Module ZColor
See Also
opals::IZColor

Aim of module

Derives color coded visualizations of grid models and stores them as geo-coded raster images.

General description

Basically, z-coloring is a classification task separating the z range of the grid model into distinct classes and assigning a specific color to each class. Thus, the process can be split up in two parts: the definition of classes and the assignment of color values to each class.

The main instrument for controlling both the class definition and color assignment is the palette file (parameter palFile). Palette files always map the entire z-range from - $\infty$ to + $\infty$, thus, the general form of the palette file is:

Z (index) Color (value) Remark
- $\infty$ color0 underflow color
index1 color1 index1 = index min
index2 color2 ...
... ... ...
indexn colorn overflow color, index n = index max

Basically, the palette file can be seen as a step function mapping z values (index) to colors (value). Thematic maps mainly using irregular class boundaries (e.g. traffic-light-maps indicating good-medium-bad quality, maps of laser scanning strip differences indicating areas with good or bad consistency) can be derived easily that way. No interpolation of intermediate colors is performed as the classes of the output raster file exactly match the classes of the palette file. Another standard application is the derivation of color coded maps using continuous/equidistant classes and smooth color transitions. Therefore, additional parameters are provided to overrule the class division as given in the palette file. Equidistant classes can be defined by specifying a class interval (parameter interval) or a desired number of classes (parameter nClasses). In this case the palette is interpreted as a piecewise linear function and intermediate colors are interpolated in the center of each class to get a look-up-table. This is the intended way for deriving e.g. hypsometric views of the terrain.

By default, the indices of the palette file equal the z-values of the grid model. Additional parameters for scaling (parameter scalePal) and offsetting (parameter offsetPal) the indices are provided. In general, the following scaling options are provided

scalePal offsetPal Description
scale (float) offset (float) The class bounds are calculated using the following formula:
class bound = offset + index * scale
low high (float float) The class bounds are calculated by stretching the palette index domain (index min to index max,) to the given reach (low to high). The same formula as above is applied (scale and offset are calculated internally). Parameter offsetPal is ignored in this case.
auto The class bounds are calculated by stretching the palette index domain (index min to index max,) to the valid z-range of the grid model. The same formula as above is applied (scale and offset are calculated internally). Parameter offsetPal is ignored in this case.
scalePal=auto is normally used together with palettes containing indices ranging from 0 to 100, allowing a percentage interpretation of the palette.

The resulting colour lookup table can be output as a scalable vector graphics (SVG) file together with the z-coded raster map itself (parameter legend). Optionally, the z-range can be clipped using parameter zRange, otherwise the entire range of grid values is used. User defined XY-limits (parameter limits) for the output grid model can either refer to the centers (=default) or corners of the outermost pixels (c.f. parameter limit description). Finally, in case the input and output grid do not coincide, the re-sampling method controlling the (internal) interpolation of input grid pixel z-values can be specified (parameter resampling).

