Soft mask fill rules

[roma86]

Can not figure out how to mask a rectangle with a text like on example image.

Rectangle should have transparent area defined by the text.

It feels like i need to combine fill_rule with soft_mask, but i can not find the right way to.

[pointlessone]

Mask is basically an alpha-channel. Black parts are transparent, white parts are fully opaque, and grey parts are semi-transparent.

Here's an example:

Prawn::Document.generate('mask.pdf', page_layout: :landscape) do
  # Drawn the background image spanning the whole page
  canvas do
    image "mountains.jpg", height: bounds.height
  end

  soft_mask do
    canvas do
      # White, we want these parts to be drawn
      fill_color 'ffffff'
      fill_rectangle([100, bounds.top - 100], bounds.width - 200, bounds.height - 200)
      # Black, we want these parts to be transparent
      fill_color '000000'
      font 'Times-Roman', size: 144, style: [:bold] do
        text_box "MASK",
          align: :center, valign: :center,
          at: [100, bounds.top - 100],
          width: bounds.width - 200, height: bounds.height - 200
      end
    end
  end

  # The mask is applied to everything that follows.
  # We just draw a full-page black rectangle. Notice how the parts outside the
  # mask are not drawn.
  canvas do
    fill_color '000000'
    fill_rectangle([0, bounds.top], bounds.width, bounds.height)
  end
end

The result: mask.pdf