From: Joe Van Dyk <joevandyk@...>
Date: 2005-08-30T06:12:40+09:00
Subject: Re: rmagick question

On 8/29/05, Timothy Hunter <cyclists@nc.rr.com> wrote:
> Joe Van Dyk wrote:
> > On 8/28/05, Timothy Hunter <cyclists@nc.rr.com> wrote:
> >
> >>No matter which approach you take let me know how it goes so I'll be
> >>able to make recommendations to other RMagick users who are working with
> >>very large images. Thanks!
> >
> >
> > Yes, my Ruby program was taking up about 500 MB of memory (for a
> > 8700x6000 pixel image).  Memory's not a problem though, all of our
> > machines have more than 2 gigabytes.
> >
> 
> Okay, first recommendation: have a honkin' great big machine :-)

(in case people forgot, the color values in @tad_data are from 0-255,
so conversion is needed if ImageMagick is using 16 bit color pixels)

The inner code loop looked something like this:

    (height - 1).downto(0) do |n|
      # Code here that displayed percent-done status to user
      width.times do |e|
        # Lookup approach with 16 bit pixel ImageMagick
        #rgb = @tad_data.read(8).unpack(TAD_FORMAT).collect! { |c|
lookup_table[c] }
        
        # Bitshifting approach with 16 bit pixel ImageMagick
        #rgb = @tad_data.read(8).unpack(TAD_FORMAT).collect! { |c| (c
<< 8) | c }

        # 8 bit ImageMagick, no conversion necessary
        rgb = @tad_data.read(8).unpack(TAD_FORMAT)

        pixel = Magick::Pixel.new(*rgb)
        @image.pixel_color(e, n, pixel)
      end
    end

Doing a 10000x10 pixel image (only 10 pixels high for unit testing
purposes), I could process:

13k pixels/second with the bitshifting approach (and 16 bit IM)
13k pixels/second with the array lookup approach (and 16 bit IM)
21k pixels/second with 256 bit IM.

I also found that building up an array of colors for one row and then
doing a @image.import_pixels had no speed improvements and the code
was uglier.