MASK_BLACK,
                                       NULL /*channel*/);
  gimp_layer_add_mask (layer,
                       layer_mask,
                       FALSE /*push_undo*/,
                       NULL /*error*/);

  /* Image compression type
   *
   * We don't do any explicit test, only implicit when we read tile
   * data in other tests
   */

  /* Guides, note we add them in reversed order */
  gimp_image_add_hguide (image,
                         GIMP_MAINIMAGE_HGUIDE2_POS,
                         FALSE /*push_undo*/);
  gimp_image_add_hguide (image,
                         GIMP_MAINIMAGE_HGUIDE1_POS,
                         FALSE /*push_undo*/);
  gimp_image_add_vguide (image,
                         GIMP_MAINIMAGE_VGUIDE2_POS,
                         FALSE /*push_undo*/);
  gimp_image_add_vguide (image,
                         GIMP_MAINIMAGE_VGUIDE1_POS,
                         FALSE /*push_undo*/);


  /* Sample points */
  gimp_image_add_sample_point_at_pos (image,
                                      GIMP_MAINIMAGE_SAMPLEPOINT1_X,
                                      GIMP_MAINIMAGE_SAMPLEPOINT1_Y,
                                      FALSE /*push_undo*/);
  gimp_image_add_sample_point_at_pos (image,
                                      GIMP_MAINIMAGE_SAMPLEPOINT2_X,
                                      GIMP_MAINIMAGE_SAMPLEPOINT2_Y,
                                      FALSE /*push_undo*/);

  /* Tattoo
   * We don't bother testing this, not yet at least
   */

  /* Resolution */
  gimp_image_set_resolution (image,
                             GIMP_MAINIMAGE_RESOLUTIONX,
                             GIMP_MAINIMAGE_RESOLUTIONY);


  /* Parasites */
  parasite = gimp_parasite_new (GIMP_MAINIMAGE_PARASITE_NAME,
                                GIMP_PARASITE_PERSISTENT,
                                GIMP_MAINIMAGE_PARASITE_SIZE,
                                GIMP_MAINIMAGE_PARASITE_DATA);
  gimp_image_parasite_attach (image,
                              parasite);
  gimp_parasite_free (parasite);
  parasite = gimp_parasite_new ("gimp-comment",
                                GIMP_PARASITE_PERSISTENT,
                                strlen (GIMP_MAINIMAGE_COMMENT) + 1,
                                GIMP_MAINIMAGE_COMMENT);
  gimp_image_parasite_attach (image, parasite);
  gimp_parasite_free (parasite);


  /* Unit */
  gimp_image_set_unit (image,
                       GIMP_MAINIMAGE_UNIT);

  /* Grid */
  grid = g_object_new (GIMP_TYPE_GRID,
                       "xspacing", GIMP_MAINIMAGE_GRIDXSPACING,
                       "yspacing", GIMP_MAINIMAGE_GRIDYSPACING,
                       NULL);
  gimp_image_set_grid (image,
                       grid,
                       FALSE /*push_undo*/);
  g_object_unref (grid);

  /* Channel */
  channel = gimp_channel_new (image,
                              GIMP_MAINIMAGE_CHANNEL1_WIDTH,
                              GIMP_MAINIMAGE_CHANNEL1_HEIGHT,
                              GIMP_MAINIMAGE_CHANNEL1_NAME,
                              &channel_color);
  gimp_image_add_channel (image,
                          channel,
                          NULL,
                          -1,
                          FALSE /*push_undo*/);

  /* Selection */
  selection = gimp_image_get_mask (image);
  gimp_channel_select_rectangle (selection,
                                 GIMP_MAINIMAGE_SELECTION_X,
                                 GIMP_MAINIMAGE_SELECTION_Y,
                                 GIMP_MAINIMAGE_SELECTION_W,
                                 GIMP_MAINIMAGE_SELECTION_H,
                                 GIMP_CHANNEL_OP_REPLACE,
                                 FALSE /*feather*/,
                                 0.0 /*feather_radius_x*/,
                                 0.0 /*feather_radius_y*/,
                                 FALSE /*push_undo*/);

  /* Vectors 1 */
  vectors = gimp_vectors_new (image,
                              GIMP_MAINIMAGE_VECTORS1_NAME);
  /* The XCF file can save vectors in two kind of ways, one old way
   * and a new way. Parameterize the way so we can test both variants,
   * i.e. gimp_vectors_compat_is_compatible() must return both TRUE
   * and FALSE.
   */
  if (! compat_paths)
    {
      gimp_item_set_visible (GIMP_ITEM (vectors),
                             TRUE,
                             FALSE /*push_undo*/);
    }
  /* TODO: Add test for non-closed stroke. The order of the anchor
   * points changes for open strokes, so it's boring to test
   */
  stroke = gimp_bezier_stroke_new_from_coords (vectors1_coords,
                                               G_N_ELEMENTS (vectors1_coords),
                                               TRUE /*closed*/);
  gimp_vectors_stroke_add (vectors, stroke);
  gimp_image_add_vectors (image,
                          vectors,
                          NULL /*parent*/,
                          -1 /*position*/,
                          FALSE /*push_undo*/);

