Histogram and Stretched Histogram

The Histogram utility is used to display the histogram and/or statistics for the files being viewed with Xvd. The Histogram uses the raw image data to gather the histogram. The Stretched Histogram differs from the Histogram in that it applies a stretch to the image data, and then gathers the histogram. The Stretched Histogram displays the data after the stretch is done. There are many options and user preferences available under the Options menu. All can be set interactively or placed in a user configuration file: which is found as ~/.Xdefaults. The histogram below was created by setting the options under the Options menu.

Stretch

The Stretch tool allows the user to perform contrast enhancements of the image being displayed. This is similar to the jstretch-* commands that were used under the older VIDS system. Each of the stretch options available from the dialog will request further information by highlighting the necessary parameters.

Stretch File Menu

The Stretch File Menu enables you to save/load an IBIS LUT (lookup table) file. A stretch can be uniquely characterized by a LUT. These LUT files are stored in an IBIS-2 format.

The Stretch Options Menu enables you to undo the most recently applied stretch. It also allows you to view the percent stretch limits for each plane, i.e., red, green, and blue, as shown in the dialog above.


Stretch Types

There are two types of stretches available, non-histogram and histogram. Non-Histogram Stretches do not require the generation of a histogram in order to perform the desired function. Histogram Stretches will require the generation of a histogram before the stretch can be performed. Refer to the table below to obtain a description of each stretch type.

These stretches can be applied to one of four combinations of the image planes, i.e., All, Red, Green, and Blue, as listed at the bottom of the dialog.

All

Apply the current stretch to all image planes. This will use only the Red/BW if the current image is an 8-bit gray scale image.

Red

Apply the current stretch to the Red plane only

Green

Apply the current stretch to the Green plane only

Blue

Apply the current stretch to the Blue plane only

Cursor Stretch

The cursor stretch allows you to use the cursor to vary a stretch. It controls a linear stretch by cursor motion, rather than by typing value changes into the stretch dialog. Vertical motion of the cursor changes the contrast; horizontal motion controls the brightness. The contrast is the slope of the line; the brightness is the Y intercept of the line. If you open the stretch dialog, you will see the values changing when the cursor stretch tool is active. To turn off the cursor stretch tool, press the right mouse button to bring up a popup tools menu.

Lookup Tables (LUT)

The Lookup Table (LUT) window presents a view of the current LUT being used to display the image. As seen below, the ramp indicates that there are no modifications, or stretches, being applied to the image. This view is constantly updated to reflect any changes which may occur during image analysis.

Such changes are usually caused by applying stretches to the image by using the facilities of the Stretch Tool (found under the Xvd Tools menu). Refer to the Stretch Lookup Tables section for an example of a stretch being applied to an image. If the Use Pseudocolor Tables option is on, a Pseudocolor lookup table is available.

(Note: For further information about the LUT resources, see the resource help.)

The current status of this view causes each of the LUTs for red, green, and blue to be plotted successively. This is why the ramp above appears blue. The blue line "hides" the green and red lines behind it. Consequently, if you were to see a red and blue plot, then the green line would lie behind the blue. Unfortunately, if only green and blue are visible, then the red line could coincide with either of them. Consult your stretch parameters to try to determine where the red plot should be.

Stretch Lookup Tables

The curve in the lookup table below indicates the image has been modified, in this case, by applying a periodic stretch to the image.

Refer to the Lookup Tables section for more general information about XVD lookup tables.

Pseudocolor Lookup Tables

The Pseudocolor Lookup Table (LUT) window presents a view of the current Pseudocolor LUT being used to display the image.

Refer to the Lookup Tables section for more general information about XVD lookup tables.

Pan

The Pan Tool is used in conjunction with XVD to better see and navigate around a large image. (For more help, refer to XVD Keys Help.) When utilizing the Pan Tool, XVD can be left at a zoom of 1, while the pan window zooms to fit the entire image in a "thumbnail" representation. An example of this would be the following:

XVD window with image displayed

Pan Tool window showing thumbnail of XVD image

In order to move about the image, simply click the left mouse button in the Pan window and drag the dotted rectangle to any arbitrary location. You may also resize the Pan window--notice that while the Pan window is resized, the aspect ratio will remain constant. So in this case, the Pan window (no matter what size set by the user) will remain square.

