How to put an image on a label using Prawn/labels? - ruby

Question

I'm using Ruby for scripting, and Prawn-labels to print ISBN labels for books, using Barby to generate the EAN-13 bar codes.
The ISBNs come out of MySQL. That works fine. I can create a PDF of 30-up labels containing the titles of the books, using Prawn/labels. That works fine.
However, I am not having much joy putting a Barby-generated PNG image of the proper EAN-13 barcode on the label.
Here's getting the info out of the database and generating EAN-13, pretty straightforward:
query = "SELECT ISBN13, Title FROM s_library WHERE Has_barcode != 'y'"
result = client.query(query, symbolize_keys: true)
tags = []
result.each do |row|
tags << [row[:ISBN13], row[:Title], Barby::EAN13.new(row[:ISBN13].chop)]
end
tags.sort! {|a, b| a[1] <=> b[1]} # Sort by book title
So now I have a two-dimensional Array, with each row containing an ISBN String, a Title String, and a Barby holding the EAN-13 coding for the ISBN (#<Barby::EAN13:0x00007fc6da13ce98 #data="978140006215">
).
Now I'm following the example for Prawn::Labels, and if I leave out trying to #embed_image the PNG data, I can successfully generate 30-up labels with the title of each book:
Prawn::Labels.generate('EAN_tags.pdf', tags, :type => 'Avery5160') do |pdf, tag|
pdf.font '/Library/Fonts/Erasdemi.TTF'
pdf.text(tag[1], :align => :center)
png = Barby::PngOutputter.new(tag[2]).to_png
# pdf.embed_image(png, Prawn::Images::PNG.new(png), {})
end
`open EAN_tags.pdf`
When I un-comment the image embedding, I get a cryptic traceback that I don't quite understand:
25: from /Users/jan/bin/Print_ISBN13.rb:73:in `<main>'
24: from /usr/local/lib/ruby/gems/2.7.0/gems/prawn-labels-1.2.6/lib/prawn/labels.rb:18:in `generate'
23: from /usr/local/lib/ruby/gems/2.7.0/gems/prawn-2.3.0/lib/prawn/document.rb:408:in `render_file'
22: from /usr/local/lib/ruby/gems/2.7.0/gems/prawn-2.3.0/lib/prawn/document.rb:408:in `open'
21: from /usr/local/lib/ruby/gems/2.7.0/gems/prawn-2.3.0/lib/prawn/document.rb:408:in `block in render_file'
20: from /usr/local/lib/ruby/gems/2.7.0/gems/prawn-2.3.0/lib/prawn/document.rb:400:in `render'
19: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/renderer.rb:167:in `render'
18: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/renderer.rb:202:in `render_body'
17: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/document_state.rb:72:in `render_body'
16: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/object_store.rb:70:in `each'
15: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/object_store.rb:70:in `each'
14: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/object_store.rb:71:in `block in each'
13: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/document_state.rb:78:in `block in render_body'
12: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/reference.rb:26:in `object'
11: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/pdf_object.rb:87:in `pdf_object'
10: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/pdf_object.rb:87:in `each'
9: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/pdf_object.rb:93:in `block in pdf_object'
8: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/pdf_object.rb:87:in `pdf_object'
7: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/pdf_object.rb:87:in `each'
6: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/pdf_object.rb:93:in `block in pdf_object'
5: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/pdf_object.rb:87:in `pdf_object'
4: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/pdf_object.rb:87:in `each'
3: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/pdf_object.rb:93:in `block in pdf_object'
2: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/pdf_object.rb:75:in `pdf_object'
1: from /usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/pdf_object.rb:25:in `utf8_to_utf16'
/usr/local/lib/ruby/gems/2.7.0/gems/pdf-core-0.8.1/lib/pdf/core/pdf_object.rb:25:in `encode': "\\x89" to UTF-8 in conversion from ASCII-8BIT to UTF-8 to UTF-16BE (Encoding::UndefinedConversionError)
I can dump the png variable to a file and see the barcode.
From the stack, it looks like it's trying to interpret the PNG data as a UTF-16 String. Ugh. Wasn't expecting that.
Any clues as to what I'm doing wrong?

Answer 1

Use pdf.image(png) instead of pdf.embed_image. This is a wrapper around pdf.embed_image that does the necessary work of getting the necessary data out of the PNG file.

Answer 2

The answer appears to be to use something called StringIO, which apparently provides a File interface to an in-memory (blob) image.
So, I added the middle line, below:
png = Barby::PngOutputter.new(tag[2]).to_png
str_png = StringIO.new(png)
pdf.image(str_png)
instead of trying to pass binary data via embed_image, and I am now getting the barcode in the final PDF output. (It still needs some tweaking, but the basic concept is now working.)
I had searched here many ways — really! But the word "blob" is what got the answer on this site.