Find the settings that change scan file size by a factor of sixteen

short · 35 min · Objective 3.3

Task

Demonstrate the resolution and format relationship by measurement, so that 'the scans are too large to email' becomes three settings rather than a complaint.

Steps

  1. Scan the same page at three resolutions -- 150, 300 and 600 dots per inch -- in the same colour mode and an UNCOMPRESSED format (BMP, or TIFF with compression off), and record file sizes in lab/scan/resolution.csv with header dpi,bytes,ratio_to_150. Compression hides the arithmetic: a JPEG or PDF of a mostly white page grows far less than the pixel count does.
  2. Scan the same page at one resolution in three colour modes -- bitonal, greyscale and colour -- recording sizes in lab/scan/colour.csv with header mode,bytes.
  3. Scan once more as PDF and once as a lossless image format, recording sizes in lab/scan/format.csv with header format,bytes.
  4. Write lab/scan/settings.csv with header purpose,dpi,colour,format,why covering at least four purposes -- filing, email, optical character recognition and archival.
  5. Write lab/scan/advice.md stating the three settings you would change for a user whose scans are too large to send, in the order you would change them.

Verify

These checks run in a POSIX shell: Terminal on macOS or Linux, and on Windows Git Bash (it comes with Git for Windows) or WSL. A stock Windows PowerShell or Command Prompt has no awk or grep, so there the first line fails.

awk -F, 'NR>1 && NF>=3 {n++} END {print n" resolution(s) measured"}' lab/scan/resolution.csv
awk -F, 'NR>1 && $1==600 {print "600 dpi is "$3" times the 150 dpi size"}' lab/scan/resolution.csv
awk -F, 'NR>1 && NF>=2 {n++} END {print n" colour mode(s)"}' lab/scan/colour.csv
awk -F, 'NR>1 && NF>=2 {n++} END {print n" format(s)"}' lab/scan/format.csv
awk -F, 'NR>1 && NF>=5 {n++} END {print n" purpose(s)"}' lab/scan/settings.csv
grep -c . lab/scan/advice.md

Three resolutions with a computed ratio, three colour modes, two formats, four purposes and the advice note. Uncompressed, the 600 dpi ratio should be near sixteen times the 150 dpi size -- four times the dots in each direction -- and a ratio near four means one of the two scans was not at the resolution recorded. Step 3's formats are where compression gets its turn.

Notes

The ratio is the memorable part. Doubling resolution quadruples the file, which is why the jump from 300 to 1200 dots per inch turns a small document into an unsendable one.

This is an independent study companion for CompTIA A+ Core 1 220-1201 and is not produced by or endorsed by CompTIA.