16.33. image_calc

This tool can be used to perform simple, per-pixel arithmetic on one or more input images. An arithmetic operation specified on the command line is parsed and applied to each pixel, then the result is written to disk. The tool supports multiple input images but each must be the same size and data type. Input images are restricted to one channel (band). For images with more than one channel, only the first channel will be read.

The following symbols are allowed in the arithmetic string: +, -, *, /, (), min(), max(), pow(), abs(), sign(), and var_N, where N is the index of one of the input images (N\(\ge\)0). The tool also supports certain conditional operations: lt, gt, lte, gte, eq (<, >, <=, >=, == respectively). These must be used in a format like “lt(var_0, 0.003, var_1, 0)”, which translates to “if var_0 < 0.003 then var_1 else 0”.

An example arithmetic string to be passed via -c is:

-abs(var_0) + min(58, var_1, var_2) / 2

The tool respects the normal PEMDAS order of operations except that it parses equal priority operations from right to left, not the expected left to right. Parentheses can be used to enforce any preferred order of evaluation.

16.33.1. Examples

16.33.1.1. Apply operation and save pixels as float32

image_calc -c "pow(var_0/3.0, 1.1)" input_image.tif \
 -o output_image.tif -d float32

16.33.1.2. Invalidate values no more than a threshold

thresh=5.2
image_calc -c "max($thresh, var_0)" -d float32 \
    --output-nodata-value $thresh              \
    input.tif -o output.tif

16.33.1.3. Apply a mask to an image

image_calc -c "var_0 * var_1" -d float32 \
    --output-nodata-value 0              \
    input.tif mask.tif -o output.tif

Here it is assumed that the image and the mask have the same dimensions, the mask has value 1 for pixels to keep and 0 for pixels to discard, and that the output pixels with value 0 are invalid.

16.33.1.4. Create a mask

image_calc -c "sign(max(var_0, 0))" -d float32 \
    input.tif -o output.tif

Positive values will become 1, and the rest will become 0.

16.33.1.5. Create an image with random values

image_calc -c "rand(var_0)" -d float32 \
    input.tif -o output.tif

The produced values will be between 0 and 1. Other operations can be combined with this one. For example, one could add a random value multiplied by a constant to the input image.

16.33.1.6. Add a value to the geoheader metadata

image_calc -c "var_0" --mo 'VAR1=VAL1' -d float32 \
  input.tif -o output.tif

If this variable already exists, its value will be overwritten. Other existing variables will be preserved. Use gdalinfo to view the metadata.

16.33.1.7. Subtract 360 degrees from the longitudes in a GeoTiff file

image_calc -c "var_0" input.tif -o output.tif \
  --longitude-offset -360 -d float32

16.33.2. Usage

image_calc [options] -c <arithmetic formula> <inputs> -o <output>

16.33.3. Command-line options

-c, --calc <string>

The arithmetic string in quotes (required).

-d, --output-data-type <type (default: float64)>

The data type of the output file. Options: uint8, uint16, uint32, int16, int32, float32, float64.

--input-nodata-value <double>

Set the nodata value for the input images, overriding the value in the images, if present.

--output-nodata-value <double>

Manually specify a nodata value for the output image. By default it is read from the first input which has it, or, if missing, it is set to data type min.

-o, --output-file <string>

Specify the output file instead of using a default.

--mo <string>

Write metadata to the output file. Provide as a string in quotes if more than one item, separated by a space, such as 'VAR1=VALUE1 VAR2=VALUE2'. Neither the variable names nor the values should contain spaces.

--longitude-offset <double (default: not specified)>

Add this value to the longitudes in the geoheader (can be used to offset the longitudes by 360 degrees).

--no-georef

Remove any georeference information (useful with subsequent GDAL-based processing).

--threads <integer (default: 0)>

Select the number of threads to use for each process. If 0, use the value in ~/.vwrc.

--cache-size-mb <integer (default = 1024)>

Set the system cache size, in MB.

--tile-size <integer (default: 256 256)>

Image tile size used for multi-threaded processing.

--no-bigtiff

Tell GDAL to not create bigtiffs.

--tif-compress <None|LZW|Deflate|Packbits (default: LZW)>

TIFF compression method.

-v, --version

Display the version of software.

-h, --help

Display this help message.