Ubuntu – Preview of CAD files in format .dwg

One workaround is to split the image generation and the PDF conversion. First convert the images via convert to A4@300dpi (i.e. 3508x2479), then use sam2p to convert them to PDF and then use sam2p_pdf_scale to convert them to A4.

convert -rotate "90>" -scale 3508x2479 -border 64x64 -bordercolor white in.png out.png
sam2p out.png out.pdf
sam2p_pdf_scale 595 842 out.pdf

Edit: A more complete script:

#!/bin/sh

A4_WIDTH=2479
A4_HEIGHT=3508

H_MARGIN=64
V_MARGIN=64
WIDTH=$((${A4_WIDTH} - ${H_MARGIN} * 2))
HEIGHT=$((${A4_HEIGHT} - ${V_MARGIN} * 2))

for i in "$@"; do
    TMP="/tmp/$(uuidgen).png"
    echo "$i"
    convert \
        -rotate "90>" \
        -scale "${WIDTH}x${HEIGHT}" \
        -border "${H_MARGIN}x${V_MARGIN}" -bordercolor white \
        -gravity center \
        -extent "${A4_WIDTH}x${A4_HEIGHT}" \
        -gravity center \
        -font helvetica -pointsize 80 \
        -fill white -draw \
        "push graphic-context
         translate $((${A4_WIDTH}/2 - 160)), 0
         rotate 90
         text -2,-2 '$i'
         text -2,2 '$i'
         text 2,-2 '$i'
         text 2,2 '$i'
         pop graphic-context
    " \
        -fill black -draw \
        "push graphic-context
         translate $((${A4_WIDTH}/2 - 160)), 0
         rotate 90
         text 0,0 '$i'
         pop graphic-context
    " \
        "$i" "$TMP"
    sam2p "$TMP" "${i}.pdf"
    sam2p_pdf_scale 595 842 "${i}.pdf"
done

# EOF #

So you are looking for raster to vector graphics converter/tracer. potrace & autotrace both are in Ubuntu repository. Myself I tried potrace before which gave nice results with default options. As I remember, both tools do not support compressed formats as input, only bitmap images.

See Potrace examples

Potrace: utility to transform bitmaps into vector graphics

potrace is a utility for tracing a bitmap, which means, transforming a bitmap into a smooth, scalable image. The input is a bitmap (PBM, PGM, PPM, or BMP format), and the default output is an encapsulated PostScript file (EPS). A typical use is to create EPS files from scanned data, such as company or university logos, handwritten notes, etc. The resulting image is not "jaggy" like a bitmap, but smooth. It can then be rendered at any resolution.

Command:

potrace -s inputfile

AutoTrace: bitmap to vector graphics converter

AutoTrace is a program for converting bitmaps to vector graphics. The aim of the AutoTrace project is the development of a freely-available application similar to CorelTrace or Adobe Streamline. In some aspects it is already better. Originally created as a plugin for the GIMP, AutoTrace is now a standalone program.

Command:

autotrace -output-format svg inputfile

References:

• man potrace
• man autotrace

Update

• There are more GUI that added tracing feature, like Gimp & Inkscape. See Hussein El Motayam's answer.

• Here also Wikipedia: Comparison of raster-to-vector conversion software, not every tool. To anyone interested in comparing results, I would like to ask reporting results to free open source tools so they can ameliorate.