Keywords: bayer, truesense, color filter array

Summary

This demonstration uses an advanced feature in the DIRSIG5 BasicPlatform plugin to simplify the setup of color filter array (CFA) focal planes. In this case, the two currently supported CFA patterns are shown: (1) the classic, 3-color RGB Bayer pattern and (2) the 4-color, Pan+RGB Truesense pattern.

Note
 This feature is not supported by the graphical platform editor. It requires hand-editing the platform file and an open/save cycle in the graphical platform editor will effectively erase the setup.

Details

Platform

The <channelpattern> element inside the <spectralresponse> of each <focalplane>. The name attribute for the <channelpattern> element defines which pattern to use and the elements inside this element define the mapping from an internally defined set of named filters to a set of named channels.

Bayer Pattern

The bayer.platform file contains the classic Bayer filter pattern that was invented by Bryce E. Bayer at Eastman Kodak in 1976 (U.S. patent 3971065) and is commonly used in many consumer electronics. This pattern utilizes a 3-color pattern with red, green and blue filters.

images/bayer_pattern.png
Figure 1. The classic, 3-color Bayer pattern.

Therefore, it expects the user to tell it the names of the channel responses that should be mapped to those three filters. The focal plane consists of a spectral response that defines 3 channels (RED, GREEN and BLUE) and the "channel pattern" that defines the mapping from the internal Bayer pattern filter names to these channel names.

  • Bayer red filter → RED channel

  • Bayer green filter → GREEN channel

  • Bayer blue filter → BLUE channel

Note
 This explicit mapping from logical (internal) filter names to the user-defined channel names allows the user to name channels something more specific or to even utilize different channels than the original filter pattern was intended for (for example, use an "orange" channel in place of the normal "red" channel). 
              <spectralresponse>
                <channellist>
                  <channel name="RED">
                    ...
                  </channel>
                  <channel name="GREEN">
                    ...
                  </channel>
                  <channel name="BLUE">
                    ...
                  </channel>
                </channellist>
                <channelpattern name="bayer">
                  <mapping filtername="red" channelname="RED"/>
                  <mapping filtername="green" channelname="GREEN"/>
                  <mapping filtername="blue" channelname="BLUE"/>
                </channelpattern>
              </spectralresponse>
Note
 The channel names are not important, as long as the names specified when the channels are defined are used in the mappings. For example, the "RED" channel could have been named "R", "red", "red4" or even "COW", "david", etc.

In the example platform files included here, the channels are simple rectangular passband shapes, but measured (tabulated) channels could be used.

Truesense Pattern

The truesense.platform file contains a Truesense filter setup, which was invented by Eastman Kodak in 2010. This pattern utilizes a 4-color pattern with pan, red, green and blue filters.

images/truesense_pattern.png
Figure 2. The 4-color Pan+RGB Truesense pattern.

Similar to the Bayer setup, it expects the user to tell it the names of the channel responses that should be mapped to those four filters. The focal plane consists of a spectral response that defines 4 channels (Pan, Red, Green and Blue) and the "channel pattern" that defines the mapping from the internal Truesense pattern filter names to these channel names.

  • Truesense pan filter → Pan channel

  • Truesense red filter → Red channel

  • Truesense green filter → Green channel

  • Truesense blue filter → Blue channel

Note
 This explicit mapping from logical (internal) filter names to the user-defined channel names allows the user to name channels something more specific or to even utilize different channels than the original filter pattern was intended for (for example, use an "orange" channel in place of the normal "red" channel). 
              <spectralresponse>
                <channellist>
                  <channel name="Pan">
                    ...
                  </channel>
                  <channel name="Red">
                    ...
                  </channel>
                  <channel name="Green">
                    ...
                  </channel>
                  <channel name="Blue">
                    ...
                  </channel>
                </channellist>
                <channelpattern name="truesense">
                  <mapping filtername="pan" channelname="Pan"/>
                  <mapping filtername="red" channelname="Red"/>
                  <mapping filtername="green" channelname="Green"/>
                  <mapping filtername="blue" channelname="Blue"/>
                </channelpattern>
              </spectralresponse>

Scene

For the scene, an RGB map defines a white background with some black text and a set of red, green and blue panels.

maps/map.png
Figure 3. The RGB image used to define the input scene.

Setup

Since the CFA setups in the platform file are not supported by the graphical platform editor, it is suggested you run the simulations from the command line:

  1. Run dirsig5 bayer.sim, which will produce bayer.img image file as the primary output.

  2. Run dirsig5 truesense.sim, which will produce truesense.img image file as the primary output.

Results

The output of the simulation is a raw pixel stream. The ENVI image header indicates that both files are single band images. Although the focal plane measured multiple colors, it did so by using different filters on different pixels. It does not measure all 3 (or 4) colors at every pixel. Hence, the color information is mosaic’ed within the array and a demosaicing algorithm must be employed to create a 3-color RGB image.

images/image_viewer_list.png
Figure 4. The built-in Image Viewer with the single-band, mosaiced output images loaded.

Mosaiced Bayer CFA Image

The image below shows the Bayer CFA raw radiance image produced by DIRSIG in the DIRSIG Image Viewer tool (note the ENVI image header was modified as described above in order to display the raw data in this 320 x 240 format). Notice that the Bayer pattern is visible in the zoom of the color panels.

images/bayer_raw.png
Figure 5. The output image from the Bayer CFA has the RGB colors mosaic’ed into a single channel.

Mosaiced Truesense CFA Image

The image below shows the Truesense CFA raw radiance image produced by DIRSIG in the DIRSIG Image Viewer tool (note the ENVI image header was modified as described above in order to display the raw data in this 320 x 240 format). Notice that the Truesense pattern is visible in the zoom of the color panels.

images/truesense_raw.png
Figure 6. The output image from the Truesense CFA has the Pan+RGB colors mosaic’ed into a single channel.

References