Use substitution values in image alt and title text

src/Elastic.Markdown/Myst/Directives/DirectiveHtmlRenderer.cs

@@ -91,7 +91,8 @@ private static void WriteImage(HtmlRenderer renderer, ImageBlock block)
 		{
 			Label = block.Label,
 			Align = block.Align,
-			Alt = block.Alt,
+			Alt = block.Alt ?? string.Empty,
+			Title = block.Title,
 			Height = block.Height,
 			Scale = block.Scale,
 			Target = block.Target,
@@ -128,7 +129,8 @@ private static void WriteFigure(HtmlRenderer renderer, ImageBlock block)
 		{
 			Label = block.Label,
 			Align = block.Align,
-			Alt = block.Alt,
+			Alt = block.Alt ?? string.Empty,
+			Title = block.Title,
 			Height = block.Height,
 			Scale = block.Scale,
 			Target = block.Target,

src/Elastic.Markdown/Myst/Directives/ImageBlock.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information
 
 using Elastic.Markdown.Diagnostics;
+using Elastic.Markdown.Helpers;
 using Elastic.Markdown.IO;
 using Elastic.Markdown.Myst.InlineParsers;
 
@@ -21,6 +22,11 @@ public class ImageBlock(DirectiveBlockParser parser, ParserContext context)
 	/// </summary>
 	public string? Alt { get; set; }
 
+	/// <summary>
+	/// Title text: a short description of the image
+	/// </summary>
+	public string? Title { get; set; }
+
 	/// <summary>
 	/// The desired height of the image. Used to reserve space or scale the image vertically. When the “scale” option
 	/// is also specified, they are combined. For example, a height of 200px and a scale of 50 is equivalent to
@@ -65,9 +71,10 @@ public class ImageBlock(DirectiveBlockParser parser, ParserContext context)
 	public override void FinalizeAndValidate(ParserContext context)
 	{
 		Label = Prop("label", "name");
-		Alt = Prop("alt");
-		Align = Prop("align");
+		Alt = Prop("alt")?.ReplaceSubstitutions(context) ?? string.Empty;
+		Title = Prop("title")?.ReplaceSubstitutions(context);
 
+		Align = Prop("align");
 		Height = Prop("height", "h");
 		Width = Prop("width", "w");
 
@@ -112,5 +119,3 @@ private void ExtractImageUrl(ParserContext context)
 		}
 	}
 }
-
-

src/Elastic.Markdown/Myst/InlineParsers/DiagnosticLinkInlineParser.cs

@@ -63,38 +63,41 @@ public override bool Match(InlineProcessor processor, ref StringSlice slice)
 
 		ValidateAndProcessLink(link, processor, context);
 
-		ParseStylingInstructions(link);
+		ParseStylingInstructions(link, context);
 
 		return match;
 	}
 
 
-	private static void ParseStylingInstructions(LinkInline link)
+	private static void ParseStylingInstructions(LinkInline link, ParserContext context)
 	{
 		if (!link.IsImage)
 			return;
 
-		if (string.IsNullOrWhiteSpace(link.Title) || link.Title.IndexOf('=') < 0)
-			return;
+		var attributes = link.GetAttributes();
+		var title = link.Title;
 
-		var matches = LinkRegexExtensions.MatchTitleStylingInstructions().Match(link.Title);
-		if (!matches.Success)
+		if (string.IsNullOrEmpty(title))
 			return;
 
-		var width = matches.Groups["width"].Value;
-		if (!width.EndsWith('%'))
-			width += "px";
-		var height = matches.Groups["height"].Value;
-		if (string.IsNullOrEmpty(height))
-			height = width;
-		else if (!height.EndsWith('%'))
-			height += "px";
-		var title = link.Title[..matches.Index];
-
-		link.Title = title;
-		var attributes = link.GetAttributes();
-		attributes.AddProperty("width", width);
-		attributes.AddProperty("height", height);
+		var matches = LinkRegexExtensions.MatchTitleStylingInstructions().Match(title);
+		if (matches.Success)
+		{
+			var width = matches.Groups["width"].Value;
+			if (!width.EndsWith('%'))
+				width += "px";
+			var height = matches.Groups["height"].Value;
+			if (string.IsNullOrEmpty(height))
+				height = width;
+			else if (!height.EndsWith('%'))
+				height += "px";
+
+			attributes.AddProperty("width", width);
+			attributes.AddProperty("height", height);
+
+			title = title[..matches.Index];
+		}
+		link.Title = title?.ReplaceSubstitutions(context);
 	}
 
