ersatztv/docs/graphics-elements.md

# Graphics elements — schema reference

Graphics elements are YAML-authored overlays composited onto a channel's **transcoded** frames by the
SkiaSharp graphics engine (`ErsatzTV.Infrastructure/Streaming/Graphics/`). They appear in the FFmpeg
filter graph as the `[vge]` stage.

`[vge]` is **not** proof that a graphics element is bound. Watermarks are a separate, older system that
falls back onto the graphics engine whenever it cannot use ffmpeg's native `overlay` shortcut — an
`Intermittent`/`OpacityExpression` mode, more than one active watermark, or an external-URL image all
route a *watermark* through `[vge]` with zero graphics-element rows. The native shortcut is only taken
for a single `Permanent` local-file watermark
(`ErsatzTV.Core/FFmpeg/FFmpegLibraryProcessService.cs`, `CanUseFFmpegNativeWatermark`).

`StreamingMode.HttpLiveStreamingDirect` renders nothing: ErsatzTV is not transcoding, so there is no
frame pipeline to draw into and `GraphicsElementSelector` returns empty.

## Discovery and attachment

- Files live under `FileSystemLayout.GraphicsElements*TemplatesFolder` (`text/`, `image/`, `motion/`,
  `subtitle/`, `script/`). `RefreshGraphicsElementsHandler` scans them and maintains one
  `GraphicsElement` row per file, removing rows whose file vanished.
- Five join tables attach an element: `ChannelGraphicsElement`, `PlayoutItemGraphicsElement`,
  `ProgramScheduleItemGraphicsElement`, `BlockItemGraphicsElement`, `DecoGraphicsElement`.
  Resolution order and the deco `Merge`/`Override`/`Disable`/`Inherit` semantics are in
  `graphics.channel-level-attachment`; the channel level is the **base layer**.

## Parsing rules that bite

- Deserialization is YamlDotNet **without** `IgnoreUnmatchedProperties`, so **an unknown key throws**
  and the whole element is silently disabled for that content with only a logged warning
  (`GraphicsElementLoader.FromYaml<T>`). A typo is not a partial render — it is no render.
- There is no schema validation (`GraphicsElementLoader` carries an explicit `TODO: validate schema`).
- Colour strings are parsed with SkiaSharp's `SKColor.TryParse` (`#RRGGBB`, `#AARRGGBB`, named
  colours). Failure handling differs by field and by whether the style is the base one. An unparseable
  **background/border** colour is warned about and that part of the box is skipped. Everywhere else it
  is silent: on the **base** style an unparseable `text_color` falls back to white and an unparseable
  `halo_color` leaves the halo unset; on a **non-base** style an unparseable `text_color` OR
  `halo_color` is simply not applied, so the value inherited from the base style stands.
- **`font_family` is required on the base style.** Non-base styles inherit it; a null one reaching
  the mapper makes `CustomFontMapper` throw on a null
  dictionary key, which the renderer swallows into "disable for this content" (#570).
- `{{ … }}` template expressions are rendered by Scriban before the YAML is parsed.

## Text element fields

Element level (`TextGraphicsElement`):

| Field | Type | Notes |
| --- | --- | --- |
| `name` | string | Display name; **never** used for identity. |
| `text` | string | Scriban-templated. `[styleName]…[/styleName]` selects a style for a span. |
| `base_style` | string | Must name an entry in `styles`, or initialization throws. |
| `styles` | list | See below. |
| `epg_entries` | int | How many EPG entries to expose as `Epg[…]`. |
| `location` | enum | `TopLeft`, `BottomLeft`, … |
| `horizontal_margin_percent` / `vertical_margin_percent` | double | Percent of frame width/height. |
| `width_percent` | double | Percent of frame width. A wrapping/scaling **budget** for the whole element, box included — not a hard clamp; see the geometry notes. |
| `text_fit` | `None` \| `Wrap` \| `Scale` | Only meaningful with `width_percent`. |
| `text_align` | `Left` \| `Center` \| `Right` | |
| `z_index` | int | |
| `opacity_percent` | int | Ignored when `opacity_expression` is set. |
| `opacity_expression` | string | NCalc expression, e.g. `LinearFadeDuration(content_seconds, 4, 1, 6)`. |
| `include_fonts_from` | string | Directory of `.ttf`/`.otf` to load. |

