# Concepts

There are a few concepts that we need to understand before we learn to apply them in Pixquare. In general, they do not have much difference between pixel art and other forms of digital art.

### Alpha-lock

If a layer is alpha-locked, we can only draw on pixels that already have non-transparent values of that layer. This has the same effect as a [selection](/tools/selection.md) of all non-transparent pixels.

### Blend mode

Layers can have special blend modes that dictate how they blend (combine) their colors with the ones below them.

If you want to learn more about the details of how blend modes work, you can give this [wiki page](https://en.wikipedia.org/wiki/Blend_modes) a visit. 

Currently, Pixquare supports these modes:

* Normal
* Darken
* Multiply
* Color burn
* Lighten
* Screen
* Color dodge
* Overlay
* Soft light
* Hard light
* Difference
* Exclusion
* Hue
* Saturation
* Color
* Luminosity

### Masking

<figure><img src="/files/gT0gBKM7wGw7nY0h2FWG" alt="" width="375"><figcaption></figcaption></figure>

There are 2 types of masks: cropping and clipping.&#x20;

* Cropping mask: when a layer becomes a cropping mask of another one, its RGB values will be ignored, only the A channel is used to determine the alpha channel of the cropped layer by multiplying the cropping mask's alpha with the cropped layer's alpha.
* Clipping mask: when a layer is clipped by another one, its alpha will be multiplied by the other layer's alpha, the RGB values are still used. In other words, the clipped layer's content will only show inside the clipping layer's content.


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