RMagick and circle problem

ruby-talk
1

Hello all, i am back with this one.

my problem is that i am trying to draw a line on a circle that will
indicate a certain angle.
kinda like the hands on a clock. I have a line image that i rotate
around and then superimpose upon the circle image that i draw.

The problem is that when the line image is rotated, it is shorter than
the circle.

here is the code.
#!/usr/bin/ruby

require 'RMagick'

def gps_image(r)
  # fetches the image file

  gps_image = Magick::Image.read('line_stop.png').first
  gps_image.background_color = 'none'

  angle = 45
  gps_image.rotate!(angle)

  factor = (r.to_i * 2)
  gps_image.crop_resized!(factor, factor, Magick::CenterGravity)
  gps_image.write("site.site_name.png")

  return gps_image
end

def circle_image
  r = 45
  #start with a bare canvas (square 2 times the radius of circle) the
extra two pixels is to not cut any of the border
  canvas = Magick::Image.new((2 * r) +2, (2*r) +2,
Magick::HatchFill.new('transparent', 'transparent'))

  # draw the circle
  gc = Magick::Draw.new
  gc.stroke('red')
  gc.stroke_width(1)
  # fill in the inside
  color = 'blue'
  gc.fill(color)
  gc.fill_opacity(0.3)
  gc.circle(r, r, 0, r)

  # put our circle on the canvas
  image = gc.draw(canvas)

  # place the line for gps circles
  gps_image = gps_image(r)
  canvas.composite!(gps_image, 0, 0, Magick::OverCompositeOp)

  # write the image out as a file
  canvas.write("circle.png")
end

circle_image()

Everything is ok if the image is 90, 180, 270, or 0 degrees, but the
more i stray toward 45 degrees or so, the smaller the line comes out
on the image.

the image used here ( line_stop.png) is just a transparent image with
a line drawn on it in the middle. ( my clock hand )

thanks for any tips.

shawn

2

shawn bright wrote:

Hello all, i am back with this one.

my problem is that i am trying to draw a line on a circle that will
indicate a certain angle.
kinda like the hands on a clock. I have a line image that i rotate
around and then superimpose upon the circle image that i draw.

The problem is that when the line image is rotated, it is shorter than
the circle.

here is the code.
#!/usr/bin/ruby

require 'RMagick'

def gps_image(r)
  # fetches the image file

  gps_image = Magick::Image.read('line_stop.png').first
  gps_image.background_color = 'none'

  angle = 45
  gps_image.rotate!(angle)

  factor = (r.to_i * 2)
  gps_image.crop_resized!(factor, factor, Magick::CenterGravity)
  gps_image.write("site.site_name.png")

  return gps_image
end

def circle_image
  r = 45
  #start with a bare canvas (square 2 times the radius of circle) the
extra two pixels is to not cut any of the border
  canvas = Magick::Image.new((2 * r) +2, (2*r) +2,
Magick::HatchFill.new('transparent', 'transparent'))

  # draw the circle
  gc = Magick::Draw.new
  gc.stroke('red')
  gc.stroke_width(1)
  # fill in the inside
  color = 'blue'
  gc.fill(color)
  gc.fill_opacity(0.3)
  gc.circle(r, r, 0, r)

  # put our circle on the canvas
  image = gc.draw(canvas)

  # place the line for gps circles
  gps_image = gps_image(r)
  canvas.composite!(gps_image, 0, 0, Magick::OverCompositeOp)

  # write the image out as a file
  canvas.write("circle.png")
end

circle_image()

Everything is ok if the image is 90, 180, 270, or 0 degrees, but the
more i stray toward 45 degrees or so, the smaller the line comes out
on the image.

the image used here ( line_stop.png) is just a transparent image with
a line drawn on it in the middle. ( my clock hand )

thanks for any tips.

shawn

Looks like you're expecting the image produced by rotate! to be the same size as the input image. The rotate! method doesn't just rotate the line, it rotates the entire image. When you rotate the image by some angle that's not a multiple of 90 degrees, the resulting image is going to be wider than the input image. The resize part of crop_resized shrinks the image to fit.

Instead of rotating the clock hand and then compositing the resulting image over the circle image, it might be simpler to use Draw's composite method to composite the clock hand over the circle with a suitable rotation. Since rotation always rotates around the origin, use translate to move the origin to the center of the circle. Here's an example.

require 'RMagick'

HAND_WIDTH = 5

def draw_hand(w)
   img = Magick::Image.new(w, HAND_WIDTH) {self.background_color = "none"}
   gc = Magick::Draw.new
   gc.stroke_linecap "round"
   gc.stroke "blue"
   gc.stroke_width HAND_WIDTH
   gc.line 0, HAND_WIDTH/2.0, w-(HAND_WIDTH*3)/4, HAND_WIDTH/2.0
   gc.draw img
   img
end

hand = draw_hand(100)

def clock(side, hand, angle)
   img = Magick::Image.new(side+5, side+5)
   gc = Magick::Draw.new
   gc.stroke "red"
   gc.fill "none"
   gc.stroke_width 5
   gc.circle side/2+2.5, side/2+2.5, side, side/2
   gc.translate((side+2)/2, (side+2)/2)
   gc.rotate angle
   gc.composite 0, 0, 0, 0, hand
   gc.draw img
   img
end

clock = Magick::ImageList.new
8.times do |n|
   clock << clock(200, hand, n*45)
end
clock.delay = 25
clock.animate

···

--
RMagick: http://rmagick.rubyforge.org/
RMagick 2: http://rmagick.rubyforge.org/rmagick2.html