Parameter description

-inFileinput grid model file
Type: opals::Path
Remarks: mandatory
Specifies the input grid model file
-outFileoutput raster file
Type: opals::Path
Remarks: estimable
Color coded raster map file name.
Estimation rule: The current directory and the name (body) of the input file are used as file name basis.
-oFormatoutput file format (e.g. GTiff, PNG, etc.)
Type: opals::String
Remarks: estimable
Output file format (GDAL driver name). Supported formats: GeoTiff, JPEG, PNG, PNM, GIF, BMP, XPM...
Estimation rule: estimated from file extension if not specified
-pixelSizepixel size of output raster image
Type: float
Remarks: estimable
Edge length of one pixel in the resulting raster image.
Estimation rule: default value = model grid size
-intervalclass width=equidistance interval
Type: float
Remarks: optional
Class width=equidistance interval
-nClassesnumber of classes
Type: unsigned int
Remarks: optional
The (equidistant) class interval is calculated from 'nClasses' and 'zRange'. Ignored if 'interval' is specified.
-palFilepalette file name
Type: opals::Path
Remarks: optional
The palette file contains the definition of a background color and a node-color-map.
-legendon / off / legend=legend file name
Type: opals::ZColorLegend
Remarks: default=on
A legend corresponding to the actually used palette will be written to the specified filename with format SVG.
-scalePalscaling factor for palette nodes [auto|scale factor|scale range]
Type: opals::ZColorScalePal
Remarks: optional
If not specified, the min-max palette node range is stretched to the z-range (given by the z-extents or 'zRange'). If two values are specified (low,high), the min-max palette node range is stretched to the given scale range (use , or ; or : as value separator and omit any whitespaces).
-offsetPalpalette node offset
Type: float
Remarks: optional
Constant offset for palette nodes
-zRangerange (zmin, zmax) of z-values considered for output
Type: opals::Array<float,2>
Remarks: optional
Range (zmin zmax) of z-values considered for output. Values outside the given 'zRange' will get the background color. If not specified, the entire z-range of the input grid model is used.
-limit2D clipping window
Type: opals::GridLimit
Remarks: optional
If no user defined limits are specified or -limit is even skipped, the entire xy-extents of the grid model are used.
-resamplingpixel resampling method.
Type: opals::ResamplingMethod
Remarks: optional

Possible values:  
  weightedAverage .... weighted average (low pass filter)
  simpleAverage ...... simple average (low pass filter)
  nearestNeighbour ... nearest neighbour
  bilinear ........... bi-linear interpolation
  bicubic ............ bi-cubic interpolation
  never .............. indicator avoiding resampling 

Method to be used for resampling the input grid to the specified pixelSize of the resulting raster image.

-bandinput band number
Type: unsigned int
Remarks: default=1
To select the desired band of a multi-layer input raster file. Bands are numbered from 1.

Palette File

A palette file is a simple XML file containing an element <OpalsPalette> which itself contain the following elements:

  • <name> Optional
  • <comment> Optional
  • <classes> This optional element has a mandatory attribute val containing the number of classes to be used by default (if neither parameter interval nor parameter nClasses will be specified).
  • <values> This element has a mandatory attribute type which value can be "RGB255" or "RGB".

and 3 to 256 elements

  • <entry> Each of these elements must have the following attributes:

    • val
      The value of this attribute may be "BG", "uf", or a number.
      • The value "BG" represents the background color which will be used for locations where no z value is available within the DTM.
      • The value "uf" represents the underflow value which will be used for z values smaller than zmin.
      • All other values have to be numeric values in ascending order and will be used for all z values equal to the value and smaller than the value of the next palette entry.
      • The palette entry with highest value will be used for all z values equal or higher than this value.
    • R red color component value
    • G green color component value
    • B blue color component value

and an optional attribute comment.

An image with indexed colors can contain no more than 256 colors. As we need a background color (BG) and one for values smaller than zmin (uf) and for values greater than zmax each, we can only have a maximum of 253 classes beetween zmin and zmax. If the number of classes specified by nClasses or calculated from other parameters is larger than 253 the resulting image will be a RGB-image.

Here is a simple example of a palette file:

<?xml version="1.0" encoding="ISO-885999" ?>
<?xml-stylesheet type="text/xsl" href="opalsPaletteStyle.xsl" ?>
<!-- Palette Definition for Opals -->
<OpalsPalette>
<name>Simple Example Palette</name>
<comment>For demonstration only.</comment>
<values type="RGB255" />
<entry val="BG" R="204" G="204" B="204" comment="background color" />
<entry val="uf" R="153" G=" 0" B=" 0" comment="underflow color" />
<entry val=" 0.000" R=" 0" G=" 68" B="144" comment="overflow color, index_max" />
</OpalsPalette>

The first two lines are only necessary to view the palette file with a WEB browser. The third line is a comment.

