add watermark opacity expression (#2266)

* add watermark opacity expression

* implement watermark opacity expression parameters

* minor fixes

CHANGELOG.md

@@ -6,8 +6,15 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 ## [Unreleased]
 ### Added
 - Add *experimental* graphics engine
-  - Permanent watermarks will use new graphics engine
-  - Intermittent watermarks will still use normal overlay pipeline (for now)
+  - `Permanent` watermarks will use new graphics engine
+  - `Opacity Expression` watermarks will use new graphics engine
+  - `Intermittent` watermarks will still use normal overlay pipeline (for now)
+- Add `Opacity Expression` watermark mode
+  - This allows specifying an expression that returns an opacity between 0.0 and 1.0
+  - The expression can use:
+    - `content_seconds` - the total number of seconds the frame is into the content
+    - `channel_seconds` - the total number of seconds the frame is from when the channel started/activated
+    - `time_of_day_seconds` - the total number of seconds the frame is since midnight
 
 ## [25.4.0] - 2025-08-05
 ### Added

ErsatzTV.Application/Streaming/HlsSessionWorker.cs

@@ -32,7 +32,7 @@ public class HlsSessionWorker : IHlsSessionWorker
     private readonly ILogger<HlsSessionWorker> _logger;
     private readonly IMediator _mediator;
     private readonly SemaphoreSlim _slim = new(1, 1);
-    private readonly object _sync = new();
+    private readonly Lock _sync = new();
     private readonly Option<int> _targetFramerate;
     private CancellationTokenSource _cancellationTokenSource;
     private string _channelNumber;
@@ -44,6 +44,7 @@ public class HlsSessionWorker : IHlsSessionWorker
     private HlsSessionState _state;
     private Timer _timer;
     private DateTimeOffset _transcodedUntil;
+    private DateTimeOffset _channelStart;
 
     public HlsSessionWorker(
         IServiceScopeFactory serviceScopeFactory,
@@ -180,6 +181,7 @@ public class HlsSessionWorker : IHlsSessionWorker
             Touch();
             _transcodedUntil = DateTimeOffset.Now;
             PlaylistStart = _transcodedUntil;
+            _channelStart =  _transcodedUntil;
 
             // time shift on-demand playout if needed
             await _mediator.Send(
@@ -428,6 +430,7 @@ public class HlsSessionWorker : IHlsSessionWorker
                 now,
                 startAtZero,
                 realtime,
+                _channelStart,
                 ptsOffset,
                 _targetFramerate);
 

ErsatzTV.Application/Streaming/HlsSessionWorkerV2.cs

@@ -35,6 +35,7 @@ public class HlsSessionWorkerV2 : IHlsSessionWorker
     private HlsSessionState _state;
     private Timer _timer;
     private DateTimeOffset _transcodedUntil;
+    private DateTimeOffset _channelStart;
 
     public HlsSessionWorkerV2(
         IServiceScopeFactory serviceScopeFactory,
@@ -127,6 +128,7 @@ public class HlsSessionWorkerV2 : IHlsSessionWorker
             Touch();
             _transcodedUntil = DateTimeOffset.Now;
             PlaylistStart = _transcodedUntil;
+            _channelStart = _transcodedUntil;
 
             // time shift on-demand playout if needed
             await _mediator.Send(
@@ -307,6 +309,7 @@ public class HlsSessionWorkerV2 : IHlsSessionWorker
             _transcodedUntil,
             startAtZero,
             realtime,
+            _channelStart,
             0,
             _targetFramerate);
 

ErsatzTV.Application/Streaming/Queries/FFmpegProcessRequest.cs

@@ -8,4 +8,5 @@ public record FFmpegProcessRequest(
     DateTimeOffset Now,
     bool StartAtZero,
     bool HlsRealtime,
+    DateTimeOffset ChannelStartTime,
     long PtsOffset) : IRequest<Either<BaseError, PlayoutItemProcessModel>>;

