Alpha Channel: When to add it or why not? How much does it add to image file size?

Question

When you load an existing image in Photoshop or Gimp, it is a single layer with no alpha channel. One reason why someone would need to add an alpha channel to that layer is if they want to cut away parts of the image and have a transparency underneath, rather than a white layer. Besides this (having a transparency layer to fall back on) What other reasons is there to add an alpha channel?

Is it actually best practice to always add an alpha channel to each newly created layer when editing a photo or image?

And technically, what really happens behind the scenes when one adds an alpha channel? Is the alpha channel actually a new layer, but not displayed as a new layer in the console, that adds additional space to the image file equal to that of its parent layer? What else is going on

Answer 1

A channel is like a single color, think of each red, blue and green as a black and white image. Not a layer. In photoshop you can view the individual channels in the channel panel. (Though it only shows agregate)

So adding alpha just ads one more row into your datatable for that layer. For a rgb image thats 1/3 more data, assuming there was no alpha. Though, what that is on disk after compression is a question in it self. Not sure ive used a computer that would care about the memory usage since maybe mid 1990's. (Workstation computers these days have about 10 000 - 50 000 times amount of memory as those computers hell even cheapest phones have 1000)

Would you always add one? Well no not if you don't need to move the image or layer it atop of another. Not sure i care of best practices, if you dont know why you do something dont do it.

Answer 2

When you load an existing image in Photoshop or Gimp, it is a single layer with no alpha channel.

Well, at least in Gimp, this is inaccurate. If the image file has an alpha channel (even unused) the loaded layer will have an alpha channel. So you don't see this alpha channel when you open a Jpeg image, since Jpeg doesn't support alpha channels. There are also some alpha-less PNG or GIF.

An alpha channel is an additional data per pixel, along side the 3 color channels. Footprint-wise it adds 25% to the layer data. You have to be very RAM-constrained for this to matter, and there are many other things that take up RAM faster, such as the number of steps in the undo history, or the number of layers.

From the file size point of view, an unused alpha channel, being uniform, compresses very well so this has little impact one file sizes (except on PNG where you get a 10% increase). Of course on formats that don't carry an alpha channel such as JPG there is no impact whatsoever.

Is it useful to add by default? Usually, yes, at least for any layer above the bottom layer because usually you want to see some of the layers below.

Answer 3

The speak more towards "other reasons": I regularly add (and store) alpha channels.

One of my jobs is photography of antique oil paintings (like you'd find in a museum), many of which have ornate frames. We keep a cropped shot of the painting itself, but for the photographs which include the frame, we carefully mask (or silhouette) the frame edge so that we can have a uniform color background which does not crop or obstruct the frame details. We use these for presentation, but we may need to place the image on a white background, black background, or random color.

We save the mask in the alpha channel to be used directly (indesign or imagemagick batch processing etc.) or so that we may recall the mask selection (ctrl+click the alpha channel to load it as a selection) and alter the matte color for uses where transparency may not be supported.