OpenGL screenshots with ChunkyPNG

gistfile1.rb

# This takes about 1.6 seconds for 800x600
def short_screenshot!(width, height)
  pixels = glReadPixels(0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE)
  image = ChunkyPNG::Image.from_rgb_stream(width, height, StringIO.new(pixels))
  image.flip_horizontally!
  image.save('screenshot.png', :fast_rgb)
end

# This takes about 0.8 seconds for 800x600
def long_screenshot!(width, height)
  pixels = glReadPixels(0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE).bytes.to_a

  image = ChunkyPNG::Image.new(width, height)

  width.times do |x|
    height.times do |y|
      color = pixels.slice((y * width + x) * 3, 3)
      image[x, height.pred - y] = ChunkyPNG::Color.rgb(*color)
    end
  end

  image.save('screenshot.png', :fast_rgb)
end

# This takes about 0.76 seconds, and of that, flipping is 0.6 seconds.
def new_screenshot!(width, height)
  gl_pixels = glReadPixels(0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE)

  File.open('pixels', 'w') { |f| f.write gl_pixels }
  
  # Flip horizontally.
  string = gl_pixels.bytes.each_slice(width * 3).to_a.reverse.flatten.map(&:chr).join
  
  string << ChunkyPNG::EXTRA_BYTE # Add a fourth byte to the last RGB triple.

  unpacker = 'NX' * (width * height)
  pixels = string.unpack(unpacker).map { |color| color | 0x000000ff }
  image = ChunkyPNG::Image.new(width, height, pixels)

  image.save('screenshot.png', :fast_rgb)
end