ErsatzTV.Application/Streaming/Queries/GetConcatProcessByChannelNumber.cs

@@ -8,6 +8,7 @@ public record GetConcatProcessByChannelNumber : FFmpegProcessRequest
         DateTimeOffset.Now,
         false,
         true,
+        DateTimeOffset.Now, // unused
         0)
     {
         Scheme = scheme;

ErsatzTV.Application/Streaming/Queries/GetConcatSegmenterProcessByChannelNumber.cs

@@ -8,6 +8,7 @@ public record GetConcatSegmenterProcessByChannelNumber : FFmpegProcessRequest
         DateTimeOffset.Now,
         false,
         true,
+        DateTimeOffset.Now, // unused
         0)
     {
         Scheme = scheme;

ErsatzTV.Application/Streaming/Queries/GetErrorProcess.cs

@@ -13,4 +13,5 @@ public record GetErrorProcess(
     DateTimeOffset.Now,
     true,
     HlsRealtime,
+    DateTimeOffset.Now, // unused
     PtsOffset);

ErsatzTV.Application/Streaming/Queries/GetPlayoutItemProcessByChannelNumber.cs

@@ -6,6 +6,7 @@ public record GetPlayoutItemProcessByChannelNumber(
     DateTimeOffset Now,
     bool StartAtZero,
     bool HlsRealtime,
+    DateTimeOffset ChannelStart,
     long PtsOffset,
     Option<int> TargetFramerate) : FFmpegProcessRequest(
     ChannelNumber,
@@ -13,4 +14,5 @@ public record GetPlayoutItemProcessByChannelNumber(
     Now,
     StartAtZero,
     HlsRealtime,
+    ChannelStart,
     PtsOffset);

ErsatzTV.Application/Streaming/Queries/GetPlayoutItemProcessByChannelNumberHandler.cs

@@ -346,6 +346,7 @@ public class GetPlayoutItemProcessByChannelNumberHandler : FFmpegProcessHandler<
                 playoutItemWithPath.PlayoutItem.FillerKind,
                 inPoint,
                 outPoint,
+                request.ChannelStartTime,
                 request.PtsOffset,
                 request.TargetFramerate,
                 disableWatermarks,

ErsatzTV.Application/Streaming/Queries/GetWrappedProcessByChannelNumber.cs

@@ -12,6 +12,7 @@ public record GetWrappedProcessByChannelNumber : FFmpegProcessRequest
         DateTimeOffset.Now,
         false,
         true,
+        DateTimeOffset.Now, // unused
         0)
     {
         Scheme = scheme;

ErsatzTV.Application/Troubleshooting/Commands/PrepareTroubleshootingPlaybackHandler.cs

@@ -143,6 +143,7 @@ public class PrepareTroubleshootingPlaybackHandler(
             FillerKind.None,
             inPoint,
             outPoint,
+            channelStartTime: DateTimeOffset.Now,
             0,
             None,
             false,

ErsatzTV.Application/Watermarks/Commands/CreateWatermark.cs

@@ -18,6 +18,7 @@ public record CreateWatermark(
     int FrequencyMinutes,
     int DurationSeconds,
     int Opacity,
-    bool PlaceWithinSourceContent) : IRequest<Either<BaseError, CreateWatermarkResult>>;
+    bool PlaceWithinSourceContent,
+    string OpacityExpression) : IRequest<Either<BaseError, CreateWatermarkResult>>;
 
 public record CreateWatermarkResult(int WatermarkId) : EntityIdResult(WatermarkId);

ErsatzTV.Application/Watermarks/Commands/CreateWatermarkHandler.cs

@@ -55,7 +55,8 @@ public class CreateWatermarkHandler : IRequestHandler<CreateWatermark, Either<Ba
                     FrequencyMinutes = request.FrequencyMinutes,
                     DurationSeconds = request.DurationSeconds,
                     Opacity = request.Opacity,
-                    PlaceWithinSourceContent = request.PlaceWithinSourceContent
+                    PlaceWithinSourceContent = request.PlaceWithinSourceContent,
+                    OpacityExpression = request.OpacityExpression
                 };
 
                 if (request.ImageSource == ChannelWatermarkImageSource.Custom)

