Gimp::Pixel - how to operate on raw pixels.

***WARNING*** this manpage is no longer up-to-date. See CWexamples/map_to_gradient for a simple raw-pixel-manipulating plug-in. If you bug me enough I'll rewrite this document.

  use Gimp;
  use PDL;              # to make sensible things with the pixels

  # Gimp::GimpDrawable  - The GimpDrawable structure
  # Gimp::Tile          - The Tile family of functions.
  # Gimp::PixelRgn      - The PixelRgn family of functions.

You can access the pixels in a drawable through tiles or pixel regions. This manpage explains how this is done in perl. All classes (Gimp::GimpDrawable, CWGimp::Tile, CWGimp::PixelRgn) are available with and without the CWGimp:: prefix.

Well, you know drawables? (also known as PARAM_DRAWABLE or Gimp::Drawable)? In the Gimp, drawables are things you can draw on: layers, channels or whole images. While most functions named CWgimp_drawable_something operate on CWdrawable_IDs, some functions (notably the ones operating on raw pixel data!) need a CWGimpDrawable instead. Every drawable has a corresponding CWGimpDrawable, you can get it with the CWgimp_drawable_get function:

 my $gdrawable = $drawable->get;

When the CW$gdrawable is destroyed, it is automatically flushed & detached, so you don't need to do this yourself.

Tiles are the basic building blocks of all drawables. Each drawable consists of a grid of tiles, each tile having the same size. The size of a tile is always the same (it's hardcoded in your Gimp program).

The CWgimp_tile_width and CWgimp_tile_height functions return the current width/height of a tile (at the moment, this is 64x64).

How do I get a tile? First, you have to grab a GimpDrawable structure. You can get one from any drawable, by calling the CWget function:

 my $gdrawable = $drawable->get;

in a sense, <$gdrawable> contains all tiles. Changes you make to them might not be reflected in the image until you destroy this variable. (Thats the reason I used my int he above example. Once CW$gdrawable gets out of scope, the drawable in the gimp automatically gets updated).

To get access to a tile, you have to call CWget_tile or CWget_tile2. CWget_tile expects row/column numbers of the tile, while CWget_tile2 expects pixel coordinates and will return the tile that pixel is in:

 my $tile = $gdrawable->get_tile2(1,75,60);

The CWdata method returns and sets the raw pixel data.

 $piddle = $tile->data; # get the tile data as a piddle
 $piddle *= 0.5;        # do sth. with the pixels
 $tile->data($piddle);  # and modify the tile

CWPixelRgns are rectangular parts of a drawable. You can access single pixels, rows, columns and rectangles within these regions. Don't expect me to explain everything now, I don't understand the mechanism too well myself..

How do I create a pixel region? First, you have to grab a GimpDrawable structure. You can get one from any drawable, by calling the CWget function:

 my $gdrawable = $drawable->get;

Now you can create as many PixelRgn structures as you want from the CWGimpDrawable:

 my $region = new PixelRgn($gdrawable,0,0,50,30,1,0);   # with "new"
 my $region = $gdrawable->pixel_rgn(0,0,50,30,1,0);     # or from a drawable

which method you choose is purely a question of style...

The following functions return packed pixel data (see Gimp::PDL for an easier way to manipulate on image data):

 $piddle = $region->get_pixel(45,60);           # return the pixel at (45|60)
 $piddle = $region->get_row(45,60,10);          # return ten horizontal pixels
 $piddle = $region->get_col(45,60,10);          # same but vertically
 $piddle = $region->get_rect(45,60,10,12);      # a 10x12 rectangle

To modify pixels, the dirty bit of the region must be set (I believe, but I don't see the reason what the dirty bit in a region is for so I might be wrong), and you can write pixel data to the region with the following functions, each one corresponding to a get-function:

 $region->set_pixel($piddle,45,60);             # set pixel at (45|60)
 $region->set_row($piddle,45,60);               # set a row
 $region->set_col($piddle,45,60);               # set a column
 $region->set_rect($piddle,45,60);              # set a whole rectangle

Please note that (unlike the C functions they call), the size arguments (width and/or height) are missing, they can be calculated from the piddle.

Marc Lehmann <pcg@goof.com>

perl(1), Gimp(1).