👁 Image
mostlylucid.llmalttext 1.0.0-preview1

Mostlylucid.LlmAltText

👁 NuGet
👁 License: Unlicense

Note: These packages are provided as-is. I'll get them working good enough to release but I can't commit to support. However they are Unlicense so have at it!

AI-powered alt text generation and OCR using Microsoft's Florence-2 Vision Language Model. Automatically generates descriptive, accessible alt text for images and extracts text content.

Features

• Automatic Alt Text Generation: Creates descriptive, accessibility-friendly alt text for any image
• OCR Text Extraction: Extracts text content from images with high accuracy
• Complete Image Analysis: Get both alt text and extracted text in one call
• Image Content Classification: Detects if image is a photograph, document, screenshot, chart, etc.
• Auto Alt Text TagHelper: Automatically populates missing alt text on <img> tags in Razor views
• Multiple Input Sources: Load images from files, URLs, byte arrays, or streams
• Automatic Image Resizing: Large images are automatically downscaled for optimal processing
• Database Caching: SQLite or PostgreSQL caching to avoid regenerating alt text
• Easy Integration: Simple dependency injection setup with .NET
• Configurable: Control model paths, prompts, logging, database, and more
• Production Ready: Built-in error handling, OpenTelemetry support, and logging
• Wide Format Support: JPEG, PNG, GIF, BMP, TIFF, WebP, and more via ImageSharp

Important: Model Downloads

This package automatically downloads AI models (~800MB) on first use.

• Models are downloaded from Microsoft's Florence-2 repository
• Download happens once and models are cached locally
• Requires ~800MB disk space and internet connectivity on first run
• Subsequent runs use cached models (no download required)
• Default model path: ./models (configurable)

Installation

dotnet add package Mostlylucid.LlmAltText

Quick Start

1. Register Services

using Mostlylucid.LlmAltText.Extensions;

// In Program.cs
builder.Services.AddAltTextGeneration();

// Or with custom configuration
builder.Services.AddAltTextGeneration(options =>
{
 options.ModelPath = "./my-models"; // Where to store models
 options.EnableDiagnosticLogging = true; // Detailed logging
 options.MaxWords = 90; // Recommended max words
 options.DefaultTaskType = "MORE_DETAILED_CAPTION";
});

2. Use the Service

using Mostlylucid.LlmAltText.Services;

public class ImageController : ControllerBase
{
 private readonly IImageAnalysisService _imageAnalysis;

 public ImageController(IImageAnalysisService imageAnalysis)
 {
 _imageAnalysis = imageAnalysis;
 }

 [HttpPost("analyze")]
 public async Task<IActionResult> AnalyzeImage(IFormFile image)
 {
 using var stream = image.OpenReadStream();

 // Get complete analysis with content classification
 var result = await _imageAnalysis.AnalyzeWithClassificationAsync(stream);

 return Ok(new
 {
 AltText = result.AltText,
 ExtractedText = result.ExtractedText,
 ContentType = result.ContentType.ToString(),
 Confidence = result.ContentTypeConfidence,
 HasText = result.HasSignificantText
 });
 }
}

API Reference

IImageAnalysisService

The main service interface for image analysis.

Properties

Stream-Based Methods

// Generate alt text from a stream
Task<string> GenerateAltTextAsync(Stream imageStream, string taskType = "MORE_DETAILED_CAPTION");

// Extract text via OCR
Task<string> ExtractTextAsync(Stream imageStream);

// Get both alt text and extracted text
Task<(string AltText, string ExtractedText)> AnalyzeImageAsync(Stream imageStream);

// Complete analysis with content type classification
Task<ImageAnalysisResult> AnalyzeWithClassificationAsync(Stream imageStream);

// Classify content type only
Task<(ImageContentType Type, double Confidence)> ClassifyContentTypeAsync(Stream imageStream);

File-Based Methods

// Generate alt text from a file path
Task<string> GenerateAltTextFromFileAsync(string filePath,
 string taskType = "MORE_DETAILED_CAPTION",
 CancellationToken cancellationToken = default);

// Extract text from a file
Task<string> ExtractTextFromFileAsync(string filePath,
 CancellationToken cancellationToken = default);

// Analyze a file
Task<(string AltText, string ExtractedText)> AnalyzeImageFromFileAsync(string filePath,
 CancellationToken cancellationToken = default);