ErsatzTV.Application/Watermarks/Commands/UpdateWatermark.cs

@@ -19,6 +19,7 @@ public record UpdateWatermark(
     int FrequencyMinutes,
     int DurationSeconds,
     int Opacity,
-    bool PlaceWithinSourceContent) : IRequest<Either<BaseError, UpdateWatermarkResult>>;
+    bool PlaceWithinSourceContent,
+    string OpacityExpression) : IRequest<Either<BaseError, UpdateWatermarkResult>>;
 
 public record UpdateWatermarkResult(int WatermarkId) : EntityIdResult(WatermarkId);

ErsatzTV.Application/Watermarks/Commands/UpdateWatermarkHandler.cs

@@ -53,6 +53,8 @@ public class UpdateWatermarkHandler : IRequestHandler<UpdateWatermark, Either<Ba
         p.DurationSeconds = update.DurationSeconds;
         p.Opacity = update.Opacity;
         p.PlaceWithinSourceContent = update.PlaceWithinSourceContent;
+        p.OpacityExpression = update.Mode is ChannelWatermarkMode.OpacityExpression ? update.OpacityExpression : null;
+
         await dbContext.SaveChangesAsync();
         _searchTargets.SearchTargetsChanged();
         return new UpdateWatermarkResult(p.Id);

ErsatzTV.Application/Watermarks/Mapper.cs

@@ -20,5 +20,6 @@ internal static class Mapper
             watermark.FrequencyMinutes,
             watermark.DurationSeconds,
             watermark.Opacity,
-            watermark.PlaceWithinSourceContent);
+            watermark.PlaceWithinSourceContent,
+            watermark.OpacityExpression);
 }

ErsatzTV.Application/Watermarks/WatermarkViewModel.cs

@@ -18,5 +18,6 @@ public record WatermarkViewModel(
     int FrequencyMinutes,
     int DurationSeconds,
     int Opacity,
-    bool PlaceWithinSourceContent
+    bool PlaceWithinSourceContent,
+    string OpacityExpression
 );

ErsatzTV.Core/Domain/ChannelWatermark.cs

@@ -19,13 +19,15 @@ public class ChannelWatermark
     public int DurationSeconds { get; set; }
     public int Opacity { get; set; }
     public bool PlaceWithinSourceContent { get; set; }
+    public string OpacityExpression { get; set; }
 }
 
 public enum ChannelWatermarkMode
 {
     None = 0,
     Permanent = 1,
-    Intermittent = 2
+    Intermittent = 2,
+    OpacityExpression = 3
 }
 
 public enum ChannelWatermarkImageSource

ErsatzTV.Core/FFmpeg/FFmpegLibraryProcessService.cs

@@ -73,6 +73,7 @@ public class FFmpegLibraryProcessService : IFFmpegProcessService
         FillerKind fillerKind,
         TimeSpan inPoint,
         TimeSpan outPoint,
+        DateTimeOffset channelStartTime,
         long ptsOffset,
         Option<int> targetFramerate,
         bool disableWatermarks,
@@ -348,8 +349,12 @@ public class FFmpegLibraryProcessService : IFFmpegProcessService
         Option<GraphicsEngineInput> graphicsEngineInput = Option<GraphicsEngineInput>.None;
         Option<GraphicsEngineContext> graphicsEngineContext = Option<GraphicsEngineContext>.None;
 
-        // use graphics engine for permanent watermarks
-        foreach (var options in watermarkOptions.Where(o => o.Watermark.Map(wm => wm.Mode is ChannelWatermarkMode.Permanent).IfNone(false)))
+        // use graphics engine for permanent watermarks, or opacity expressions
+        var maybeGraphicsEngineWatermark = watermarkOptions
+            .Where(o => o.Watermark
+                .Map(wm => wm.Mode is ChannelWatermarkMode.Permanent or ChannelWatermarkMode.OpacityExpression)
+                .IfNone(false));
+        foreach (var options in maybeGraphicsEngineWatermark)
         {
             watermarkInputFile = Option<WatermarkInputFile>.None;
 