Background box (#732) — element level, because the engine lays the whole element out as one text block
and rasterises it into one bitmap, so there is exactly one box per element:

| Field | Type | Default | Notes |
| --- | --- | --- | --- |
| `background_color` | string | unset | Unset means no **fill**. A `border_color` alone still draws an outlined box; with neither, there is no box and no insets. |
| `background_opacity_percent` | int | 100 | Clamped to 0–100; multiplies the colour's own alpha, so `#AARRGGBB` and this field compose. |
| `background_padding` | double px | 0 | Space between the text and the box edge, on all four sides. |
| `background_corner_radius` | double px | 0 | Clamped to half the shorter side. |
| `border_color` | string | unset | |
| `border_width` | double px | 1 when `border_color` is set | A border colour with no width draws a hairline rather than nothing. |

Geometry notes:

- Padding and border grow the element's bitmap and are subtracted from the text's wrap/scale budget, so
  `width_percent` applies to the visible box rather than to the text inside it. The inset is rounded up
  to a whole pixel once and the same integer is used on both sides, so the box cannot exceed the budget
  by a rounding remainder.
- **`width_percent` is a budget, not a guarantee.** Wrapping cannot break below a single glyph, so a
  narrow `width_percent` overflows with or without a box — that is pre-existing behaviour, not
  something the box introduced. `text_fit: Scale` shrinks the font instead, down to a 5px floor.
- If the inset does not fit inside the budget at all, the **inset is clamped** (with a logged warning)
  rather than the text being squeezed to nothing, so an oversized `background_padding` stops growing
  its box at the budget instead of producing one several times the requested width.
- The inset is **also capped at `min(frameWidth, frameHeight) / 2`**, with a logged warning, whether or
  not `width_percent` is set. Each field is bounded on its own, but padding and border add up, so
  without this a two-field mistake could allocate a bitmap far larger than the frame it is drawn onto.
- With no box fields set the wrap/scale budget is passed through untouched, so existing overlays keep
  their exact pre-#732 geometry.
- The box is drawn behind the text and fades with the element, so `opacity_expression` dims box and
  text together.
- Sizes are in **pixels at frame scale**, matching `font_size`. They do not scale with resolution.

Per-style (`StyleDefinition`): `name`, `font_family` (required), `font_size`, `font_weight`,
`font_italic`, `text_color`, `letter_spacing`, `line_height`, `halo_color`, `halo_width`, `halo_blur`.
Every one of these overrides the `base_style`'s value when the style declares it — the halo fields
did not until #732, so before that a non-base style silently rendered the base style's halo. The
seeded `on-now-next.yml` is unaffected because all three of its styles declare the same halo: its
rendered bitmap is byte-identical either way. That invariant holds on any host; a specific hash would
not, because the render depends on the host's fonts (see `docs/testing.md`). Re-derive it by rendering
`OnNowNextYaml` through `TextElement` at 1920x1080 with and without the per-style halo merge and
comparing the encoded PNGs — they must be equal, whatever their hash is.

**This is an operator-visible change and belongs in the release notes.** A custom template whose
non-base style declares its own `halo_color` / `halo_width` / `halo_blur` renders differently after
this version — correctly, but differently. So does one using `line_height` with `text_fit: Scale`:
the Scale path now carries line height through, which changes measured height and therefore where the
element anchors.
Halo is drawn by RichTextKit as part of the text, not by the box code — a wide halo **on top of** a
background box over-darkens the glyphs, which is why the seeded template cuts it to 1px.

## The built-in On Now / Next element

`GraphicsElementSeeder` writes `on-now-next.yml` once per database (`graphics.on_now_next_seeded`) and
never overwrites an operator's file. Two rules govern it after that:

- Updating the shipped default → `graphics.seeded-template-upgrade-by-fingerprint`.
- It is attached to channels by default → `graphics.on-now-next-on-by-default`.

Identity is the **filename** (`GraphicsElementDefaults.OnNowNextFileName`), never the editable `Name`;
`GraphicsElementResponseModel.BuiltIn` is derived from it server-side so the SPA never name-matches.

## Tests

`ErsatzTV.Tests/Infrastructure/Graphics/TextElementBackgroundBoxTests.cs` renders real bitmaps and
asserts pixels — the box's failure mode is a silent no-op, which a model-level test cannot see.
Geometry assertions are relative to a no-box baseline so they do not depend on the host's typeface,
and `Baseline_Renders_A_Non_Empty_Bitmap` turns a fontless host into a loud red instead of a suite
that passes vacuously on a 0x0 bitmap (see `docs/testing.md` → font dependency).

Attachment behaviour is covered by `GraphicsElementDefaultAttachTests`,
`GraphicsElementSeederUpgradeTests` and `CreateChannelDefaultGraphicsElementTests`.

