Quick black-and-white cleanup with Photoshop


#1


#2

For the CorelDRAW/PHOTO-PAINT users, the procedure is almost identical.


#3

also, you can do this quickly by bringing up the levels dialog (cmd/ctrl-L) and selecting the white point and clicking on your white background.


#4

that’s how I have always done it too.


#5

I have no idea what this stuff means…Whats going on here?


#6

if you have a black and white picture with limited greys, you can quickly clean it up and whiten the background (making it easier to cut or engrave) with one of these steps.


#7

both tutorials are showing ways to use a photo editing program to take a scanned image and adjust it so there are only black areas and white areas. no grays.


#8

So what does that box with the diagonal line mean, and what does the box with the sharp increase and flat top mean?

Is this something that is possible in GIMP?


#9

They are graphs with the original pixel value on the X axis and the new pixel value on the Y axis. The straight one has no effect. The kinked one makes things close to black absolutely black and anything vaguely white absolutely white. A small range of greys in the middle get mapped to grey but with more contrast because of the steep slope.

I don’t know anything about GIMP but I expect it will.


#10

Thanks, it’s another small step in the direction of figuring out how to put pictures/tracings/artwork onto material via the laser :slight_smile:


#11

Yes but it it also useful for enhancing pictures for legibility as well. For example, you can bring out detail lost in shadows without altering the bright bits.


#12

I’d like to understand those curves. I’ve used them a lot of times but I don’t know exactly what I’m doing :sweat_smile:


#13

They map the pixel values in the image to a new value by using the old value as the X ordinate and looking up the corresponding Y value of the line as the new pixel value. So the height of the line on the left specifies what happens to the dark values and on the right the light values.

A 45 degree diagonal line is a one to one mapping. If you make part of it steeper then that section of X values gets stretched so the contrast is increased for those values of grey. Conversely if you make the line less steep in an area those greys get compressed and contrast is reduced.

A horizontal section means some values of grey all map to a single value, so all detail in those tones will be lost. A vertical line makes a threshold, so above a certain grey it abruptly gets brighter.

A single vertical line in the middle would make a black and white image.

Sloping the line downwards makes a negative. In general you want it always sloping upwards unless you want to remove some detail.


#14

Thanks! Now I get it (a little). I’m going to experiment with some images :sunglasses:


#15

Yes. Same process.

It’s how I sharpened the heirloom recipes for the cutting boards.


#16

Better in fact because it overlays the histogram, so you can relate the X values to the image contents.


#17

Yup, just colors, curves in GIMP, then same.


#18

photoshop shows the histogram under the curves as well by default.


#20

Where is that curves dialog in Inkscape?


#21

I don’t know that Inkscape has one…it’s more of a vector program.