@@ -361,6 +366,9 @@ public class FFmpegLibraryProcessService : IFFmpegProcessService
                 [watermark],
                 channel.FFmpegProfile.Resolution,
                 await playbackSettings.FrameRate.IfNoneAsync(24),
+                ChannelStartTime: channelStartTime,
+                ContentStartTime: start,
+                await playbackSettings.StreamSeek.IfNoneAsync(TimeSpan.Zero),
                 finish - now);
         }
 

ErsatzTV.Core/Interfaces/FFmpeg/IFFmpegProcessService.cs

@@ -37,6 +37,7 @@ public interface IFFmpegProcessService
         FillerKind fillerKind,
         TimeSpan inPoint,
         TimeSpan outPoint,
+        DateTimeOffset channelStartTime,
         long ptsOffset,
         Option<int> targetFramerate,
         bool disableWatermarks,

ErsatzTV.Core/Interfaces/Streaming/GraphicsEngineContext.cs

@@ -7,6 +7,9 @@ public record GraphicsEngineContext(
     List<GraphicsElementContext> Elements,
     Resolution FrameSize,
     int FrameRate,
+    DateTimeOffset ChannelStartTime,
+    DateTimeOffset ContentStartTime,
+    TimeSpan Seek,
     TimeSpan Duration);
 
 public abstract record GraphicsElementContext;

ErsatzTV.Core/Interfaces/Streaming/IGraphicsElement.cs

@@ -4,7 +4,14 @@ namespace ErsatzTV.Core.Interfaces.Streaming;
 
 public interface IGraphicsElement
 {
+    bool IsFailed { get; set; }
+
     Task InitializeAsync(Resolution frameSize, int frameRate, CancellationToken cancellationToken);
 
-    void Draw(object context, TimeSpan timestamp);
+    void Draw(
+        object context,
+        TimeSpan timeOfDay,
+        TimeSpan contentTime,
+        TimeSpan channelTime,
+        CancellationToken cancellationToken);
 }

ErsatzTV.Infrastructure.MySql/Migrations/20250806161001_Add_ChannelWatermarkOpacityExpression.cs

@@ -0,0 +1,29 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ErsatzTV.Infrastructure.MySql.Migrations
+{
+    /// <inheritdoc />
+    public partial class Add_ChannelWatermarkOpacityExpression : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "OpacityExpression",
+                table: "ChannelWatermark",
+                type: "longtext",
+                nullable: true)
+                .Annotation("MySql:CharSet", "utf8mb4");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "OpacityExpression",
+                table: "ChannelWatermark");
+        }
+    }
+}

ErsatzTV.Infrastructure.MySql/Migrations/TvContextModelSnapshot.cs

@@ -379,6 +379,9 @@ namespace ErsatzTV.Infrastructure.MySql.Migrations
                     b.Property<int>("Opacity")
                         .HasColumnType("int");
 
+                    b.Property<string>("OpacityExpression")
+                        .HasColumnType("longtext");
+
                     b.Property<string>("OriginalContentType")
                         .HasColumnType("longtext");
 

ErsatzTV.Infrastructure.Sqlite/Migrations/20250806160643_Add_ChannelWatermarkOpacityExpression.cs

@@ -0,0 +1,28 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ErsatzTV.Infrastructure.Sqlite.Migrations
+{
+    /// <inheritdoc />
+    public partial class Add_ChannelWatermarkOpacityExpression : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "OpacityExpression",
+                table: "ChannelWatermark",
+                type: "TEXT",
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "OpacityExpression",
+                table: "ChannelWatermark");
+        }
+    }
+}

ErsatzTV.Infrastructure.Sqlite/Migrations/TvContextModelSnapshot.cs

@@ -364,6 +364,9 @@ namespace ErsatzTV.Infrastructure.Sqlite.Migrations
                     b.Property<int>("Opacity")
                         .HasColumnType("INTEGER");
 
+                    b.Property<string>("OpacityExpression")
+                        .HasColumnType("TEXT");
+
                     b.Property<string>("OriginalContentType")
                         .HasColumnType("TEXT");
 

