Layers

Layers can be thought of as different images placed in front of one another. Imagine a pane of glass with a tree painted on it, and then another with a mountain painted on it, and then a third with a blue sky painted on it. This is akin to the way layers work in most digital painting and design programs. Keep in mind that the order of the layers in your layer list will make a difference. Traditionally the lower entries in the layer list are behind the upper list positions which are shown in the front.

Within the digital basics section of this site you can find an expanded explanation of layer basics. Click this sentence to go there now if you like.

Opacity

Opacity is a word almost synonymous with "transparency". It describes how much an image allows images and colors behind it to come through to the visible side. It is typically measured in percentages with 0% being fully transparent and 100% being fully opaque.

Blending modes

Most all programs will allow you to change the way different layers interact with one another. Obviously layers that are higher up the list will be drawn "above" other layers, and layers that have transparent areas will show the content of lower layers, but there also ways to actually combine the result of them.

And that will be an important thing to keep in mind when looking at these examples. We are not changing the color information on the actual layer. We are ONLY changing the COLORS DISPLAYED on the screen.

Also note that these blending modes are not universal. Different programs will have different options so your results may differ.

Multiply

The "Multiply" blending mode can be thought of as a kind of procedural mask. This means it allows certain values through while retaining complete opacity with others. With "Multiply" the darker the RGB value the more an individual pixel will be opaque while the lighter an RGB value the more transparent it will be.

It's not a stretch to say that this is one of the more popular blending modes for it's multitude of uses. For instance...

  • Applying it to a layer containing a black and white photo will aid you in colorizing the photo by letting through any colors you place on lower layers while retaining the dark values of the photo layer above.
  • Artists can use the multiply blending mode to retain line art for illustrations, cartoons, and caricatures that use dark linework to define shapes.

Screen

"Screen" can be thought of as the inverse of the "Multiply" blending mode. It allows lighter tones to remain while knocking out the darker tones of an image.

Overlay

Overlay works almost as a combination of "multiply" and "screen" in that it doesn't necessarily remove light or dark values from the layer but allows colors below to combine with both.

Masks

A Mask is any image that is not meant to be seen but is meant to store opacity information for another image. They are typically greyscale (256 shades from white to black).

Masks are a way of hiding certain parts of an image. Masks are typically just raster images (though they can be vectors as well) that are the same size as the canvas. They are always greyscale images and can use the full range of 256 values assigned to an 8-bit, single channel image.

Let's look at a visual represenation of how masks work.

Admire this peanut butter cup. Go ahead. Admire it. Admire the black and white image that comes second while we're at it.

+ =

In this case the original image of the candy has had the background hidden, not deleted , just hidden. The black and white second image is our mask which dictates what part of the image should be hidden.

The white and grey checkerboard around the candy in the third image denotes transparency. This is the default "indicator" of most image editing software. If you were to export this image as a PNG, GIF, or any file type that supports transparency then this part of the image would not be seen. The checkerboard will only be see *within* the image editing software.

It's a near universal convention in image editing programs that within the greyscale range of the mask lower values HIDE the image and higher values SHOW the will image. In other words the black parts of the mask denote hidden areas, white parts denote shown areas, and scales of grey inbetween partially show or hide the image.

Raster masks

Raster masks are the default form of masking in most image editors. The mask is an actual multidemnsional array of pixels that only has one channel (it's a greyscale image). This means 256 possible levels of opacity that can be applied to the visible layer.

Raster masks are useful for images that are not made up entirely of hard edges or require some level of transparency. For instance foliage, clothing, and hair are good examples of when to use a raster mask. Also shadows, glass, semi-translucent fabrics, and light effects such as lens flairs might also involve raster masks.

For the image below note how the mask makes use of grey tones around the edge to smooth the transition from opaque to transparent.



In this image we are able to use a raster mask to mask out the subject's hair. Hair is one of the most common requirements of a raster mask since the fine strands simulate a semi-translucent surface which you can only "kind of" see through. (photo)

Vector masks

Vector masks work and act the same exact way that raster masks do but have a few specific properties.

  • Just like visible vector shapes vector masks can be scaled up or down without loss of quality. If you are using vector masks to define shapes edges then you can use a raster layer to give a shape "texture" but keep the edges sharp regardless of the scale.
  • As they are not raster images vector masks can NOT have shades of grey beyond black or white. Either the image is fully visible or it is not (though most programs will anti-alias the edge).

Chances are you've already used Vector masks. If you've used vectorized shape objects in any way than you've created paths that dictate the visible and invisible areas of a solid color.

Clipping masks

Clipping masks are different than the other two types of masks in that they don't use unique geryscale image to hide parts of the image. Instead they actually use a second layer, raster or vector, and the transparency of that second layer.

Clipping masks are not available in every image editing program. They are easy to set up in Adobe Photoshop and will be discussed in depth there.

Alpha Channels

While looking at the channels palette within your image editor you may have seen a reference to an "alpha" channel.

Premulitplied alphas

If you ever deal with 3D graphics, whether they're pre-rendered or real time (such as a video game), then you're inevitably going to run into the option of utilizing "premultiplied" alpha channels.