Lock Sign Issue

[MahaB82A]

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

[davescm · Accepted solution]

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

[jane-e · Accepted solution]

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

[MahaB82A]

Is there any special usage for layers without transperency? 

[jane-e]

No. They were required through PS 5.0 (not CS5.0). JPEGs still require a Background layer, but otherwise they are unnecessary.

Jane

[MahaB82A]

My question is incorrect. I use PNG layer often, if there is no need for background image. But I am not clear about JPG layer. Today I watched a video, video shows about using Leves & Curves. Presenter used JPG layer. I wish to know is there any special purpose of using JPG layer?   

[jane-e]

@MahaB82A 

If you have a JPEG and add a layer of any kind (including a Levels and Curves Adjustment Layer) , then it no longer meets the requirements for a jpeg. Doing a regular Save As will try to save the image as a PSD. You can intentionally choose jpeg, but note that the image will be flattened, meaning there will be one Background layer.

PNGs also have only one layer, although they do support transparency.

If you want transparency and layers, save as a PSD or TIFF. 

Jane

[Glenn 8675309]

just click on the lock to be able to do stuff to that image.  

[MahaB82A]

Thanks for all