ErsatzTV.Infrastructure/Streaming/GraphicsEngine.cs

@@ -1,12 +1,13 @@
 using System.IO.Pipelines;
 using ErsatzTV.Core.Interfaces.Streaming;
+using Microsoft.Extensions.Logging;
 using SixLabors.ImageSharp;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing;
 
 namespace ErsatzTV.Infrastructure.Streaming;
 
-public class GraphicsEngine : IGraphicsEngine
+public class GraphicsEngine(ILogger<GraphicsEngine> logger) : IGraphicsEngine
 {
     public async Task Run(GraphicsEngineContext context, PipeWriter pipeWriter, CancellationToken cancellationToken)
     {
@@ -36,7 +37,18 @@ public class GraphicsEngine : IGraphicsEngine
         {
             while (!cancellationToken.IsCancellationRequested && frameCount < totalFrames)
             {
-                var timestamp = TimeSpan.FromSeconds(frameCount / context.FrameRate);
+                // seconds since this specific stream started
+                double streamTimeSeconds = (double)frameCount / context.FrameRate;
+                var streamTime = TimeSpan.FromSeconds(streamTimeSeconds);
+
+                // `content_seconds` - the total number of seconds the frame is into the content
+                var contentTime = context.Seek + streamTime;
+
+                // `time_of_day_seconds` - the total number of seconds the frame is since midnight
+                var frameTime = context.ContentStartTime + contentTime;
+
+                // `channel_seconds` - the total number of seconds the frame is from when the channel started/activated
+                var channelTime = frameTime - context.ChannelStartTime;
 
                 using var outputFrame = new Image<Bgra32>(
                     context.FrameSize.Width,
@@ -48,7 +60,20 @@ public class GraphicsEngine : IGraphicsEngine
                 {
                     foreach (var element in elements)
                     {
-                        element.Draw(ctx, timestamp);
+                        try
+                        {
+                            if (!element.IsFailed)
+                            {
+                                element.Draw(ctx, frameTime.TimeOfDay, contentTime, channelTime, cancellationToken);
+                            }
+                        }
+                        catch (Exception ex)
+                        {
+                            element.IsFailed = true;
+                            logger.LogWarning(ex,
+                                "Failed to draw graphics element of type {Type}; will disable for this content",
+                                element.GetType().Name);
+                        }
                     }
                 });
 

ErsatzTV.Infrastructure/Streaming/WatermarkElement.cs

@@ -1,7 +1,9 @@
+using System.Globalization;
 using ErsatzTV.Core.Domain;
 using ErsatzTV.Core.FFmpeg;
 using ErsatzTV.Core.Interfaces.Streaming;
 using ErsatzTV.FFmpeg.State;
+using NCalc;
 using SixLabors.ImageSharp;
 using SixLabors.ImageSharp.Formats.Gif;
 using SixLabors.ImageSharp.Processing;
@@ -16,6 +18,7 @@ public class WatermarkElement : IGraphicsElement, IDisposable
     private readonly List<Image> _scaledFrames = [];
     private readonly List<double> _frameDelays = [];
 
+    private Expression _expression;
     private double _animatedDurationSeconds;
     private Image _sourceImage;
     private Point _location;
@@ -36,8 +39,20 @@ public class WatermarkElement : IGraphicsElement, IDisposable
 
     public bool IsValid => _imagePath != null && _watermark != null;
 
+    public bool IsFailed { get; set; }
+
     public async Task InitializeAsync(Resolution frameSize, int frameRate, CancellationToken cancellationToken)
     {
+        if (_watermark.Mode is ChannelWatermarkMode.OpacityExpression && !string.IsNullOrWhiteSpace(_watermark.OpacityExpression))
+        {
+            _expression = new Expression(_watermark.OpacityExpression);
+        }
+        else
+        {
+            float opacity = _watermark.Opacity / 100.0f;
+            _expression = new Expression(opacity.ToString(CultureInfo.InvariantCulture));
+        }
+
         bool isRemoteUri = Uri.TryCreate(_imagePath, UriKind.Absolute, out var uriResult)
                            && (uriResult.Scheme == Uri.UriSchemeHttp || uriResult.Scheme == Uri.UriSchemeHttps);
 
