Photo To Sketch

| Before going into these, maybe it is a good idea to first discuss
| convert's built-in -charcoal operator, just to make it a full
| story, because there might be people who don't know of the operator.

I'll add a link to my own discussion… http://www.imagemagick.org/Usage/transform/#charcoal :-)

But the two methods you mention are not really using charcoal as such. In that case it is used more as a edge detection method, to generate disgonal pencil lines.

I have added a new 'photo transform' for charcoal. http://www.imagemagick.org/Usage/photos/#charcoal Give it a day to so for it to appear on the main IM exmaple site.

| Moreover, when I apply the Childrens Color-In Image method to a
| portrait photo, it did not work well.  ... So I'm
| thinking, let's try another algorithm.

Good, because I am certainly not happy with it!

| ... I found a very good algorithm from the gimp
| tutorials section.
|
| http://www.gimp.org/tutorials/Sketch_Effect/[]

Now that is a great effect!!!

In fact such an effect of removing unimportant lines may be a good one for image comparision too!

| To be honest, I was just stunned by how good the effect can be.
| I had been trying to achieve the same result in gimp long time ago
| using my limited understanding/knowledge but failed. Looking at
| how a machine can do in just several simples steps, I have to say I
| was amazed. I think that's a very effective algorithm, but I don't
| know how to do it in IM.

Hmmm

highpass fiter:   -- not sure...  Anyone?
blur:     -blur 0x7
invert    -negate

we need the original then do a opacity 50% and add..

Command so far…

convert image.png \( +clone -median 2 -blur 0x7 -negate \) \
        -matte -channel A -evaluate set 50% -compose plus -composite \
        ...

Hmmm our image has some color in it but it is close. prehaps I have the point of cloning wrong, or I have the high pass filter wrong (likely)

Continuing increase contrast

-sigmoidal-contrast 7x50%

or -normalize prehaps with a controled 'denormalize' or greying… http://www.imagemagick.org/Usage/color/#greying

After that is simular to using various stech and edge detection.

Of course some manual (sic) handling was also done.

Anyone like to improve this?

Another technique I would like to see converted to IM is….

Watercolors from photos… http://www.state-of-entropy.com/watercolor.html

Though the title image is great looking, the example image used is not so good.

Anthony Thyssen

Newsgroups: gmane.comp.video.image-magick.user
Date: Tue, 24 Jul 2007 12:44:50 +1000

| The background of pic1.png has been cleared (using gimp). After the
| following command (to produce the sketch effect), the cleared background
| shows up again in pic1.gif. I'm wondering why the cleared background can
| still managed to show up, and how can I fix the problem.=20
|
|   convert pic1.png \( +clone -median 2 -blur 0x7 -negate \) \
|     -matte -channel A -evaluate set 50% -compose plus -composite pic1.gif

Of course the background shows up again. You replaced all transparnency values with 50% in BOTH images, added together that is 100% opaque!

Try using

-evaluate multiply .5

or

-evaluate divide 2

Instead. Do not use a percentage, as that is a percentage of color and not 1/2:

Also note that GIF does not allow semi-transparency, so your source image better not be using any semi-transparency.

| I've also tried to specify -channel RGBA, but the result is all grayed.
|
|   convert pic1.png \( +clone -median 2 -blur 0x7 -negate \) \
|     -matte -channel RGBA -evaluate set 50% -compose plus -composite pic1.gif

Naturally you just SET every color to 50% grey and 50% transparent!

See IM Examples Evaluate, Faster Simple Math Operations http://www.imagemagick.org/Usage/transform/#evaluate

Anthony Thyssen