So, the first step is to convert the pdf files to pngs. For that we use:
```
$CONVERT -density $PDF_DENSITY "$pdffile" -background white -alpha remove
-colorspace Gray -channel R -separate "$directory/page-%d.png”
```
Variables are defined as:
$CONVERT: the name (perhaps with path) of ImageMagick’s conversion tool on your
computer. If using ImageMagick 6, this was convert. In ImageMagick 7 it’s
magick.
$PDF_DENSITY: The dpi at which the file will be converted. The higher the
number, the more pixels the visual comparison will have to work from. We
default to
$pdffile: The pdf file being converted.
$directory: Where to put the pngs. This command handles multi-page scores and
each page gets its own image.
Once the scores are converted to png, they are compared with:
```
$COMPARE -metric NCC "$name" "$expected”
```
Variables are defined as:
$COMPARE: the name (perhaps with path) of ImageMagick’s comparison tool on your
computer along with some. If using ImageMagick 6, this was compare. In
ImageMagick 7 it’s magick compare.
This gives a numeric result (embedded within a longer report in ImageMagick 7)
that is used to decide whether the images are close enough to be considered
“the same” or “different.” I.e. this is where we determine if a test is PASS
or FAIL. If they are different, we then generate the diff with:
```
$CONVERT "$name" "$expected" \
\( -clone 0,1 -compose difference
-composite \) \
\( -clone 0 -clone 2 -compose minus
-composite \
-background blue -alpha shape \) \
\( -clone 1 -clone 2 -compose minus \
-composite -background red -alpha
shape \) \
\( -clone 0,1 -fill white -colorize 80%
\) \
-delete 0-2 -reverse -background white \
-compose over -flatten "diff-$name”
```
Variables are defined as:
$CONVERT: as above
$name: the “new” file. Stuff present here but not in the old file will be
blue. This will also be used to name the diff file.
$expected: the “old” file. Stuff present here but not in the new file will be
red.
For multi-page scores, we iterate over each page using a for loop in bash.
You can skip the `$COMPARE` step if you know the scores are different (or don’t
mind a diff that is just gray). Our test bank includes around 600 image
comparison tests, so we don’t want to be wasting cycles generating diffs for
files that aren’t actually different.
✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝
Fr. Samuel, OSB
(R. Padraic Springuel)
St. Anselm’s Abbey
4501 South Dakota Ave, NE
Washington, DC, 20017
202-269-2300
(c) 202-853-7036
PAX ☧ ΧΡΙΣΤΟΣ
> On Aug 14, 2026, at 1:36 PM, Mikko Kouhia <[email protected]> wrote:
>
> On Thursday, 13 August 2026 21:17:32 Eastern European Summer Time Fr. Samuel
> Springuel wrote:
>> The Gregorio project uses ImageMagik to visually compare scores as part of
>> our test suite. [...]
>>
>> [...] If you’re interested though, I’ll post the series of ImageMagik
>> commands that it takes to generate the attached [...]
>
> Hi, I'm interested to learn also this solution. When you have time, please do
> post your script.
>
>
> --
> Mikko Kouhia
>
>
>