@@ -73,13 +88,12 @@ public class WatermarkElement : IGraphicsElement, IDisposable
             horizontalMargin,
             verticalMargin);
 
-        float opacity = _watermark.Opacity / 100.0f;
         _animatedDurationSeconds = 0;
 
         for (int i = 0; i < _sourceImage.Frames.Count; i++)
         {
             var frame = _sourceImage.Frames.CloneFrame(i);
-            frame.Mutate(ctx => ctx.Resize(scaledWidth, scaledHeight).Opacity(opacity));
+            frame.Mutate(ctx => ctx.Resize(scaledWidth, scaledHeight));
             _scaledFrames.Add(frame);
 
             var frameDelay = _sourceImage.Frames[i].Metadata.GetFormatMetadata(GifFormat.Instance).FrameDelay / 100.0;
@@ -88,17 +102,32 @@ public class WatermarkElement : IGraphicsElement, IDisposable
         }
     }
 
-    public void Draw(object context, TimeSpan timestamp)
+    public void Draw(
+        object context,
+        TimeSpan timeOfDay,
+        TimeSpan contentTime,
+        TimeSpan channelTime,
+        CancellationToken cancellationToken)
     {
         if (context is not IImageProcessingContext imageProcessingContext)
         {
             return;
         }
 
-        Image frameForTimestamp = GetFrameForTimestamp(timestamp);
+        _expression.Parameters["content_seconds"] = contentTime.TotalSeconds;
+        _expression.Parameters["channel_seconds"] = channelTime.TotalSeconds;
+        _expression.Parameters["time_of_day_seconds"] = timeOfDay.TotalSeconds;
+
+        object expressionResult = _expression.Evaluate();
+        float opacity = Convert.ToSingle(expressionResult, CultureInfo.InvariantCulture);
+        Console.WriteLine("opacity is " + opacity);
+        if (opacity == 0)
+        {
+            return;
+        }
 
-        // scaled frames already have opacity set
-        imageProcessingContext.DrawImage(frameForTimestamp, _location, 1f);
+        Image frameForTimestamp = GetFrameForTimestamp(contentTime);
+        imageProcessingContext.DrawImage(frameForTimestamp, _location, opacity);
     }
 
     private Image GetFrameForTimestamp(TimeSpan timestamp)

ErsatzTV.Scanner.Tests/Core/FFmpeg/TranscodingTests.cs

@@ -378,6 +378,7 @@ public class TranscodingTests
             FillerKind.None,
             TimeSpan.Zero,
             TimeSpan.FromSeconds(3),
+            DateTimeOffset.Now,
             0,
             None,
             false,
@@ -656,6 +657,7 @@ public class TranscodingTests
             FillerKind.None,
             TimeSpan.Zero,
             TimeSpan.FromSeconds(3),
+            DateTimeOffset.Now,
             0,
             None,
             false,

ErsatzTV/Controllers/InternalController.cs

@@ -272,6 +272,7 @@ public class InternalController : ControllerBase
             DateTimeOffset.Now,
             false,
             true,
+            DateTimeOffset.Now,
             0,
             Option<int>.None);
 

