Showing Partial Icon Sprite

[Pizza Sun]

I can't figure this out.

Suppose I had this sprite:

and this sprite:

How would I code it to display this

using only the sprites above?
I want to show only a partial image and using another sprite as a "window."

 

[Luka S.J.]

There is no way for you to do this by default in neither RMXP itself, or Essentials. You would have to code an entirely new function that would compare the alpha values of two different bitmaps, and use those to "cut" out one from the other. Luckily for you, I required this for my own stuff, and wrote a function to integrate into the Sprite class.

#===============================================================================
# ** Sprite Mask
#        by Luka S.J.
#-------------------------------------------------------------------------------
# Allows you to use a bitmap, sprite or image path to a bitmap to use as a mask
# for a sprite's original bitmap.
#===============================================================================
# handled as an extension of the Sprite class
class Sprite
  # function used to to mask the Sprite's current bitmap with another
  def mask(mask = nil,xpush = 0,ypush = 0)
    # exits out of the function if the sprite currently has no bitmap to mask
    return false if !self.bitmap
    # backs up the Sprites current bitmap
    bitmap = self.bitmap.clone
    # check for mask types
    if mask.is_a?(Bitmap) # accepts Bitmap.new
      mbmp = mask
    elsif mask.is_a?(Sprite) # accepts other Sprite.new
      mbmp = mask.bitmap
    elsif mask.is_a?(String) # accepts Strings
      mbmp = BitmapCache.load_bitmap(mask)
    else # exits if non-matching type
      return false
    end
    # creates a new bitmap
    self.bitmap = Bitmap.new(mbmp.width, mbmp.height)
    # creates the main mask
    mask = mbmp.clone
    # calculates the dimension metrics for pixel transfer
    ox = (bitmap.width - mbmp.width) / 2
    oy = (bitmap.height - mbmp.height) / 2
    width = mbmp.width + ox
    height = mbmp.height + oy
    # draws pixels to mask bitmap
    for y in oy...height
      for x in ox...width
        # gets pixel of mask for analysis
        pixel = mask.get_pixel(x - ox, y - oy)
        # gets pixel of current bitmap for analysis
        color = bitmap.get_pixel(x - xpush, y - ypush)
        # sets the new alpha to use the value of the mask alpha
        alpha = pixel.alpha
        alpha = color.alpha if color.alpha < pixel.alpha
        # draws new pixels onto the Sprite's bitmap
        self.bitmap.set_pixel(x - ox, y - oy, Color.new(color.red, color.green,color.blue, alpha))
      end
    end
    # returns finalized bitmap to be used elsewhere
    return self.bitmap
  end
end

This piece of code is very simple to use (and I've gone ahead and written plenty of comments, so that you can see what part does exactly what), and an example would look something like this:

sprite = Sprite.new(viewport)
sprite.bitmap = Bitmap.new("main_sprite.png")
sprite.mask("mask_sprite.png")

And just for the sake of illustrating this even further, here's a visual representation of what a Sprite being masked with this function would yield:

EDIT: This is a resource intensive task/function. So I wouldn't recommend using this in conjuncture with large sprites/masks (and you especially shouldn't be using this in some main scene loops), as that will probably cause your performance to drop.

 

[Pizza Sun]

Thank you very much! I appreciate the help.