From: Timothy Hunter <cyclists@...>
Date: 2005-08-31T07:31:23+09:00
Subject: Re: rmagick question

Joe Van Dyk wrote:
> On 8/29/05, Joe Van Dyk <joevandyk@gmail.com> wrote:
> 
>>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.
> 
> 
> Hm... I don't think that those pixels/second numbers are correct.  I
> was doing the timings inside a Benchmark.measure { ... } block and
> perhaps that makes things slower?
> 
> I was able to process a 8700x6000 pixel image when using 'time ./script.rb'
> 440.005u 9.917s 8:27.20 88.7%       0+0k 0+0io 52012pf+0w
> 
> So, around 7 minutes for 5 million pixels.. that's around 118000
> pixels per second.  Hm.  Oh well, it's fast enough for me.  This
> operation isn't done all that often.   Although, a while ago I did
> something like this in C with the GD library, and it took around 10-20
> seconds to process a similar sized image.
> 
> 

Joe, what is the format of the pixel data in your input file? Is it just 
8-bit R,G,B values in top-down scanline order?