### Mutation record (#732)

Every clause below was mutated in turn against this tree and **the whole `ErsatzTV.Tests` project**
re-run; the "Reddens" column is the measured failure set. The working tree was confirmed clean after
each run. Reproduce one by making the edit and running
`dotnet test ErsatzTV.Tests/ErsatzTV.Tests.csproj` — **not** a `--filter`ed run.

Three traps this table is built to avoid:

- **Measure against the whole project, never a per-fixture filter.** A filtered run structurally
  cannot observe a red in another fixture, so it under-reports the failure set while looking precise.
  Thirteen rows below redden more than one test, and two of them span two fixture classes — those two
  are exactly what a per-fixture filter cannot see.
- **A mutation that fails to COMPILE is not a result.** Warnings-as-error turn the obvious mutation
  shapes into build failures — `CS0162` for `if (true) { return; }`, `CS1717` for self-assignment,
  Sonar `S3981` for a constant-folded condition — and a build failure emits no test summary at all,
  which reads exactly like a pass in a scrolled log. Require a summary line, and detect build failure
  with `: error `, not `error CS`.
- **Re-measure the red set; never carry it over.** Test renames and added cases both change which
  tests a clause reddens, so a row written from anything but a fresh run describes a tree that no
  longer exists.

**This table is not an enumeration of the change.** It lists the clauses that have a measured proof;
it makes no claim about the ones that do not appear. A clause absent from it has no proof — which is a
reason to go and check, never evidence that none is possible. A "the only uncovered clause is X"
sentence was tried here and abandoned: it was falsified three times, each time by a different clause,
because it silently becomes false the moment a guard is added without its row.

Known clauses with no red, as examples rather than a closed set. Each was measured, and each is
recorded with why a test could not reach it — an unreachable clause is a reason to say so, not to ship
a test that cannot fail:

- the `float.IsFinite` guard on the `width_percent` budget — .NET saturates float→int conversion, so
  removing it changes nothing observable;
- the `insetPixels == 0 ? maxWidth : …` compatibility branch — a `width_percent` that rounds to zero
  renders identically whether RichTextKit is given a `MaxWidth` of 0 or 1;
- the absent-target early return and the temp-file existence check in `UpgradeUnmodifiedTemplate` —
  behaviourally redundant; the first re-raises as a caught `IOException`, the second guards a delete
  that is already a no-op;
- the inner `catch` around that cleanup delete — it stops a failing delete from replacing the
  exception being unwound (which C# otherwise does, and which would downgrade a real cancellation to a
  swallowed warning). `MockFileSystem` does not propagate an interceptor throw raised during the
  delete, so the clause is correct by language semantics but not reachable from a test here.

Rows are written per clause, not per block: a row naming a whole `if` or a whole style-merge block
hides which individual fields inside it are actually reachable from a test.