  /* Vectors 2 */
  vectors = gimp_vectors_new (image,
                              GIMP_MAINIMAGE_VECTORS2_NAME);

  stroke = gimp_bezier_stroke_new_from_coords (vectors2_coords,
                                               G_N_ELEMENTS (vectors2_coords),
                                               TRUE /*closed*/);
  gimp_vectors_stroke_add (vectors, stroke);
  gimp_image_add_vectors (image,
                          vectors,
                          NULL /*parent*/,
                          -1 /*position*/,
                          FALSE /*push_undo*/);

  /* Some of these things are pretty unusual, parameterize the
   * inclusion of this in the written file so we can do our test both
   * with and without
   */
  if (with_unusual_stuff)
    {
      /* Floating selection */
      gimp_selection_float (GIMP_SELECTION (gimp_image_get_mask (image)),
                            gimp_image_get_active_drawable (image),
                            gimp_get_user_context (gimp),
                            TRUE /*cut_image*/,
                            0 /*off_x*/,
                            0 /*off_y*/,
                            NULL /*error*/);
    }

  /* Adds stuff like layer groups */
  if (use_gimp_2_8_features)
    {
      GimpLayer *parent;

      /* Add a layer group and some layers:
       *
       *  group1
       *    layer3
       *    layer4
       *    group2
       *      layer5
       */

      /* group1 */
      layer = gimp_group_layer_new (image);
      gimp_object_set_name (GIMP_OBJECT (layer), GIMP_MAINIMAGE_GROUP1_NAME);
      gimp_image_add_layer (image,
                            layer,
                            NULL /*parent*/,
                            -1 /*position*/,
                            FALSE /*push_undo*/);
      parent = layer;

      /* layer3 */
      layer = gimp_layer_new (image,
                              GIMP_MAINIMAGE_LAYER1_WIDTH,
                              GIMP_MAINIMAGE_LAYER1_HEIGHT,
                              GIMP_MAINIMAGE_LAYER1_FORMAT,
                              GIMP_MAINIMAGE_LAYER3_NAME,
                              GIMP_MAINIMAGE_LAYER1_OPACITY,
                              GIMP_MAINIMAGE_LAYER1_MODE);
      gimp_image_add_layer (image,
                            layer,
                            parent,
                            -1 /*position*/,
                            FALSE /*push_undo*/);

      /* layer4 */
      layer = gimp_layer_new (image,
                              GIMP_MAINIMAGE_LAYER1_WIDTH,
                              GIMP_MAINIMAGE_LAYER1_HEIGHT,
                              GIMP_MAINIMAGE_LAYER1_FORMAT,
                              GIMP_MAINIMAGE_LAYER4_NAME,
                              GIMP_MAINIMAGE_LAYER1_OPACITY,
                              GIMP_MAINIMAGE_LAYER1_MODE);
      gimp_image_add_layer (image,
                            layer,
                            parent,
                            -1 /*position*/,
                            FALSE /*push_undo*/);

      /* group2 */
      layer = gimp_group_layer_new (image);
      gimp_object_set_name (GIMP_OBJECT (layer), GIMP_MAINIMAGE_GROUP2_NAME);
      gimp_image_add_layer (image,
                            layer,
                            parent,
                            -1 /*position*/,
                            FALSE /*push_undo*/);
      parent = layer;

      /* layer5 */
      layer = gimp_layer_new (image,
                              GIMP_MAINIMAGE_LAYER1_WIDTH,
                              GIMP_MAINIMAGE_LAYER1_HEIGHT,
                              GIMP_MAINIMAGE_LAYER1_FORMAT,
                              GIMP_MAINIMAGE_LAYER5_NAME,
                              GIMP_MAINIMAGE_LAYER1_OPACITY,
                              GIMP_MAINIMAGE_LAYER1_MODE);
      gimp_image_add_layer (image,
                            layer,
                            parent,
                            -1 /*position*/,
                            FALSE /*push_undo*/);
    }

  /* Todo, should be tested somehow:
   *
   * - Color maps
   * - Custom user units
   * - Text layers
   * - Layer parasites
   * - Channel parasites
   * - Different tile compression methods
   */

  return image;
}

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.