What is locking transparent pixel and how do I 'lock' and 'unlock' it?

Question

I am very afraid of photoshop; I don't know why. So I started from the basics, like digital coloring, coloring children art, etc, where the basics of Photoshop are discussed and also demonstrated. I was following one tutorial I found useful. There the tutor says 'I use lock transparent pixel for shading'.

This was a time-lapse tutorial. The tutor doing things so fast like a bee! I slowed him down to see what he actually meant, but I saw nothing! I saw some questions here, but they already seem to know what 'locking transparent pixel' is. I don't know.

Please can anyone tell me: 1- What is locking transparent pixel? 2- How do I lock and unlock it? 3- Why people do it during coloring? 4- Why it is needed for shading?

Please use easy words, I dnt wanna quit learning photoshop for 1000th time.

Thank you.

Answer 1

It's an icon in the Layers panel, just next to the word Lock. In the next image I have turned it ON and drew an ugly black brush stroke from bottom to top over the layer:

The painted area kept the same transparency as the pixels already had - no visible paint over the 100% transparent background.

In the next image the transparency lock is OFF and a new black stroke is drawn:

Why to use it? One may have a layer which contains a geometrically good shape which is surrounded by transparent areas that must be kept as is. But he wants to paint the colors better, maybe only to airbrush something dark or bright to create apparent 3D shading. By locking the transparency there's no worry of spoiling the edge when one brushes over the colored area. It works also with smoothly fading (=blurry) edges.

Some of us may prefer to use layer masks to get the same, but this leaves the layer mask free for other purposes that an advanced user may well see useful.

One trick does quite the same as locking the transparent pixels. Make a selection by Ctrl+Click on the layer icon in the Layers panel. Painting is limited to the selection. If there's a blurry edge the paint on it stays also blurry.

BTW If you work tutorials you surely meet unknown (for you) keywords that the instructor assumes to be known and you cannot work the shown trick. In that case search immediately what the keyword means and progress faster. Searching Photoshop lock transparent yields numerous results which contain everything said above and more.