SVG images with small dimension detected as zero pixels, ignoring units

[mMerlin]

Discourse uses fastimage as an initial check on image sizes. Fastimage detects svg images that have the "height" or "width" attribute value less than "1" as zero pixels. That ignores the units specified with the attribute value. If it was less than "1px", zero could be a reasonable interpretation. However less than "1in" is considerably different. Especially when the "viewBox" value shows that 1 inch is actually 1000 units. The (current) associated discourse code is quoted here

  def extract_image_info!
    @image_info = FastImage.new(@file) rescue nil
    @file.rewind

    if @image_info.nil?That is
      @upload.errors.add(:base, I18n.t("upload.images.size_not_found"))
    elsif max_image_pixels > 0 && pixels >= max_image_pixels * 2
      @upload.errors.add(:base, I18n.t("upload.images.larger_than_x_megapixels", max_image_megapixels: SiteSetting.max_image_megapixels * 2))
    end
  end

For the problem images, the size_not_found error is displayed, indicating pixels == 0.

Here is a minimal failing example

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg
  xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg"
  width="1.2in" height="0.9in"
  viewBox="0 0 1200 900"
>
  <line x1="1100" x2="100" y1="100" y2="800" stroke="#000000" stroke-width="50"/>
</svg>

github does support uploading svg files, so I renamed to append ".txt" to the name.

image1.svg.txt

Increasing the size and adjusting viewbox to maintain aspect ratio to height="1in" viewBox="0 0 1200 1000" works.

image2.svg.txt

Doubling the height and width to width="2.4in" height="1.8in", then scaling back down in the post works.

image3.svg.txt

Changing the 0.9in height to the equivalent 22.86mm works.

image4.svg.txt

None of those workarounds are helpful for a typical forum user (https://forum.fritzing.org/). The images are associated with an application, where dimensions have meaning beyond the display size. Typical users works with inkscape, without knowing anything about the raw svg xml. Or the image is out of a part file created by other users or application code. The units are typically inches, because the convention is 0.1 spacing for many things.

[SamSaffron]

Looks like we have a fix in Discourse. Not sure if we need to change anything with the gem.

[mMerlin]

Looks like we have a fix in Discourse. Not sure if we need to change anything with the gem.

That would depend on whether anyone else is using fastimage for something similar. The latest information for the Discourse fix is still tentative.

[sdsykes]

Yes, thanks for the detailed description. I do think it would be useful to somehow fix this case in FastImage.