Overlays

Inheritance answers "what am I built on" and composites behind a slide.

A logo, a footer, a conference badge ask a different question: "what goes on top of every slide, whatever it is built on". That is an overlay, and it is the second composition axis.

Keeping them separate is what lets chrome reach every layout without a duplicate layout for each combination.

Using one

An overlay is an SVG at slide dimensions, drawn above the finished slide. Declare them on the deck:

from inkflow import Deck, Overlay, Slide

Deck(
    overlays=[Overlay("footer"), Overlay("logo")],
    slides=[
        Slide("title.svg", overlays=[]),  # a bare title, no chrome
        Slide("content", md="intro"),  # inherits the deck's two
    ],
)

List order is paint order.

Resolution runs Slide.overlays → Deck.overlays → Theme.overlays. Each level overrides rather than merges: None inherits the level above, and [] means no chrome at all. That is the same rule as transition and font_size.

What overlays can do

Overlays are composited before slide numbering and content injection, so they behave like any other part of the slide:

• A zone-slide-number inside a footer is filled like any other zone.
• A zone the overlay declares can be filled from zones={...} on a slide, and is pruned on slides that leave it empty.
• Animations can target elements inside an overlay, so chrome can be revealed on a step.

They also travel with the slide during a transition. A footer slides along with a Push and dips through a Crossfade.

Finding an overlay by name

Overlays live in overlays/ and use the same three-level search and prefix grammar as layouts, against overlays/ instead of layouts/:

Written as	Resolves to
"footer"	The three-level search
"local:footer"	{project}/overlays/footer.svg
"theme:footer"	{theme}/overlays/footer.svg
"builtin:footer"	A built-in overlay
"./chrome/footer.svg"	Relative to the project directory

Layouts and overlays are separate namespaces, so a bare name resolves to one or the other and never both.

Overlays can inherit

inkflow:parent on an overlay means "drawn behind me, within this overlay". The overlay as a whole still lands on top of the slide, so the two axes stay independent:

overlays/brand.svg     the rule line and the mark
overlays/chrome.svg    inkflow:parent="brand", adds the event name

This is how a theme ships theme:brand that a project extends without copying it.

A bare name in an overlay's inkflow:parent resolves in the overlay namespace, so chrome can only ever inherit chrome.

Do not point an overlay's parent at a layout

Layouts paint a full-bleed background. On top of a slide, that hides the entire deck.

A bare name cannot reach a layout from an overlay, so this needs an explicit path to happen at all. inkflow verify reports it as an error and names the file.

Shipping chrome with a theme

Set overlays on the theme class and every deck using it gets the chrome:

class Corporate(Theme):
    overlays = [Overlay("theme:brand")]

A deck opts out with Deck(overlays=[]), a slide with Slide(overlays=[]).

Drawing one

An overlay file previews differently from a slide, since it has no idea what it will land on. See drawing an overlay for the backdrop attribute that makes it workable in Inkscape.