```ruby
#! /usr/local/bin/ruby -w
require 'RMagick'

canvas = Magick::Image.new(260, 125)
gc = Magick::Draw.new
gc.fill('black')
gc.rectangle(10,20, 250,90)

gc.stroke('blue')
gc.fill('yellow')
gc.stroke_width(10)

gc.opacity('25%')
gc.roundrectangle(20,20, 60,90, 5,5)

gc.opacity('50%')
gc.roundrectangle(80,20, 120,90, 5,5)

gc.opacity(0.75)
gc.roundrectangle(140,20, 180,90, 5,5)

gc.opacity(1)
gc.roundrectangle(200,20, 240,90, 5,5)

gc.font_weight(Magick::NormalWeight)
gc.font_style(Magick::NormalStyle)
gc.stroke('transparent')
gc.fill('black')
gc.gravity(Magick::SouthGravity)
gc.text(-90,15, '"25%"')
gc.text(-30,15, '"50%"')
gc.text( 30,15, '"75%"')
gc.text( 90,15, '"100%"')

gc.draw(canvas)

canvas.write("opacity.gif")
exit
```

Reset matte channel for better rendering
user
```ruby
#! /usr/local/bin/ruby -w
require 'RMagick'

canvas = Magick::Image.new(260, 125)
gc = Magick::Draw.new
gc.fill('black')
gc.rectangle(10,20, 250,90)

gc.stroke('blue')
gc.fill('yellow')
gc.stroke_width(10)

gc.opacity('25%')
gc.roundrectangle(20,20, 60,90, 5,5)

gc.opacity('50%')
gc.roundrectangle(80,20, 120,90, 5,5)

gc.opacity(0.75)
gc.roundrectangle(140,20, 180,90, 5,5)

gc.opacity(1)
gc.roundrectangle(200,20, 240,90, 5,5)

gc.font_weight(Magick::NormalWeight)
gc.font_style(Magick::NormalStyle)
gc.stroke('transparent')
gc.fill('black')
gc.gravity(Magick::SouthGravity)
gc.text(-90,15, '"25%"')
gc.text(-30,15, '"50%"')
gc.text( 30,15, '"75%"')
gc.text( 90,15, '"100%"')

gc.draw(canvas)
canvas.matte = false
canvas.write("opacity.gif")
exit
```
assistant