Legacy ImageMagick Discussions Archive

Hi!

When I generate a histogram for an image with a command like:

convert xpto.jpg histogram:output.png

The output.png file tends to be huge, something like 300k or more...

Now, if I do this:

convert xpto.jpg histogram:- | convert - output.png

The same exact histogram is generated (of course!) but now the file size is only about 2KB as expected.

Am I doing something wrong here?

Found the answer myself: the image contains, in comments, the entire list of colors with the color count for each one!

The correct way to sort this out in a single "convert" is something like this:

convert input.jpg -write histogram:mpr:xpto +delete mpr:xpto -strip -evaluate Divide 2 -evaluate Add 25% output.png

One trouble I'm still having is calculation time... damn it, 2.8 seconds in a 1.8GHz CPU for calculating an histogram of a 480*320 image?!? This is SLOW! Any ideas?

"identify -verbose" shows output.png from the first command carries a huge comment, which seems to be the numbers behind the graph, a written form of the histogram.

output.png also has a comment, but now it seems to be the histogram of the histogram image, which is (of course) much simpler.

I would expect "-strip" in the first command to remove that comment. Strangely, it doesn't.

Add +set comment on the command line to remove a comment from an image, for example:

• convert image.png +set comment image.jpg

Nice, now is there someway to make it run faster without -filter Point -resize 50% or similar extreme measures?

The algorithm spends most of its time generating the histogram comment. We'll make it an option in ImageMagick 6.6.1-4 Beta by sometime tomorrow. Add -define histogram:unique-colors=true to generate the unique colors comment. Without it, generating a histogram is lightning fast.

Thanks guys, once again!

Okay I get it.

Adding -define histogram:unique-colors=false to a command like turn off histogram comment generation.