Get to Know a Feature: Color Table Wave Creation

In "Get to Know a Feature: Color Table Wave Basics", we showed how Igor Pro 7's color table waves are more flexible than Igor Pro 6's color index waves because multiple uses of the same color table wave can cover differing Z value ranges.

We previously showed that you can use Data→Packages→Color Wave Editor to create color table waves by modifying a copy of a built-in color table, or by directly choosing the color for each row of a color wave.

COLOR TABLE WAVE VALUES

Color table (and color index) waves have one row for each color, and 3 or 4 columns for red, green, blue and optionally alpha components containing values that range from 0-65535.

It is useful to realize that any color index wave can also be used as a color table wave; the difference is simply that Igor 7 ignores the X scaling when using a wave as a color table wave.

Here are some other ways to create color table waves:

IMPORTING RGB VALUES FROM A CSV FILE

Sometimes a researcher is kind enough to provide a data file containing the red, green, blue values in a data file, which we can import and rescale to Igor's color range.

For example, from Color Schemes Appropriate for Scientific Data Graphics, you can download a "color number text file", such as this Blue-to-Green 14-step table:

You could just copy the values from the text file and paste them into an Igor table, but for larger color tables that would be too painful, and what are computers for, anyway?

Let's load the file (from the Clipboard in my case) as delimited text into a "color0" matrix wave:

The last three columns are the Red, Green, and Blue components.

An easy way to make a color wave is to Duplicate columns 11, 12, and 13 into a new wave, which we will name "BlueGreen", using Igor 7's handy new /RMD flag for the Duplicate operation:

Igor uses a range of 0-65535 for colors, instead of the 0-255 used here.

It is not obvious, but multiplying by 257 (not the 256 you might expect) scales to Igor's 0-65535 rather nicely:

Let's convert the wave's numeric type to unsigned 16-bit integer and then multiply by 257.

It wouldn't hurt to reset the Y scaling to match the column number, and even set dimension labels just for clarity:

This is what this color table wave looks like in an image plot and ColorScale:

Which compares nicely to the original:

RECREATING A COLOR TABLE FROM A SCREEN CAPTURE

A captured screen shot can be used to reproduce a color table when the numeric values aren't available.

Here's an example of reproducing the many-valued "Ametrine" color table from École Polytechnique Fédérale de Lausanne:

Screen capture of the Ametrine color table

Loading this screen capture as an Igor RGB Image plot is straightfoward; use a screen capture utility to save a .TIFF or .PNG file, and load it as an image.

STEP 1: SAVE A SCREEN CAPTURE TO A FILE

On Macintosh, the built-in screen capture to a file is done by pressing Cmd+Shift+4, using the mouse to drag out a marquee around the to-be-captured element.

Upon release of the mouse button, a .tiff file is saved to the desktop (some versions of Mac OS X save a .png file).

STEP 2: LOAD THE FILE AS AN IMAGE PLOT

Use Igor 7's Data->Load Image dialog and load the TIFF or PNG screen shot file as an RGB image:

STEP 3: EXTRACT THE COLOR TABLE WAVE FROM THE IMAGE PLOT

We want only a portion of the image plot's values for our color table. We can use the marquee to indicate where the colors are, and then create a GraphMarquee menu definition and associated code to extract the colors from that region:

(GraphMarquee code for extracting the color table values, ready to be copied and pasted into an Igor procedure file, follows at the end of this article.)

The resulting color table wave is displayed in an image plot so that you can check that no unintended pixels crept into the color table wave. Adjusting the marquee and selecting Color Table from Selection again updates the same color table wave and image plot.

COMPARING THE EXTRACTED THE COLOR TABLE WAVE WITH IDEAL VALUES

We can check the results by downloading the corresponding csv file:

•LoadWave/J/M/A=ametrine/P=text/K=0 "ametrine.txt"
  Delimited text load from "ametrine.txt"
  Matrix size: (256,3), wave: ametrine0

Then we can scale the ametrine0 values from 0-255 to 0-65535 and apply the resulting color table wave to a copy of the image plot with the extracted color table wave for comparison:

Pretty tough to tell the difference:

REUSING THE EXTRACTED THE COLOR TABLE WAVE

A good way to make the color table wave available to multiple experiments is to save the color table wave as an Igor Binary wave in an easily-accessed folder.

Simply load the binary wave and choose it in the Modify Image Appearance dialog.

Whether you copy the binary wave into your experiment file or leave it "shared" (saved separate from the experiment on disk) is up to you.

Igor 7.01 or later will ship with many color table waves in a Color Table Waves folder for convenience. They have been created using the sources listed below.

REFERENCES

• Color Map Advice for Scientific Visualization
• Colormaps - Data Science at Scale
• How to display data by color schemes compatible with red-green color perception deficiencies, by Matthias Geissbuehler and Theo Lasser
• Color Schemes Appropriate for Scientific Data Graphics
• Matplotlib: show colormaps

COLOR TABLE FROM SELECTION CODE