An OPALS distribution contains the following standard palettes:

  • standardPal.xml: used for terrain elevation codings (green land to brown mountains)

  • differencePal.xml: used for visualizations of (strip) differences (negative differences: blue, positive differences: red)

  • densityPal.xml: used for point density maps (poor density: red, yellow, satisfying density: green)

  • aspectPal.xml: terrain aspect

  • slopePal.xml: terrain slope

  • greyPal.xml: simple black-to-white palette for multiple purposes

  • Additional ColorBrewer palettes: ColorBrewer is providing cartographic color schemes

Examples

The data used in the subsequent examples are located in the $OPALS_ROOT/demo/ directory and the follwing preprocessing steps (data import, surface grid and difference model calculation) are required:

opalsImport -inFile strip11.laz
opalsImport -inFile strip21.laz
opalsImport -inFile fullwave.fwf
opalsGrid -inFile strip11.odm -outFile strip11.tif -interpolation movingPlanes -gridSize 0.5
opalsGrid -inFile strip21.odm -outFile strip21.tif -interpolation movingPlanes -gridSize 0.5
opalsGrid -inFile fullwave.odm -outFile fullwave.tif -interpolation movingPlanes -filter Echo[last]
opalsDiff -inFile strip21.tif strip11.tif -outFile diff_11_21.tif

Example 1: Visualization of vertical strip differences

opalsZColor -inFile diff_11_21.tif -palFile differencePal.xml -scalePal 0.02

This is a typical example, visualizing the height differences of two laserscanning strips (strip19 and strip20) using the predefined palette differencePal.xml (stored in the default palette directory $OPALS_ROOT/addons/pal). Since no output raster file is specified, the default file name (diff_11_21_zco.tif) is constructed automatically and GeoTiff is used as output format. The pixel size is automatically set equal to the grid size of the input grid (diff_11_21.tif) as the parameter pixelSize is not specified.

No parameters for continuous class division are specified, thus, the indices of the palette (scaled by the given factor of 0.02) define the class bounds. Therefore the used class bounds are: -0.10 -0.08 -0.06 -0.04 -0.02 0.00 +0.02 +0.04 +0.06 +0.08 +0.10. Differences smaller +/-2 cm are shown in a light grey, differences between 2-6cm appear in light colours, and for larger differences dark colours are used. Pixels with absolute strip differences greater than 10cm (0.02 * 5) will get the overflow color and underflow color respectively.

zco_stripdiff.png
Fig. 1a: Colour coded visualization of vertical strip differences (strip11/strip21, legend cf Fig. 1b)
zco_stripdiff_legend.png
Fig. 1b: Legend for colours used in Fig. 1a)

Example 2: Hypsometric terrain visualizations

One of the typical applications of ZColor is the derivation of color coded terrain visualizations.

opalsZColor -inFile fullwave.tif -outFile terrain-hypso.tif -pixelSize 0.25 -interval 0.5 -zRange 270 300 -scalePal auto
zco_hypso.png
Fig. 2: Hypsometric terrain visualization

Equidistant 0.5m class intervals are used overruling the class bounds given in the palette file. Automatic stretching of the user defined 270-330m z-range to the index range 09900(%) is performed due to scalePal=auto. Based on the input grid file (fullwave.tif) a hypsometric raster image map (terrain-hypso.tif) is derived in GeoTiff file format (automatically estimated from the .tif extension) with a pixel size of 0.25m. No palette file is specified, therefore the built-in standard palette will be used, which is equivalent to the following palette

<?xml version="1.0" encoding="ISO-8859-1" ?>
<?xml-stylesheet type="text/xsl" href="opalsPaletteStyle.xsl" ?>
<!-- Palette Definition for Opals -->
<OpalsPalette>
<name>Standard Palette</name>
<comment>Green land to mountains</comment>
<classes val="253" />
<values type="RGB255" />
<entry val="BG" R="204" G="204" B="204" comment="background color" />
<entry val="uf" R=" 0" G="255" B="255" comment="underflow color" />
<entry val="0" R=" 0" G="153" B=" 51" comment="index_min" />
<entry val="33" R="153" G="230" B=" 0" />
<entry val="60" R="222" G="222" B=" 31" />
<entry val="100" R="135" G=" 87" B=" 18" comment="overflow color, index_max" />
</OpalsPalette>
Author
B. Wöhrer
Date
26.05.2016