// Complete analysis of a file
Task<ImageAnalysisResult> AnalyzeWithClassificationFromFileAsync(string filePath,
 CancellationToken cancellationToken = default);

URL-Based Methods

// Generate alt text from a URL (string or Uri overloads)
Task<string> GenerateAltTextFromUrlAsync(string imageUrl,
 string taskType = "MORE_DETAILED_CAPTION",
 CancellationToken cancellationToken = default);

Task<string> GenerateAltTextFromUrlAsync(Uri imageUrl,
 string taskType = "MORE_DETAILED_CAPTION",
 CancellationToken cancellationToken = default);

// Extract text from URL
Task<string> ExtractTextFromUrlAsync(string imageUrl,
 CancellationToken cancellationToken = default);

// Analyze image from URL
Task<(string AltText, string ExtractedText)> AnalyzeImageFromUrlAsync(string imageUrl,
 CancellationToken cancellationToken = default);

// Complete analysis from URL
Task<ImageAnalysisResult> AnalyzeWithClassificationFromUrlAsync(string imageUrl,
 CancellationToken cancellationToken = default);

Byte Array Methods

// Generate alt text from byte array
Task<string> GenerateAltTextAsync(byte[] imageData, string taskType = "MORE_DETAILED_CAPTION");

// Extract text from byte array
Task<string> ExtractTextAsync(byte[] imageData);

// Analyze byte array
Task<(string AltText, string ExtractedText)> AnalyzeImageAsync(byte[] imageData);

// Complete analysis of byte array
Task<ImageAnalysisResult> AnalyzeWithClassificationAsync(byte[] imageData);

Task Types

ImageAnalysisResult

public class ImageAnalysisResult
{
 public required string AltText { get; set; }
 public required string ExtractedText { get; set; }
 public ImageContentType ContentType { get; set; }
 public double ContentTypeConfidence { get; set; } // 0-1
 public bool HasSignificantText { get; set; }
}

Content Types

Usage Examples

Basic Usage with Streams

public async Task<string> GetAltTextFromFormFile(IFormFile file)
{
 using var stream = file.OpenReadStream();
 return await _imageAnalysis.GenerateAltTextAsync(stream);
}

Processing Local Files

// Single file
var altText = await _imageAnalysis.GenerateAltTextFromFileAsync("/path/to/image.jpg");

// Batch processing
var files = Directory.GetFiles("/images", "*.jpg");
foreach (var file in files)
{
 var result = await _imageAnalysis.AnalyzeWithClassificationFromFileAsync(file);
 Console.WriteLine($"{Path.GetFileName(file)}: {result.AltText}");
}

Processing Images from URLs

// From string URL
var altText = await _imageAnalysis.GenerateAltTextFromUrlAsync(
 "https://example.com/photo.jpg");

// From Uri with cancellation
var cts = new CancellationTokenSource(TimeSpan.FromSeconds(30));
var result = await _imageAnalysis.AnalyzeWithClassificationFromUrlAsync(
 new Uri("https://example.com/photo.jpg"),
 cts.Token);

Processing Byte Arrays

// From HttpClient response
var httpClient = new HttpClient();
var imageBytes = await httpClient.GetByteArrayAsync("https://example.com/image.png");
var altText = await _imageAnalysis.GenerateAltTextAsync(imageBytes);

// From base64
var base64 = "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJ...";
var bytes = Convert.FromBase64String(base64);
var result = await _imageAnalysis.AnalyzeWithClassificationAsync(bytes);

Different Task Types

using var stream = file.OpenReadStream();

// Brief caption
var brief = await _imageAnalysis.GenerateAltTextAsync(stream, "CAPTION");
// Result: "A dog sitting on grass"

stream.Position = 0;

// Detailed caption
var detailed = await _imageAnalysis.GenerateAltTextAsync(stream, "DETAILED_CAPTION");
// Result: "A golden retriever sitting on green grass in a park"

stream.Position = 0;

// Most detailed (default, best for accessibility)
var full = await _imageAnalysis.GenerateAltTextAsync(stream, "MORE_DETAILED_CAPTION");
// Result: "A happy golden retriever with light fur sitting on lush green grass
// in a sunny park, with trees visible in the background."

