Hi,
I have many of these drawings (see file attached) and would like to know if there is a trick to make all the lines the same or similar dark?
thank you

Try the Threshold tool (with possibly several selections to use different thresholds for different image parts (such as the cross).



But is this is the original image or a scaled down version? The original image could have more information to play with and better techniques could possibly be used.

thanks for your answer (PS: that's the only size I have, they were all small small drawings)

There is also that picture which is in color (see attachment :"blues"),
is there a possibility to make the blues of the picture in a similar darkness?
for example the arm came out too dark.

1) Right click the layer the image is on and select "Add Alpha Channel"
2) Go to Colors>Colors to Alpha and select the color "white"
3) Now just keep duplicating the drawing layer until you see what you like contrast wise.
4) Right click the layer and select > "Merge Visible Layers" Use expand as necessary
5) Right click the layer and select "Alpha To Selection"
6) Go to Edit>Fill with FG color (black)
7) Go to Colors>Threshold and just use 127 and 255


This could probably be scripted to use on batch images.

also quite a good idea, worked for the pencil drawings
but for the painting I couldnt get it to kind of balance the blues in there so they come to a similar darkness

my try
select by color the area with a too intense cyan
decrease saturation, increase brightness

Yep, good solution