Class: ImageVise::EllipseStencil

Inherits:
Object
  • Object
show all
Defined in:
lib/image_vise/operators/ellipse_stencil.rb

Overview

Applies an elliptic stencil around the entire image. The stencil will fit inside the image boundaries, with about 1 pixel cushion on each side to provide smooth anti-aliased edges. If the input image to be provessed is square, the ellipse will turn into a neat circle.

This adds an alpha channel to the image being processed (and premultiplies the RGB channels by it). This will force the RenderEngine to return the processed image as a PNG in all cases, instead of keeping it in the original format.

The corresponding Pipeline method is ‘ellipse_stencil`.

Instance Method Summary collapse

Instance Method Details

#apply!(magick_image) ⇒ Object 



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# File 'lib/image_vise/operators/ellipse_stencil.rb', line 15

def apply!(magick_image)
  width, height = magick_image.columns, magick_image.rows
  
  # This is a bit involved. We need to do a manual composite. Here is what it entails.
  #
  # Given a premultiplied RGB image B, and a grayscale mask A, we need to do the following
  # operation:
  #
  #    BrBgBb / Ba * (Ba * A)
  #
  # Since ImageMagick works with unpremultiplied alphas, it is doable - but special care
  # must be taken not to overmult or overdivide.
  #
  # To begin,generate a black and white image for the stencil
  mask = Magick::Image.new(width, height)
  draw_circle(mask, width, height)
  
  # At this stage the mask contains a B/W image of the circle, black outside, white inside.
  # Retain the alpha of the original in a separate image
  only_alpha = magick_image.copy
  only_alpha.alpha(Magick::ExtractAlphaChannel)
  mask.composite!(only_alpha, Magick::CenterGravity, Magick::MultiplyCompositeOp)
  
  # With this composite op, enabling alpha on the destination image is
  # not required - it will be enabled automatically.
  # The CopyOpacityCompositeOp implies that we copy the grayscale version
  # of the RGB channels as the alpha channel, so for some weird reason we need
  # to disable the alpha on our mask image
  mask.alpha(Magick::DeactivateAlphaChannel)
  # And perform the operation (set gray(RGB) of mask as the A of magick_image)
  magick_image.composite!(mask, Magick::CenterGravity, Magick::CopyOpacityCompositeOp)
ensure
  [mask, only_alpha].each do |maybe_image|
    ImageVise.destroy(maybe_image)
  end
end

#draw_circle(into_image, width, height) ⇒ Object 



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# File 'lib/image_vise/operators/ellipse_stencil.rb', line 52

def draw_circle(into_image, width, height)
  center_x = (width / 2.0)
  center_y = (height / 2.0)
  # Make sure all the edges are anti-aliased
  radius_width = center_x - 1.5
  radius_height = center_y - 1.5

  gc = Magick::Draw.new
  gc.fill C_black
  gc.rectangle(0, 0, width, height)
  gc.fill C_white
  gc.ellipse(center_x, center_y, radius_width, radius_height, deg_start=0, deg_end=360)
  gc.draw(into_image)
ensure
  ImageVise.destroy(gc)
end