 	private static bool IsInCommentBlock(LinkInline link) =>
@@ -183,7 +186,7 @@ private static void ProcessCrossLink(LinkInline link, InlineProcessor processor,
 				s => processor.EmitError(link, s),
 				s => processor.EmitWarning(link, s),
 				uri, out var resolvedUri)
-		   )
+			 )
 			link.Url = resolvedUri.ToString();
 	}
 

src/Elastic.Markdown/Slices/Directives/Image.cshtml

@@ -1,6 +1,7 @@
+@using Microsoft.AspNetCore.Mvc.ModelBinding.Validation
 @inherits RazorSlice<ImageViewModel>
 <a class="reference internal image-reference" href="javascript:void(0)" onclick="document.getElementById('modal-@Model.UniqueImageId').style.display='flex'">
-	<img loading="lazy" alt="@Model.Alt" src="@Model.ImageUrl" style="@Model.Style" class="@Model.Screenshot" />
+	<img loading="lazy" title="@Model.Title" alt="@(Model.Alt == string.Empty ? HtmlString.Empty : new HtmlString(Model.Alt))" src="@Model.ImageUrl" style="@Model.Style" class="@Model.Screenshot" />
 	[CONTENT]
 </a>
 
@@ -14,7 +15,7 @@
 			</a>
 		</span>
 		<a class="reference internal image-reference" href="@Model.ImageUrl" target="_blank">
-			<img loading="lazy" alt="@Model.Alt" src="@Model.ImageUrl" />
+			<img loading="lazy" title="@Model.Title"  alt="@(Model.Alt == string.Empty ? HtmlString.Empty : new HtmlString(Model.Alt))" src="@Model.ImageUrl" />
 		</a>
 	</div>
 </div>

src/Elastic.Markdown/Slices/Directives/_ViewModels.cs

@@ -49,7 +49,8 @@ public class ImageViewModel
 {
 	public required string? Label { get; init; }
 	public required string? Align { get; init; }
-	public required string? Alt { get; init; }
+	public required string Alt { get; init; }
+	public required string? Title { get; init; }
 	public required string? Height { get; init; }
 	public required string? Scale { get; init; }
 	public required string? Target { get; init; }

tests/Elastic.Markdown.Tests/Inline/SubstitutionTest.cs

@@ -165,3 +165,43 @@ public void OnlySeesGlobalVariable() =>
 	public void HasNoErrors() => Collector.Diagnostics.Should().HaveCount(0);
 
 }
+
+public class ReplaceInImageAlt(ITestOutputHelper output) : InlineTest(output,
+"""
+---
+sub:
+  hello-world: Hello World
+---
+
+# Testing ReplaceInImageAlt
+
+![{{hello-world}}](_static/img/observability.png)
+"""
+)
+{
+
+	[Fact]
+	public void OnlySeesGlobalVariable() =>
+		Html.Should().NotContain("alt=\"{{hello-world}}\"")
+			.And.Contain("alt=\"Hello World\"");
+}
+
+public class ReplaceInImageTitle(ITestOutputHelper output) : InlineTest(output,
+"""
+---
+sub:
+  hello-world: Hello World
+---
+
+# Testing ReplaceInImageTitle
+
+![Observability](_static/img/observability.png "{{hello-world}}")
+"""
+)
+{
+
+	[Fact]
+	public void OnlySeesGlobalVariable() =>
+		Html.Should().NotContain("title=\"{{hello-world}}\"")
+			.And.Contain("title=\"Hello World\"");
+}

tests/authoring/Blocks/ImageBlocks.fs

@@ -79,3 +79,17 @@ type ``image ref out of scope`` () =
     [<Fact>]
     let ``emits an error image reference is outside of documentation scope`` () =
         docs |> hasError "./img/observability.png` does not exist. resolved to"
+
+type ``empty alt attribute`` () =
+    static let markdown = Setup.Markdown """
+:::{image} img/some-image.png
+:alt:
+:width: 250px
+:::
+"""
+
+    [<Fact>]
+    let ``validate empty alt attribute`` () =
+        markdown |> convertsToContainingHtml """
+            <img loading="lazy" alt src="/img/some-image.png" style="width: 250px;">
+       """