Content Type Classification

var result = await _imageAnalysis.AnalyzeWithClassificationAsync(stream);

switch (result.ContentType)
{
 case ImageContentType.Document:
 // Prioritize extracted text for screen readers
 Console.WriteLine($"Document text: {result.ExtractedText}");
 break;

 case ImageContentType.Screenshot:
 // Include both alt text and extracted UI text
 Console.WriteLine($"Screenshot: {result.AltText}");
 if (result.HasSignificantText)
 Console.WriteLine($"UI Text: {result.ExtractedText}");
 break;

 case ImageContentType.Photograph:
 // Focus on descriptive alt text
 Console.WriteLine($"Photo: {result.AltText}");
 break;

 case ImageContentType.Chart:
 // Include data if extracted
 Console.WriteLine($"Chart: {result.AltText}");
 Console.WriteLine($"Data: {result.ExtractedText}");
 break;
}

Auto Alt Text TagHelper

The TagHelper automatically generates alt text for <img> tags that don't have one.

Enable the TagHelper

// In Program.cs
builder.Services.AddAltTextGeneration(options =>
{
 options.EnableTagHelper = true;
 options.EnableDatabase = true; // Enable caching (recommended)
 options.DbProvider = AltTextDbProvider.Sqlite;
 options.SqliteDbPath = "./alttext.db";
 options.CacheDurationMinutes = 60;
});

// After building the app, migrate the database
var app = builder.Build();
await app.Services.MigrateAltTextDatabaseAsync();

Register TagHelpers in Views

In your _ViewImports.cshtml:

@addTagHelper *, Mostlylucid.LlmAltText

TagHelper Behavior

<img src="https://example.com/photo.jpg" />


<img src="https://example.com/photo.jpg" alt="My custom alt text" />


<img src="https://example.com/decorative.jpg" alt="" />


<img src="https://example.com/photo.jpg" data-skip-alt="true" />


<img src="data:image/png;base64,iVBORw0KGgo..." />

TagHelper Rules

Domain Restrictions

options.AllowedImageDomains = new List<string>
{
 "mycdn.example.com",
 "images.example.org"
};
// Only images from these domains will be processed

PostgreSQL Configuration

builder.Services.AddAltTextGeneration(options =>
{
 options.EnableTagHelper = true;
 options.EnableDatabase = true;
 options.DbProvider = AltTextDbProvider.PostgreSql;
 options.ConnectionString = "Host=localhost;Database=alttext;Username=user;Password=pass";
});

Configuration Options

AltTextOptions

public class AltTextOptions
{
 // Model storage location
 public string ModelPath { get; set; } = "./models";

 // Custom prompt for alt text generation
 public string AltTextPrompt { get; set; } = "Provide 2-3 complete...";

 // Default task type: CAPTION, DETAILED_CAPTION, MORE_DETAILED_CAPTION
 public string DefaultTaskType { get; set; } = "MORE_DETAILED_CAPTION";

 // Enable detailed diagnostic logging
 public bool EnableDiagnosticLogging { get; set; } = true;

 // Maximum recommended word count for alt text
 public int MaxWords { get; set; } = 90;

 // TagHelper settings
 public bool EnableTagHelper { get; set; } = false;
 public bool EnableDatabase { get; set; } = false;
 public bool AutoMigrateDatabase { get; set; } = true;
 public AltTextDbProvider DbProvider { get; set; } = AltTextDbProvider.Sqlite;
 public string SqliteDbPath { get; set; } = "alttext.db";
 public string? ConnectionString { get; set; }

 // Security
 public List<string> AllowedImageDomains { get; set; } = new();
 public List<string> SkipSrcPrefixes { get; set; } = new() { "data:", "blob:" };

 // Caching
 public int CacheDurationMinutes { get; set; } = 60;
}

Configuration Examples

// Production configuration
builder.Services.AddAltTextGeneration(options =>
{
 options.ModelPath = "/var/app/ai-models";
 options.EnableDiagnosticLogging = false;
 options.MaxWords = 100;
 options.EnableTagHelper = true;
 options.EnableDatabase = true;
 options.DbProvider = AltTextDbProvider.PostgreSql;
 options.ConnectionString = Environment.GetEnvironmentVariable("ALTTEXT_DB");
});

// Development configuration
builder.Services.AddAltTextGeneration(options =>
{
 options.ModelPath = "./models";
 options.EnableDiagnosticLogging = true;
 options.EnableTagHelper = true;
 options.EnableDatabase = true;
 options.DbProvider = AltTextDbProvider.Sqlite;
 options.SqliteDbPath = "./dev-alttext.db";
});

// Minimal (no database, no TagHelper)
builder.Services.AddAltTextGeneration();

Image Processing

Automatic Resizing

Large images are automatically downscaled before processing:

• Max dimension: 2048px (width or height)
• Max file size trigger: 20MB
• Resize algorithm: Lanczos3 (high quality)
• Output format: PNG (lossless)

This ensures consistent processing times and prevents memory issues with very large images.

