Lock Sign Issue

Question

What is the reason JPG comes with a lock sign? PNG does not come.

jane-e · Accepted Answer

Adding a little history to Dave's correct reply, Photoshop used to require a Background layer, which is a unique layer that could not be deleted, renamed, reordered, or have transparency.

Background layers have not been required since Alvy Ray Smith came up with the formula for transparency:

"αA + (1-α)B"

and in 1999 Photoshop 5.5 was released. Details here:

https://www.cs.princeton.edu/courses/archive/spring05/cos426/papers/smith95c.pdf

Jane

davescm · Answer

It indicates a background layer, which is a specific type of layer that cannot be transparent. That matches the file it came from as a jpeg cannot contain transparency either.

A png can contain transparency, so opens as an ordinary layer - Layer 0.

You can convert a background layer to an ordinary layer, but remember if you make any pixels transparent then that will not be supported in a jpeg.

Dave

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