Difficulty transferring prawn pdf image to frontend

Question

I am streaming a prawn PDF to the frontend of my application, but it shows the image as 100% black. The image is a gif file that I convert to png using rmagick. If I save the pdf file on the backend, it looks correct. I have tried other png files instead, and they render properly. What aspects of the png file could cause it to work fine on the backend, but show up as a black rectangle on the frontend?

Backend code:

png_blobs = []
      gif_blobs.each do |gif_blob|
        gif = ImageList.new
        # gif_blob is streamed from UPS' API
        gif = gif.from_blob(Base64.decode64(gif_blob))
        gif.crop!(0, 0, 1202, 800) # get rid of whitespace that comes with UPS label
        gif.rotate!(-90.0)
        gif.format = 'png'
        gif.profile!("8bim",nil)
        blob = gif.to_blob
        png_blobs.push(StringIO.open(blob)) # I have tried using the blob and the file
        gif.write('/tmp/myfile.png'){ |options| options.quality = 50 }
        # png_blobs.push('/tmp/myfile.png')
      end

Prawn::Document.new(page_size: [386, 578], margin: 0) do |pdf|
        pdf.define_grid(columns: 38, rows: 57, gutter: 0)
        png_blobs.each_with_index do |gif_png_blob, idx|
          pdf.grid([0, 0], [57, 38]).bounding_box do
            pdf.image gif_png_blob, scale: 0.2
            # Same problem with this attempt below, except it turns 100% gray due to transparency
            # pdf.transparent(0.5) do
            #   pdf.image '/tmp/myfile.png', width: 386, scale: 0.48
            # end
            # This actually works, but it is a completely different png file
            pdf.image Rails.root.join('app/assets', 'images', 'ups_barcode.png'), scale: 0.2
          end
          pdf.start_new_page if idx < png_blobs.length - 1
        end

        pdf.render_file('/tmp/myfile.pdf') # This looks good
        # this shows a black rectangle on the frontend
        body['ShipmentResponse']['ShipmentResults']['PackageResults']['ShippingLabel'] = Base64.encode64(pdf.render)
end

Answer 1

Using send_data as opposed to render (along with other json) in rails seemed to fix this problem. That said, all of my other images and pdfs that didn't use ImageMagick were fine in the render function.