The Canvas and its optional views

Most nodes and inputs use a canvas to display some bands of an image as RGB. This will take up a large proportion of their view - in some cases all of it. It is worth discussing in some detail. A canvas is shown below as the entire control area for an input node, which brings one of the four inputs into the graph.

You can pan the canvas using the scroll bars at the edge, and zoom with the mouse wheel. The button at bottom right will reset to show the entire image.

To the left of the canvas image itself is the canvas control area. This has quite a lot in it, so is scrollable with collapsible sections, rather like the palette.

At the top - and not collapsible - are three combo boxes which determine the image mapping : how the bands within the (probably) multispectral image map onto RGB channels for viewing. Each band in the combo box shows the input number (i.e. which of the four global inputs the data is sourced from), a colon, and typically the name, position or wavelength of the band. Exactly what is shown depends on the image being loaded and the Caption global control. The Guess RGB button tries to guess appropriate channels for the RGB canvas image.

Data section

This is the first of the collapsible sections.

• Show ROIs will mark any regions of interest the image has - these are added using nodes like rect (for a rectangle) and are carried forward through subsequent nodes (where appropriate), delimiting the area upon which calculations are performed. They also control the regions used for calculating spectra. Normally an ROI is only shown in the node which adds it.
• Show spectrum opens a side pane, and dragging the cursor across the image will plot the spectrum of the pixel under the cursor in this pane. If no filter wavelengths are available, a list of the values for each band is shown. The show spectrum pane looks like this:

  The screenshot isn't showing the cursor, unfortunately, but the spectrum is for the pixel under the cursor.

When a spectrum view is opened the image pane can be tiny - to fix this you can resize the PCOT window (or undocked node window), or drag the separator between the image and the spectrum (the two vertical bars). Each dot is shown with an approximation of its wavelength colour (using Dan Bruton's algorithm) or black if the wavelength is not visible.

• Export image saves the RGB-mapped image as a PDF, SVG or PNG. If a gradient has been plotted using the gradient node, it may add a legend to the image.
• Cursor coordinates are shown next, followed by number of pixels and ROIs in the image and the image dimensions (width x height x bands).
• The hide DQ disables the data quality overlays and hides their sections - data quality can be slow to draw.

Normalisation section

Nominally, the canvas channels are 0-1 with 0 being zero intensity and 1 being full intensity (so RGB 1,1,1 is white). Values outside the 0-1 range are clamped. However, this can be changed.

• The norm box selects the normalisation range:
  • to all bands means that the normalisation range of each band for each pixel is the minimum and maximum of all the bands as a whole.
  • to RGB means that the normalisation range is taken from the channels mapped to RGB in the canvas.
  • independent means that each band is normalised independently, to its own range.
  • none means no normalisation is done.
• If to cropped area is on, the range used for normalisation is taken from the portion of the image visible in the canvas.

Data quality layers

Each image has a set of data quality bits and an uncertainty value associated with each pixel of each band. Viewing this can be challenging. We make three "layers" of DQ data available: each works the same way:

• SRC specifies which band we are viewing the DQ or uncertainty for. If max is specified and we are viewing uncertainty, the maximum uncertainty across all bands is used. If sum, then the sum of the uncertainties is used. If a DQ bit is being shown, the intersection of those bits across all bands is used for both max and sum.
• DATA specifies what data is being shown:
  • NONE specifies that the DQ layer is inactive.
  • BIT:name options specify a particular DQ bit. These are subject to change, but are likely to include nodata, nounc (no uncertainty data), sat (saturated).
  • UNC specifies that the uncertainty data should be shown.
  • UNC>THRESH specifies that the layer should be full intensity for pixels where the uncertainty is above a threshold (see below).
  • UNC<THRESH specifies that the layer should be full intensity for pixels where the uncertainty is below a threshold (see below).
• COL specifies the colour for the DQ layer.
• transp indicates the transparency of the layer
• contrast may help improve visibility (it is a simple power function)
• thresh is the threshold for the threshold DATA modes
• additive indicates that the data quality layers should be added to the RGB image rather than blended.