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New to photo editing, so I've got some conceptional confusion about the colors and channels,

sample

  1. In photoshop, we can create text layer, we can have text on top of an "empty space" the checker board, then what is the color of the checker board, from what I can see the checker board does not have color information, so what exactly is the empty area?

  2. Photoshop treats the fourth channel as Transparency channel, but even if I do not have this channel I still have Transparency in the empty area, so I can control an area's transparency with transparency channel, or set it to "totally empty", really do not understand the difference between "Empty area" and "Transparency channel" and "Alpha channel"

So when I want to have transparency background, do I need to erease the part that should be transparent, or make a "Transparency channel" or "Alpha channel"?

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  • Technically in the final bitmap format there is no such thing as a transparent pixel.
    – joojaa
    Jan 29, 2021 at 15:32

2 Answers 2

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All documents in Photoshop must have a width and a height. If there is a width and a height, but no pixel data, the area is defined as "empty". "Empty area" encompasses the document canvas without pixel data and is used to describe areas between pixel data and the canvas edges.

When exporting/saving Photoshop does not save/export the "empty area".

All "empty areas" are transparent.

enter image description here

Transparency is defined as an area which has no pixel data. No color, no value, nothing. If you want to control transparency in Photoshop, that is done by adding or removing pixel data in Layers or color Channels.

There is no "transparency channel" in Photoshop. The pixel data on layers defines transparency.

An Alpha Channel is a saved selection. Similar to a layer mask, an Alpha channel merely indicates a specific area on the open document. There's some liberty taken by Adobe - using "Alpha" in a more broad sense where customarily "Alpha" has meant transparency.. that's not always the case within Photoshop. Alpha Channel, in Photoshop, merely means "extra".

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  • Hello thanks for replying, I saw your demo image says the text area has pixel data, then what about the area between the letters does it have data as well?
    – Dew
    Jan 28, 2021 at 11:27
  • the "pixel data area" is going to be the smallest rectangle that can contains image information, often called the "extents." This is a memory optimization, and a layer may actually have two or more areas/extents if they do not overlap (depending on the software). I expect there is some form of "alpha information" stored for these areas. In this way, the non-pixel area is "empty" and the image can be reconstructed by storing the width and height, and the origin point of each pixel-area. instead of storing a full byte per channel per pixel for empty areas.
    – Yorik
    Jan 28, 2021 at 17:30
  • @Dew As Yorik points out the non-empty area consists of the smallest rectangle which encompasses pixel data. Areas such as those between type glyphs are not considered "empty" but rather simply transparent. All empty areas are transparent, but not all transparent areas are empty.
    – Scott
    Jan 28, 2021 at 18:23
  • I am use this texture in blender, it treated the empty space as black color so I was wondering what value is stored in this area
    – Dew
    Jan 28, 2021 at 22:56
  • @Scott After some trials I found that my PNG file's transparent area is R:0,G:0,B:0 and Alpha:0, this is essentially black clolor plus Alpha 0, I'm not sure why.
    – Dew
    Jan 30, 2021 at 7:48
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A mask an and an alpha channel are the same thing. they are grayscale images from 0 to 1 multiplied by the rgb channels. Black (value of 0 luminance) will give you transparency, white (value of full luminance) gives you the full rgb channel values. The word mask (also stencil) refers to how photos were manipulated in the dark room before computers. Alpha channel is the programmer name for how the same functionality is achieved within a computer.

Now photoshop (not all, or even most graphic software has made this horrible choice) has shifted this to "emptiness" and hidden the process from the end user.

The problem with this is that an empty area has to be told to be "empty". How does photoshop tell something to be "empty"? save a png as a jpg and you will find that emptiness filled. That emptiness is a number in the fourth channel, which is the alpha channel of a png.

This is why your Alpha is 0. Before "emptiness", that 0 was multiplied by the lumninance (RGB) and therefore you got transparency. If indeed there is emptiness, the description above makes it sounds like there is run length encoding setting all the empty area to 0 luminance and then within the non-empty transparent areas there is an alpha creating 0 luminance by multiplying the rgb by 0. Sadly photoshop has chosen to conceal this from the end user.

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