Module: GDAL::RasterBand::AlgorithmExtensions

Included in:
GDAL::RasterBand
Defined in:
lib/gdal/extensions/raster_band/algorithm_extensions.rb

Overview

RasterBand methods for doing warp-like things, but not using GDAL’s Warp API.

Instance Method Summary collapse

Instance Method Details

#simple_erase! ⇒ Integer

A method that doesn’t use GDAL’s Warp API for erasing pixels (setting to NODATA) in the method-owning RasterBand. It simply iterates over all of the valued pixels (ones that aren’t NODATA pixels) and yields them to a block. The return value of the block tells the method whether to keep the pixel or not: a truthy return value keeps the pixel; a false value sets the pixel value to the band’s NODATA value. If there’s no NODATA value set, it will raise an exception.

A typical clip operation would be from checking to see if the RasterBand’s pixel is within some Polygon. Doing so would look like:

You could, however use any other criteria in the block for erasing points away. The method also the pixel value in case you can use it for erasing. Perhaps, for example, you want to remove all pixels in the upper left quadrant of your raster that have a value less than 0. That would look like:

Examples:

Clipping by polygon


contrived_wkt = 'POLYGON ((0 0, 0 20, 20 20, 20 0, 0 0))'
polygon = OGR::Geometry.create_from_wkt(contrived_wkt)

# Note the 'w' to open for writing!
dataset = GDAL::Dataset.open('my.tif', 'w')
geo_transform = dataset.geo_transform
test_point = OGR::Point.new

dataset.raster_band(1).simple_erase! do |pixel_num, line_num|
  # Project the pixel using the dataset's GeoTransform
  projected_point_values = geo_transform.apply_geo_transform(pixel_num, line_num)
  test_point.set_point(projected_point_values[:x_geo], projected_point_values[:y_geo])

  test_point.within? polygon
end

# Make sure to close the dataset so the changes get flushed to disk.
dataset.close

Erasing using pixel location and values


dataset = GDAL::Dataset.open('my.tif', 'w')
raster_band = dataset.raster_band(1)

raster_band.simple_erase! do |x, y, value|
  if x < (raster_band.x_size / 2) && y < (raster_band.y_size / 2) && value.negative?
    false
  else
    true
  end
end

Returns:

  • (Integer) —

    The number of pixels that were erased.



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# File 'lib/gdal/extensions/raster_band/algorithm_extensions.rb', line 62

def simple_erase!
  erase_value = no_data_value[:value]
  write_buffer = GDAL._buffer_from_data_type(data_type, x_size)
  erased_pixel_count = 0

  unless erase_value
    raise GDAL::NoRasterEraseValue,
          "Cannot erase values, RasterBand does not have a NODATA value set"
  end

  y_size.times do |line_num|
    pixel_row = raster_io("r", x_size: x_size, y_size: 1, x_offset: 0, y_offset: line_num)
    pixel_values = GDAL._read_pointer(pixel_row, data_type, x_size)
    row_changed = false
    pixel_num = 0

    while pixel_num < pixel_values.length
      pixel_value = pixel_values[pixel_num]
      next if pixel_value == no_data_value[:value]

      keep_in_raster = yield(pixel_num, line_num, pixel_value)
      next if keep_in_raster

      erased_pixel_count += 1
      pixel_values[pixel_num] = erase_value
      row_changed = true
      pixel_num += 1
    end

    rewrite_pixel_row(write_buffer, pixel_values, line_num) if row_changed
  end

  erased_pixel_count
end