ErsatzTV/Pages/WatermarkEditor.razor

@@ -14,7 +14,7 @@
 
 <MudForm Model="@_model" @ref="@_form" Validation="@(_validator.ValidateValue)" ValidationDelay="0" Style="max-height: 100%">
     <MudPaper Square="true" Style="display: flex; height: 64px; min-height: 64px; width: 100%; z-index: 100; align-items: center">
-        <MudButton Variant="Variant.Filled" Color="Color.Primary" Class="ml-6" OnClick="HandleSubmitAsync" StartIcon="@(IsEdit ? Icons.Material.Filled.Save : Icons.Material.Filled.Add)">@(IsEdit ? "Save Watermark" : "Add Watermark")</MudButton>
+        <MudButton Variant="Variant.Filled" Color="Color.Primary" Class="ml-6" OnClick="@HandleSubmitAsync" StartIcon="@(IsEdit ? Icons.Material.Filled.Save : Icons.Material.Filled.Add)">@(IsEdit ? "Save Watermark" : "Add Watermark")</MudButton>
     </MudPaper>
     <div class="d-flex flex-column" style="height: 100vh; overflow-x: auto">
         <MudContainer MaxWidth="MaxWidth.ExtraLarge" Class="pt-8">
@@ -34,6 +34,7 @@
                     <MudSelectItem Value="@(ChannelWatermarkMode.None)">None</MudSelectItem>
                     <MudSelectItem Value="@(ChannelWatermarkMode.Permanent)">Permanent</MudSelectItem>
                     <MudSelectItem Value="@(ChannelWatermarkMode.Intermittent)">Intermittent</MudSelectItem>
+                    <MudSelectItem Value="@(ChannelWatermarkMode.OpacityExpression)">Opacity Expression</MudSelectItem>
                 </MudSelect>
             </MudStack>
             <MudStack Row="true" Breakpoint="Breakpoint.SmAndDown" Class="form-field-stack gap-md-8 mb-5">
@@ -51,7 +52,7 @@
                 <div class="d-flex">
                     <MudText>Image</MudText>
                 </div>
-                <InputFile id="watermarkFileInput" OnChange="UploadWatermark" style="display: none;"/>
+                <InputFile id="watermarkFileInput" OnChange="@UploadWatermark" style="display: none;"/>
                 <MudButton HtmlTag="label"
                            Variant="Variant.Filled"
                            Color="Color.Primary"
@@ -176,9 +177,17 @@
                               For="@(() => _model.Opacity)"
                               Adornment="Adornment.End"
                               AdornmentText="%"
-                              Disabled="@(_model.Mode == ChannelWatermarkMode.None)"
+                              Disabled="@(_model.Mode is ChannelWatermarkMode.None or ChannelWatermarkMode.OpacityExpression)"
                               Immediate="true"/>
             </MudStack>
+            <MudStack Row="true" Breakpoint="Breakpoint.SmAndDown" Class="form-field-stack gap-md-8 mb-5">
+                <div class="d-flex">
+                    <MudText>Opacity Expression</MudText>
+                </div>
+                <MudTextField @bind-Value="_model.OpacityExpression"
+                              For="@(() => _model.OpacityExpression)"
+                              Disabled="@(_model.Mode != ChannelWatermarkMode.OpacityExpression)"/>
+            </MudStack>
         </MudContainer>
     </div>
 </MudForm>
@@ -223,7 +232,8 @@
                 FrequencyMinutes = 15,
                 DurationSeconds = 15,
                 Opacity = 100,
-                PlaceWithinSourceContent = false
+                PlaceWithinSourceContent = false,
+                OpacityExpression = string.Empty
             };
         }
     }

ErsatzTV/ViewModels/WatermarkEditViewModel.cs

@@ -27,6 +27,7 @@ public class WatermarkEditViewModel
         DurationSeconds = vm.DurationSeconds;
         Opacity = vm.Opacity;
         PlaceWithinSourceContent = vm.PlaceWithinSourceContent;
+        OpacityExpression = vm.OpacityExpression;
     }
 
     public int Id { get; set; }
@@ -43,6 +44,7 @@ public class WatermarkEditViewModel
     public int DurationSeconds { get; set; }
     public int Opacity { get; set; }
     public bool PlaceWithinSourceContent { get; set; }
+    public string OpacityExpression { get; set; }
 
     public CreateWatermark ToCreate() =>
         new(
@@ -58,7 +60,8 @@ public class WatermarkEditViewModel
             FrequencyMinutes,
             DurationSeconds,
             Opacity,
-            PlaceWithinSourceContent);
+            PlaceWithinSourceContent,
+            OpacityExpression);
 
     public UpdateWatermark ToUpdate() =>
         new(
@@ -75,5 +78,6 @@ public class WatermarkEditViewModel
             FrequencyMinutes,
             DurationSeconds,
             Opacity,
-            PlaceWithinSourceContent);
+            PlaceWithinSourceContent,
+            OpacityExpression);
 }