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So I was checking on how to display PDF thumbnails in Rails since for some reason creating a thumbnail version of my file in my uploader doesn't work, and it lead me to this: Convert a .doc or .pdf to an image and display a thumbnail in Ruby?

So I got up to this:

def show_thumbnail
    require 'rmagick'
    pdf = Magick::ImageList.new(self.pdf_file.file.path)
    first_page = pdf.first
    scaled_page = first_page.scale(300, 450)
end

But how do I display scaled_page to a webpage?

I added this function in the decorator so I can do something like this:

= image_tag(pdf.pdf_file.show_thumbnail)

But it results in this error:

Can't resolve image into URL: undefined method `to_model' for #<Magick::Image:0x0000000122c4b300>
IBG
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1 Answers1

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To display the image, the browser only need a URL to the image. If you don't want to store the image on your HDD, you can encode the image into a data URL.

...
scaled_page = first_page.scale(300, 450)

# Set the image format to png so that we can quantize it to reduce its size
scaled_page.format('png')

# Quantize the image
scaled_page = scaled_page.quantize

# A blob is just a binary string
blob = scaled_page.to_blob

# Base64 encode the blob and remove all line feeds
base64 = Base64.encode64(blob).tr("\n", "")

data_url = "data:image/png;base64,#{base64}"

# You need to find a way to send the data URL to the browser,
# e.g. `<%= image_tag data_url %>`

But I highly recommend that you persist the thumbnails on your HDD or better on a CDN because such images are hard to generate but are frequently accessed by the browsers. If you decided to do so, you need a strategy to generate unique URLs to those thumbnails and a way to associate the URLs to your PDF files.

Aetherus
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  • Thanks for the answer. I've managed to make it work using versions in the uploader (been working with the WRONG uploader file ughh) so the file persists now. I hope someone else finds this answer handy. – IBG Apr 30 '20 at 09:09