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This file was just moved as content/core/specifications/locks.md in the pat/bootstrap branch of the gimp-web-devel repository. This branch will soon be merged and become our new website. Removing the now duplicate in our source repo.
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This file details the logic for item locks (on layers, channels, paths,
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etc.)
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[TOC]
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## The 4 types of locks
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### Lock pixels
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Locking pixels means you cannot change any value in any of the channels
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of the image. In other words, you cannot paint, erase, apply any
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filters, neither can you apply most transformations.
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The only transformation which you can apply is moving the whole layer at
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once (not moving a selection contents), because then you only change the
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offsets. The contents of each channel stay the same.
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### Lock position
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Locking position is the opposite of the "Lock pixels" as you can do
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anything on your item except moving it, i.e. changing its offsets.
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### Lock alpha
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Locking alpha is in a way a subcase of "Lock pixels" as the only channel
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which cannot change is the alpha channel, i.e. how transparent is each
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pixel. You can still change the colors of pixels, but not their
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transparency. In other words, if you paint over a layer with "Lock
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alpha", this would only affect the color.
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This use case can sometimes be frustrating/confusing if you paint over
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full transparent pixels as it would look it's not working, yet you would
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get neither statusbar error nor blinking locks to explain why you see no
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changes on your layer.
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This lock is useless over an item with no alpha channel, though it is
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currently still allowed to set such a lock. It just does nothing useful.
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Note that you can still change the opacity of the item as a whole
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through the Opacity slider (for instance in Layers or Channels
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dockables) or through actions affecting the layer's opacity. It's only
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individual pixel alpha values which are locked.
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### Lock visibility
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Locking visibility means you cannot change the item's visibility
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(typically the "eye" icon on dockables).
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This is especially useful for when you [shift-click or
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alt-click](#exclusive-visibility-locks-switch) the eye icons to
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massively toggle item visibility, yet you want to exclude some items.
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For instance, say you have a background you want to always show and
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several foreground object you want to conditionally show alone or
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together. You could lock the background to keep it always visible and
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shift-click foreground objects.
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## Effects on layer groups
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Layers in particular can be grouped in "Layer groups". Though the effect
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of locks is obvious on non-group items, we tried to specify their
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behaviour when set on layer groups to be the most useful and also
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hopefully expected possible.
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Below are what the effects are for various locks.
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### Lock pixels and alpha
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Since it involves painting, it means nothing on layer groups as you
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can't paint on them. There are 2 exceptions:
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#### Layer group masks
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A layer group can have a mask. A "Lock pixels" would prevent from
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drawing in it. "Lock alpha" is meaningless there as masks don't have
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alpha channels (well they are kind of separate alpha value themselves,
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but as said, we use the "Lock pixels" to block this type of editing).
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#### Child layers
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These 2 locks when set on a group apply on all children. In other words,
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you can lock painting and drawing on many layers organized in groups by
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locking an ancestor.
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### Lock position
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Moving the group as a whole is equivalent to moving each and every of
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its children layers (recursively if there are sub-groups). Locking the
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layer group forbid this.
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Locking a group also forbid moving any child layer individually, as do
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"Lock pixels" and "Lock alpha".
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Note that locking the position of a child layer also prevents the whole
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group contents to be moved.
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So "Lock position" is really working both ways: any lock in the tree
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prevents the whole direct genealogy of layers to be moved. Sister layers
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are fine though.
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### Lock visibility
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Locking a layer group visibility has the exact semantic as locking a
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non-group layer since this is the only feature which is actually
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normally applicable to layer groups.
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Also it means that unlike all other lock features, locking the
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visibility of the layer group does not forbid changing visibility of a
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child layer.
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## Layer mask
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The layer mask is also its own particular case. Right now, the "Lock
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alpha" and "Lock visibility" don't affect them (masks are somehow their
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own alpha channel, but in the same time, we use "Lock pixels" to forbid
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painting; as for visibility, they have their own feature for this).
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"Lock pixels" prevents from painting in a mask.
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Note that both "Lock pixels" and "Lock position" prevent from moving a
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mask. Indeed masks dimensions are currently tied to their associated
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layer (or layer group) dimensions. It means than moving a mask does not
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just change the offset. You actually also delete some of the mask
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pixels/data and create new ones. Therefore moving changes the contents
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of the mask, which is why "Lock pixels" also prevent moving the mask.
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## Exclusive visibility/locks switch
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GIMP provides a way to massively toggles visibility and locks.
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### Exclusive switch on same level
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When `Shift-clicking` either a visibility icon or a lock icon, all other
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visibility or locks respectively on the same level will be switched OFF
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(leaving only the clicked switch ON). `Shift-clicking` again, all
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visibility or locks on same level will be back on.
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Say the state is:
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```
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* Layer 1
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* Layer group
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* Layer 2
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* Layer 3
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* Layer 4
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* Layer 5
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```
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Then `Shift-click` the eye icon of Layer 3: Layer 2 and Layer 4's
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visibility will be turned OFF. Shift-click again and it goes back to
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initial state.
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### Exclusive switch of selected items
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When `Alt-clicking` a visibility or lock icon, the same happens as
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`Shift-click` except among selected items. The level in the item tree
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doesn't matter.
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### Exclusive visibility switch with a "Lock visibility" set
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If any "Lock visibility" is set on an item, its visibility status won't
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be switched along other items when `Shift-clicking` or `Alt-clicking`.
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Being able to exclude some items from such massive switch is one of the
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main use case of the "Lock visibility".
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