What are Layers

The standard explanation of layers in image-editing is “think of layers as like a pile of acetates” or “layers are like transparencies laid on top of each other”.

The problem with this description is that anyone under 30 (I guess) is unlikely to have encountered the type of overhead projector (OHP) which uses transparencies.  In fact, the last time someone asked me if I knew where to find an OHP for an elderly presenter whose material was all on transparencies, my honest answer was “Try the museum”.

Understanding the overhead-projectors / transparency / acetate answer

An overhead project was a light box with a lens mounted above it which rotated light from the light-box by 90 or so degrees (using mirrors), to allow it to be projected onto a wall or screen

A transparency was an A4-size sheet of rigid plastic, made out of a chemical called acetate, and able to cope with the high temperatures generated by the bulbs in early overhead projectors.
So it was possible to use the projector to show text or an image from a transparency onto a wall or screen. They were fantastic pieces of technology in their day: they freed teachers from writing the same stuff on blackboards over and over again, they gave business people and trainers far more options for explaining things to groups of people, and they freed church-goers from being stuck to the same old hymns from their hymn book. But they were expensive: acetate transparencies cost a lot, as did the special pens that were needed to write on them. Eventually there were transparencies which could be put through laser printers, but the printed text tended to chip off them over time. as laptops got cheaper and data-projectors became more widely available.

A better explanation of layers

Yes, you can even get them in
pretty coloured edgings like this

Think of layers as similar to plastic page-protectors – the high-quality ones that you can see through properly, not the cheaper ones that look a little fuzzy.

And think of a file in your image-editing program as being like a ring-binder folder.

Now imagine putting a pile of page-protectors in a folder, putting some parts of an image into each of them

If you look into the pile from the above, then the picture that you see is the sum of all the picture-parts in the individual page-protectors:

• If there is a part of the page which isn’t covered up by the contents of any page-protector, then that part will look empty, AKA transparent.
• If one page protector has something which covers the whole page, then you won’t be able to see anything from page-protectors which are underneath that one. So most likely you want to put that “background” page-protector at the very bottom of the pile.

And that is how layers work in pretty much all image-editing programs (Photoshop, GIMP, Paint.net, etc)

Of course you can do more with layers in a computer programme than you can with page-protectors: you can be certain that items which you put into a layer will stay exactly where you put around and not wiggle out of place if someone moved the folder. But you can also you can make them semi-opaque, re-arrange the order of the layers using drag-and-drop, apply a colour filter to an entire layer, etc.

But the general idea is the same.


Next: Working with layers in Photoshop