Also, notice whether or not the scroll bars are enabled around the XVD image area. These scroll bars appear whenever the area being displayed does not contain the full image. These scroll bars allow you to move about the image as would be intuitively expected. As mentioned above, changes made using the scroll bars will be reflected in the Pan window.

For a look at the Pan tool's resources and settings, see the Pan resources help.

Magnifying Glass

(DEFINITION COMING SOON!)

Zoom

For further information about the Zoom View's resources and settings, please refer to the resource help.

There are two ways to perform any arbitrary zoom factor you could want. First, the Zoom Factor Option menu is avaliable on the Side Bar. Note: This means that the Display Side Bar option found in the Preferences dialog must be enabled. The Zoom Factor Option menu appears as the following:

As soon as the first mouse button is clicked on this option button, you will be presented with a detailed list of available options as seen below:

This menu will appear under your pointer ready for an immediate selection. Once a selection is made, the choice is placed on the Zoom Option button to reflect that change. In this case, if the user selected say, a zoom of 1/2, the Zoom Option title would be changed from None to 1/2. This is beneficial in that it will be a constant reminder as to what the current zoom factor is.

Special and Zoom to Fit

In addition to the many prefabricated zoom, you can select the Special and Zoom to Fit options. All selections other than Special will immediately perform the desired operation. The Zoom to Fit option will size the image such that it will just fit within the current Image Display Window. The original aspect ratio is maintained.

If you select Special, you will be presented with the following dialog:

This dialog allows the user to specify the same stretches as are available from the Zoom Factor Options menu in the Side Bar, as well as, the Special zooms. These Special settings allow for fractional and integer zooms not available from the Zoom Factor menu, and unequal zooms for the sample and line dimensions. By entering the desired values for

• Zoom X In
• Zoom X Out
• Zoom Y In
• Zoom Y Out

For example, if the parameters are set as follows:

• Zoom X In = 3
• Zoom X Out = 1
• Zoom Y In = 3
• Zoom Y Out = 2

Warning: Excessive zooming in may eventually crash xvd, as the memory required to hold a zoomed tile expands beyond system memory limits. Attempts are made to trap out-of-memory errors but these are not successful on all platforms. Because the limit is very system-dependent, it was decided not to place an arbitrary limit on the zoom factor. You should be safe to at least 10x on most systems.

Pseudocolor Dialog

Pseudocolor refers to mapping a monochrome image into a color image. The mapping is determined such that different features of the data set can be distinguished by different colors. Color coding brings out information that is often difficult to detect and interpret in a monochrome image.

Pseudocolor File Menu

You can load your own pseudocolor table and save the created table by using the File menu.

Pseudocolor Options Menu

The Pseudocolor dialog is set by using the Options menu.

There are seven defined color tables to choose from. Table 1 is the default. You can load any of the installed color tables (tables 1-7) by selecting from the menu. Use Pseudocolor Tables enables or disables the pseudocolor table.

Note: For further information about the Pseudo Color View's resources and settings, please refer to the resource help.

Pseudocolor and monochrome modes use an 8 bit visual type even if you have a 24 bit display. The reasoning for this is the speed factor and color flashing (if your hardware only has one 8 bit colormap you may encounter color flashing). Because various windows have different color maps color flashing can occur. This can be eleminated by changing the settings in the Preferences. It is recommended to change the Color Map Policy to Half and the Dither Mode to Ordered. These and other modes are explained in the Preferences help.

In the Pseudocolor window the gray scale along with its corresponding color scale are shown. You set your marks or colortable values by selecting values with the left mouse button on the gray scale. The selected color value is shown in the square box on the right and the exact DN value is in the upper left of the window. The DN values of the selected mark in the red, green and blue planes are also listed above the gray scale.

The green mark is always the current or active mark. To delete a mark, select and drag it off the end of either side of the scale. To create an interval, select a mark with the middle mouse button and drag it to your end point. Clicking between two marks with the middle mouse button will also create an interval. The resulting bar on the gray scale boxes indicates an interval has been created.

The red, green and blue sliders allows you to manipulate the color of the mark or interval.

Once the mark or interval is set and the color has been selected an interpolation mode can be selected.

None

No interpolation. Each value between two selected marks will retain their default value from the initial color table. The selected marks will be set with their new value.

Flat

Flat sets the value between two marks. Each value between the two marks is assigned the left marks value.

Linear

Each value between the two marks will be the result of interpolation of the two marks.

Cubic Spline

Not implemented yet.