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 --filtered 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 Background_Color_Fills_The_Box Background_Opacity_Percent_Is_Clamped_To_Its_Documented_Range Background_Opacity_Percent_Scales_The_Alpha Background_Padding_Actually_Insets_The_Text Border_Color_Draws_A_Border_Distinct_From_The_Fill Border_Color_Without_An_Explicit_Width_Draws_A_Hairline Corner_Radius_Rounds_The_Corner_Away
2	inset not subtracted from the wrap budget	Fractional_Padding_Still_Respects_Width_Percent 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 Background_Opacity_Percent_Scales_The_Alpha
5	UpgradeUnmodifiedTemplate call removed	A_Failed_Replace_After_A_Complete_Temp_Write_Leaves_The_Original_Intact A_Failed_Write_Leaves_The_Original_Template_Intact An_Unwritable_Template_Does_Not_Fail_Startup The_Upgraded_Template_Still_Deserializes_With_A_Resolvable_Base_Style Upgrades_An_Untouched_Previous_Default 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 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 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 A_Failed_Write_Leaves_The_Original_Template_Intact A_Read_Failure_On_The_Template_Does_Not_Fail_Startup An_Unwritable_Template_Does_Not_Fail_Startup
15	FitTextBlock drops HaloColor again	The_Scale_Path_Preserves_Halo_Blur 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 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 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 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 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 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