Supported Formats

All formats supported by SixLabors.ImageSharp:

• JPEG/JPG
• PNG
• GIF (first frame)
• BMP
• TIFF
• WebP
• PBM
• TGA
• QOI

OpenTelemetry Support

The package includes built-in OpenTelemetry instrumentation:

// Add tracing
builder.Services.AddOpenTelemetry()
 .WithTracing(tracing =>
 {
 tracing.AddSource("Mostlylucid.LlmAltText");
 });

Traced Activities

• llmalttext.generate_alt_text
• llmalttext.extract_text
• llmalttext.analyze_image
• llmalttext.analyze_with_classification
• llmalttext.classify_content_type

Recorded Metrics

• Image size (bytes)
• Processing duration (ms)
• Alt text length
• Extracted text length
• Content type classification
• Confidence scores
• Success/failure status

Health Checks

public class ImageAnalysisHealthCheck : IHealthCheck
{
 private readonly IImageAnalysisService _service;

 public ImageAnalysisHealthCheck(IImageAnalysisService service)
 {
 _service = service;
 }

 public Task<HealthCheckResult> CheckHealthAsync(
 HealthCheckContext context,
 CancellationToken cancellationToken = default)
 {
 if (_service.IsReady)
 return Task.FromResult(HealthCheckResult.Healthy("AI model ready"));

 return Task.FromResult(HealthCheckResult.Unhealthy("AI model not initialized"));
 }
}

Performance

Tips

• Register as Singleton (model initialization is expensive)
• Enable database caching for repeated images
• Use cancellation tokens for timeout handling
• Process images in parallel with caution (memory usage)

Troubleshooting

Models Not Downloading

Problem: Model download fails or times out

Solutions:

• Check internet connectivity
• Ensure firewall allows downloads from Hugging Face
• Verify sufficient disk space (~800MB)
• Check write permissions on model directory
• Set a custom ModelPath to a writable location

Service Not Ready

Problem: IsReady returns false

Solutions:

• Check logs for initialization errors
• Verify model files exist in ModelPath
• Ensure sufficient memory available (2GB+)
• Check for exceptions during startup

Poor Quality Alt Text

Problem: Generated alt text is not descriptive enough

Solutions:

• Use "MORE_DETAILED_CAPTION" task type (default)
• Customize AltTextPrompt in options
• Ensure input images are clear and well-lit
• Check if image is too small or blurry

TagHelper Not Working

Problem: Alt text not generated for images

Solutions:

• Verify EnableTagHelper = true in options
• Check @addTagHelper in _ViewImports.cshtml
• Ensure images have absolute URLs (relative not supported)
• Check AllowedImageDomains if configured
• Look for data-skip-alt attribute
• Verify database migration ran

Accessibility Best Practices

When using generated alt text:

1. Review AI Output: Always review generated alt text for accuracy
2. Keep It Concise: Aim for 90-100 words maximum
3. Be Descriptive: Include context, subjects, actions, and relationships
4. Avoid Redundancy: Don't start with "Image of..." or "Picture of..."
5. Consider Context: Alt text should serve the image's purpose on the page
6. Use Empty Alt for Decorative: Set alt="" for purely decorative images
7. Include Text in Images: If image contains text, include it in alt or nearby

Requirements

• .NET 8.0 or .NET 9.0
• ~800MB disk space for AI models (one-time download)
• Internet connectivity for initial model download
• Memory: Recommended 2GB+ RAM for model operations
• Optional: SQLite or PostgreSQL for TagHelper caching

License

Unlicense - Public Domain

Contributing

Contributions welcome! Please see the main repository: https://github.com/scottgal/mostlylucidweb

Credits

Built on:

• Florence-2 - Microsoft's Vision Language Model
• SixLabors.ImageSharp - Cross-platform image processing
• Entity Framework Core - Database access
• Microsoft.Extensions.* - Dependency injection and configuration

Support

For issues, questions, or contributions: https://github.com/scottgal/mostlylucidweb/issues