| # | Clause mutated | Reddens (measured) |
|---|---|---|
| 1 | DrawBackgroundBox returns immediately | `An_Oversized_Border_Is_Clamped_And_Does_Not_Flood_The_Element`<br>`Background_Color_Fills_The_Box`<br>`Background_Opacity_Percent_Is_Clamped_To_Its_Documented_Range`<br>`Background_Opacity_Percent_Scales_The_Alpha`<br>`Background_Padding_Actually_Insets_The_Text`<br>`Border_Color_Draws_A_Border_Distinct_From_The_Fill`<br>`Border_Color_Without_An_Explicit_Width_Draws_A_Hairline`<br>`Corner_Radius_Rounds_The_Corner_Away` |
| 2 | inset not subtracted from the wrap budget | `Fractional_Padding_Still_Respects_Width_Percent`<br>`Width_Percent_Bounds_The_Whole_Box_Including_Padding` |
| 3 | text painted at (0,0) instead of the inset | `Background_Padding_Actually_Insets_The_Text` |
| 4 | ApplyOpacityPercent returns the colour unchanged | `Background_Opacity_Percent_Is_Clamped_To_Its_Documented_Range`<br>`Background_Opacity_Percent_Scales_The_Alpha` |
| 5 | UpgradeUnmodifiedTemplate call removed | `A_Failed_Replace_After_A_Complete_Temp_Write_Leaves_The_Original_Intact`<br>`A_Failed_Write_Leaves_The_Original_Template_Intact`<br>`An_Unwritable_Template_Does_Not_Fail_Startup`<br>`The_Upgraded_Template_Still_Deserializes_With_A_Resolvable_Base_Style`<br>`Upgrades_An_Untouched_Previous_Default`<br>`Upgrades_An_Untouched_Previous_Default_With_Windows_Line_Endings` |
| 6 | fingerprint check ignored (upgrade unconditionally) | `Leaves_An_Operator_Modified_File_Alone` |
| 7 | line-ending normalisation dropped from Normalize | `Upgrades_An_Untouched_Previous_Default_With_Windows_Line_Endings` |
| 8 | HLS-Direct exclusion removed from the backfill | `Skips_Hls_Direct_Channels_Where_The_Overlay_Cannot_Render` |
| 9 | already-attached filter removed | `Does_Not_Duplicate_An_Existing_Attachment`<br>`While_Armed_A_Restored_Element_Is_Attached_To_Every_Eligible_Channel` |
| 10 | filename check removed from the built-in lookup | `Ignores_A_Non_Built_In_Element_With_A_Different_Filename` |
| 11 | graphics.on_now_next_default_attached guard never fires | `Does_Not_Re_Attach_After_An_Operator_Clears_It` |
| 12 | create-time `ChannelGraphicsDefaults.Attach` call removed from `CreateChannelHandler` | `Attaches_The_Built_In_Element_To_A_New_Channel` |
| 13 | HLS-Direct skip removed from the create path | `Leaves_An_Hls_Direct_Channel_Alone_Because_Nothing_Can_Render_There`<br>`Should_Not_Attach_The_Overlay_To_An_Hls_Direct_Channel` |
| 14 | upgrade no longer catches filesystem exceptions | `A_Failed_Replace_After_A_Complete_Temp_Write_Leaves_The_Original_Intact`<br>`A_Failed_Write_Leaves_The_Original_Template_Intact`<br>`A_Read_Failure_On_The_Template_Does_Not_Fail_Startup`<br>`An_Unwritable_Template_Does_Not_Fail_Startup` |
| 15 | FitTextBlock drops HaloColor again | `The_Scale_Path_Preserves_Halo_Blur`<br>`The_Scale_Path_Preserves_The_Halo` |
| 16 | unrounded inset subtracted from the budget | `Fractional_Padding_Still_Respects_Width_Percent` |
| 17 | inset clamp removed | `An_Oversized_Padding_Is_Clamped_To_The_Largest_That_Fits` |
| 18 | Kind == Text filter dropped from the lookup | `Ignores_A_Same_Named_Element_Of_A_Different_Kind` |
| 19 | Sanitize passes values through unchecked | `Non_Finite_And_Absurd_Box_Values_Do_Not_Corrupt_The_Geometry` |
| 20 | the BOX itself is no longer clamped (only insetPixels) | `An_Oversized_Border_Is_Clamped_And_Does_Not_Flood_The_Element` |
| 21 | duplicate guard removed from EnsureBuiltInElementRow | `Repeated_Seeding_Does_Not_Accumulate_Element_Rows` |
| 22 | EnsureBuiltInElementRow removed from the ALREADY-SEEDED branch | `An_Already_Seeded_Install_Missing_Its_Element_Row_Gets_One`<br>`Repeated_Seeding_Does_Not_Accumulate_Element_Rows` |
| 23 | non-atomic in-place write restored | `A_Failed_Replace_After_A_Complete_Temp_Write_Leaves_The_Original_Intact`<br>`A_Failed_Write_Leaves_The_Original_Template_Intact` |
| 24 | marker written even with nothing resolved | `Stays_Armed_When_There_Is_No_Built_In_Element_To_Attach`<br>`While_Armed_A_Restored_Element_Is_Attached_To_Every_Eligible_Channel` |
| 25 | the atomic replace becomes a non-atomic copy | `A_Failed_Replace_After_A_Complete_Temp_Write_Leaves_The_Original_Intact` |
| 26 | the frame cap on the inset is disarmed | `Non_Finite_And_Absurd_Box_Values_Do_Not_Corrupt_The_Geometry`<br>`The_Inset_Is_Capped_Against_The_Frame_Even_With_No_Width_Percent` |
| 27 | per-style halo overrides dropped from the style merge | `A_Non_Base_Style_Uses_Its_Own_Halo_Blur`<br>`A_Non_Base_Style_Uses_Its_Own_Halo_Not_The_Base_Styles` |
| 28 | FitTextBlock drops HaloBlur from the rebuilt style | `The_Scale_Path_Preserves_Halo_Blur` |
| 29 | FitTextBlock drops LineHeight from the rebuilt style | `The_Scale_Path_Preserves_Line_Height` |
| 30 | per-style halo_blur override dropped from the style merge | `A_Non_Base_Style_Uses_Its_Own_Halo_Blur` |
| 31 | the 0-100 opacity clamp is removed | `Background_Opacity_Percent_Is_Clamped_To_Its_Documented_Range` |
| 32 | the lineup create path stops attaching the built-in element | `Should_Attach_The_Built_In_On_Now_Next_Element` |