# soft_mask.rb : Implements soft-masking
#
# Copyright September 2012, Alexander Mankuta. All Rights Reserved.
#
# This is free software. Please see the LICENSE and COPYING files for details.
#

module Prawn
  # The Prawn::SoftMask module is used to create arbitrary transparency in
  # document. Using a soft mask allows creating more visually rich documents.
  #
  # You must group soft mask and graphics it's applied to under
  # save_graphics_state because soft mask is a part of graphic state in PDF.
  #
  # Example:
  #   pdf.save_graphics_state do
  #     pdf.soft_mask do
  #       pdf.fill_color "444444"
  #       pdf.fill_polygon [0, 40], [60, 10], [120, 40], [60, 68]
  #     end
  #     pdf.fill_color '000000'
  #     pdf.fill_rectangle [0, 50], 120, 68
  #   end
  #
  module SoftMask
    # @group Stable API

    def soft_mask(&block)
      renderer.min_version(1.4)

      group_attrs = ref!(
        Type: :Group,
        S: :Transparency,
        CS: :DeviceRGB,
        I: false,
        K: false
      )

      group = ref!(
        Type: :XObject,
        Subtype: :Form,
        BBox: state.page.dimensions,
        Group: group_attrs
      )

      state.page.stamp_stream(group, &block)

      mask = ref!(
        Type: :Mask,
        S: :Luminosity,
        G: group
      )

      g_state = ref!(
        Type: :ExtGState,
        SMask: mask,

        AIS: false,
        BM: :Normal,
        OP: false,
        op: false,
        OPM: 1,
        SA: true
      )

      registry_key = {
        bbox: state.page.dimensions,
        mask: [group.stream.filters.normalized, group.stream.filtered_stream],
        page: state.page_count
      }.hash

      if soft_mask_registry[registry_key]
        renderer.add_content "/#{soft_mask_registry[registry_key]} gs"
      else
        masks = page.resources[:ExtGState] ||= {}
        id = masks.empty? ? 'GS1' : masks.keys.sort.last.succ
        masks[id] = g_state

        soft_mask_registry[registry_key] = id

        renderer.add_content "/#{id} gs"
      end
    end

    private

    def soft_mask_registry
      @soft_mask_registry ||= {}
    end
  end
end
