internals.rst

Ginga Internals

This chapter explains the secret inner workings of Ginga and its classes so that you can subclass them and use them in your own applications.

Introduction

Ginga uses a version of the Model-View-Controller design pattern. The MVC pattern spells out a division of responsibilities and encapsulation where the Model provides various ways to access and interface to the data, the View provides ways to display the data and the Controller provides the methods and user interface hooks for controlling the view.

The Model

Hierarchy of Ginga AstroImage class

The Model classes are rooted in the base class BaseImage. The basic interface to the data is expected to be a Numpy-like array object that is obtained via the get_data() method on the model. It also provides methods for obtaining scaled, cutouts and transformed views of the data, and methods for getting and setting key-value like metadata.

There are two subclasses defined on BaseImage: RGBImage and AstroImage. RGBImage is used for displaying 3 channel RGB type images such as JPEG, TIFF, PNG, etc. AstroImage is the subclass used to represent astronomical images and its organization is shown in Figure fig-astroimage. It has two delegate objects devoted to handling World Coordinate System transformations and file IO. There is also a mixin class, LayerImage that can be used to create layered images with alpha compositing on each layer.

New models can be created, subclassing from BaseImage or AstroImage. As long as the model duck types like a BaseImage it can be loaded into a view object with the set_image() method. AstroImage provides convenience methods for accessing WCS information that may be necessary when using the model in canvas subclasses of a View that allow graphics drawing.

The View

Class structure of Ginga basic widget viewer

Figure fig-imageviewzoom shows the class inheritance of the ImageViewZoom class, which is a typical end class to use in a program if one is not planning to do any graphical overplotting. The figure key indicates the base class verses the widget specific classes.

The View classes are rooted in the base class ImageView, which handles image display, scaling (zooming), panning, manual cut levels, auto cut levels, color mapping, transformations, and rotation. The ImageView is quite powerful compared to base classes in most inheritance designs, as it actually renders the view all the way out to RGB image planes in the appropriate sizes for the widget target window. Ginga supports "backends" for different widget sets (Gtk, Qt, Tk, etc.) through various subclasses of this base class, which do the actual painting of the resulting RGB image into a widget in the native widget set.

In this example, ImageViewXYZ is a class that renders to a native widget in the "XYZ" toolkit. ImageViewEvent adds event handlers for various pointing and keyboard events, but without connecting them to any particular handling scheme. Finally, ImageViewZoom provides a concrete implementation of event handling by connecting the handlers in the ImageViewEvent class with the logic in the BindingMapper and Bindings delegate objects as will as some logic in the UIMixin class. This event handling scheme is described in more detail in the section on the Controller. With this layered class construction, it is possible to minimize the widget specific code and reuse a large amount of code across widget sets and platforms. Because the vast majority of work is done in the base class, and the outer classes simply inherit the widget-specific ones and mix in the others, it is a fairly simple matter to port the basic Ginga functionality to a new widget set. All that is required is that the new widget set have some kind of native widget that supports painting an RGB image (like a canvas or image widget) and a way to register for user interaction events on that widget.

The Controller

The control interface is a combination of methods on the view object and a pluggable Bindings class which handles the mapping of user input events such as mouse, gesture and keystrokes into commands on the view. There are many callback functions that can be registered, allowing the user to create their own custom user interface for manipulating the view.

Graphics on Ginga

Class structure of Ginga DrawingCanvas class.

Ginga's graphics are all rendered from objects placed on a DrawingCanvas. All objects that can be put on a DrawingCanvas are rooted in the CanvasObject type (including DrawingCanvas itself).

Miscellaneous Topics

I want to use my own World Coordinate System!

No problem. Ginga encapsulates the WCS behind a pluggable object used in the AstroImage class. Your WCS should implement this abstract class:

def MyWCS(object):
    def __init__(self, logger):
        self.logger = logger

    def get_keyword(self, key):
        return self.header[key]

    def get_keywords(self, *args):
        return [self.header[key] for key in args]

    def load_header(self, header, fobj=None):
        pass

    def pixtoradec(self, idxs, coords='data'):
        # calculate ra_deg, dec_deg
        return (ra_deg, dec_deg)

    def radectopix(self, ra_deg, dec_deg, coords='data', naxispath=None):
        # calculate x, y
        return (x, y)

    def pixtosystem(self, idxs, system=None, coords='data'):
        return (deg1, deg2)

    def datapt_to_wcspt(self, datapt, coords='data', naxispath=None):
        return [[ra_deg_0, dec_deg_0], [ra_deg_1, dec_deg_1], ...,
                [ra_deg_n, dec_deg_n]]

    def wcspt_to_datapt(self, wcspt, coords='data', naxispath=None):
        return [[x0, y0], [x1, y1], ..., [xn, yn]]

To use your WCS with Ginga create your images like this:

from ginga.AstroImage import AstroImage
AstroImage.set_wcsClass(MyWCS)
...

image = AstroImage()
...
view.set_image(image)

or you can override the WCS on a case-by-case basis:

from ginga.AstroImage import AstroImage
...

image = AstroImage(wcsclass=MyWCS)
...
view.set_image(image)

You could also subclass AstroImage or BaseImage and implement your own WCS handling. There are certain methods in AstroImage used for graphics plotting and plugins, however, so these would need to be supported if you expect the same functionality.

I want to use my own file storage format, not FITS!

First of all, you can always create an AstroImage and assign its components for wcs and data explicitly. Assuming you have your data loaded into an numpy array named data:

from ginga import AstroImage
...

image = AstroImage()
image.set_data(data)

To create a valid WCS for this image, you can set the header in the image (this assumes header is a valid mapping of keywords to values):

image.update_keywords(header)

An AstroImage can then be loaded into a viewer object with set_dataobj(). If you need a custom WCS see the notes in Section sec-custom-wcs. If, however, you want to add a new type of custom loader into Ginga's file loading framework, you can do so using the following instructions.

Adding a new kind of file opener

Ginga's general file loading facility breaks the loading down into two phases: first, the file is identified by its magic signature (requires the optional Python module python-magic be installed), MIME type, or filename extension. Once the general category of file is known, methods in the specific I/O module devoted to that type are called to load the file data.

The ginga.util.loader module is used to register file openers. An opener is a class that understand how to load data objects from a particular kind of file format. You'll want to start by examining this module and especially looking at the examples at the bottom of that file for how openers are registered.

For implementing your own special opener, take a look at the BaseIOHandler class in ginga.util.io.io_base. This is the base class for all I/O openers for Ginga. Subclass this class, and implement all of the methods that raise NotImplementedError and optionally implement any other methods marked with the comment "subclass should override as needed". You can study the io_fits and io_rgb modules to see how these methods are implemented for specific formats. Here is an example opener class for HDF5 standard image files:

code/io_hdf5.py

Once you have created your opener class (e.g. HDF5FileHandler), you register it as follows:

from ginga.util import loader
import io_hdf5
loader.add_opener(io_hdf5.HDF5FileHandler, ['application/x-hdf'])

If you want to use this with the Ginga reference viewer, a good place to register the opener is in your ginga_config.py as discussed in Section sec-workspaceconfig of the Reference Viewer Manual. The best place is probably by implementing pre_gui_config and registering it as shown above in that function. Once your loader is registered, you will be able to drag and drop files and use the reference viewers regular loading facilities to load your data.

Porting Ginga to a New Widget Set

[TBD]