Optimizing Image Cropping and Writing Performance with Ruby Vips

[JohnAnon9771]

Hi,

I’m working on a project that uses Ruby Vips to crop and adjust images for printing. However, I’ve encountered performance issues, specifically with the speed of writing the cropped image to a file. The writing process is quite slow, and I’d like to know how I can optimize this further to speed up the entire operation, as I need to transmit the processed image as quickly as possible.

Here’s the relevant code I’m using for cropping the image:

class ImageCropVips
  def self.crop(image_path)
    img = Vips::Image.new_from_file(image_path)
    bbox = Trimer.find_trim(img)
    dpi = (img.xres > 0 ? img.xres : 11.81) * 25.4

    puts "\n VIPS bbox: #{bbox}, dpi: #{dpi} \n"

    if bbox
      tmp = Tempfile.new(["tmp", '.png'])
      img.crop(*bbox).write_to_file(tmp.path, Q: 100)
    else
      nil
    end
  end
end

class Trimer
  def self.find_trim(image)
    return [0, 0, image.width, image.height] if image.bands < 4

    alpha_channel = image.extract_band(3)
    _, first_row = alpha_channel.profile
    left = first_row.min

    _, first_row = alpha_channel.fliphor.profile
    right = image.width - first_row.min

    first_column, = alpha_channel.profile
    top = first_column.min

    first_column, = alpha_channel.flipver.profile
    bottom = image.height - first_column.min

    width = right - left
    height = bottom - top

    [left, top, width, height]
  end
end

The trimming works fine, but the file writing step (write_to_file) takes a significant amount of time. Do you have any suggestions for speeding up the writing process or improving overall performance? Any insights would be greatly appreciated!

Thank you!

[jcupitt]

Hi @JohnAnon9771,

find_trim is built into libvips, and you can use [n] for band extract. You can probably write (untested):

    img = Vips::Image.new_from_file(image_path)
    bbox = img[3].find_trim
    img.crop(*bbox).write_to_file("x.png")

How large are these files? PNG is a very slow format, so if the images are large, write will take a long time. Are there any other formats you could use? webp or tiff would probably be quicker.

[JohnAnon9771]

Yes, they are 4200x4800 png images, with an average size of 10mb. Unfortunately, I can only use png, as the image will be sent to a printer that only accepts png.

[jcupitt]

Ah, shame. Yes, it'll be slow:

$ time vips crop wtc.jpg x.png 0 0 4200 4800 

real	0m1.755s
user	0m1.883s
sys	0m0.191s

So 1.75s just for a 4800 x 4200 PNG write on this Ubuntu 24.04 PC. PNG write is single-threaded, sadly.

Most PNG write time is spent in libz, so it might be worth experimenting with other versions. It's such a critical library that many people have attempted faster implementations, for example libz-ng:

https://github.com/zlib-ng/zlib-ng

You can also disable PNG zip compression. This makes it a lot quicker, though files will be larger:

$ time vips crop wtc.jpg x.png[compression=0] 0 0 4200 4800 

real	0m0.434s
user	0m0.328s
sys	0m0.279s
$ ls -l x.png x0.png 
-rw-r--r-- 1 john john 60584302 Oct 18 14:03 x0.png
-rw-r--r-- 1 john john 46732001 Oct 18 14:03 x.png

600k vs. 470k.

[jcupitt]

Or tiff with zlib compression:

$ time vips crop wtc.jpg x.tif[tile,compression=deflate] 0 0 4200 4800 

real	0m1.009s
user	0m1.128s
sys	0m0.194s
$ ls -l x.tif
-rw-r--r-- 1 john john 37114608 Oct 18 14:07 x.tif

Almost twice as fast as PNG, with much better compression. PNG really is a poorly designed format.

[JohnAnon9771]

Thank you very much for the super detailed answer! I'll try to do it without compression, it really did increase the speed a lot. One question: does zlib-ng work automatically with libvips? How can I use it?

[jcupitt]

You might need to rebuild libpng, or libspng. You'd have to experiment.