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The personal website of Scott W Harden

Rename Image Files by Capture Date with C#

How to rename a folder of JPEGs by the creation date stored in their metadata
October 12, 2021
csharp

I have hundreds of folders containing thousands of photographs I wish to move to a single directory. Images are all JPEG format, but unfortunately they have filenames like DSC_0123.JPG. This means there are many identical filenames (e.g., every folder has a DSC_0001.JPG) and also the sorted list of filenames would not be in chronological order.

JPEG files have an Exif header and most cameras store the acquisition date of photographs as metadata. I wrote a C# application to scan a folder of images and rename them all by the date and time in their header. This problem has been solved many ways (googling reveals many solutions), but I thought I’d see what it looks like to solve this problem with C#. I reached for the MetadataExtractor package on NuGet. My solution isn’t fancy but it gets the job done.

foreach (string sourceImage in System.IO.Directory.GetFiles("../sourceFolder", "*.jpg"))
{
    DateTime dt = GetJpegDate(sourceImage);
    string fileName = $"{dt:yyyy-MM-dd HH.mm.ss}.jpg";
    string filePath = Path.Combine("../outputFolder", fileName);
    System.IO.File.Copy(sourceImage, filePath, true);
    Console.WriteLine($"{sourceImage} -> {filePath}");
}

DateTime GetJpegDate(string filePath)
{
    var directories = MetadataExtractor.ImageMetadataReader.ReadMetadata(filePath);

    foreach (var directory in directories)
    {
        foreach (var tag in directory.Tags)
        {
            if (tag.Name == "Date/Time Original")
            {
                if (string.IsNullOrEmpty(tag.Description))
                    continue;
                string d = tag.Description.Split(" ")[0].Replace(":", "-");
                string t = tag.Description.Split(" ")[1];
                return DateTime.Parse($"{d} {t}");
            }
        }
    }

    throw new InvalidOperationException($"Date not found in {filePath}");
}

> dotnet run
DSC_0121.JPG -> 2016-09-30 21.51.25.jpg
DSC_0122.JPG -> 2016-10-03 21.42.05.jpg
DSC_0123.JPG -> 2016-10-09 23.04.05.jpg
DSC_0124.JPG -> 2016-10-09 23.05.48.jpg
DSC_0125.JPG -> 2016-11-30 23.07.56.jpg
DSC_0126.JPG -> 2017-01-01 19.16.56.jpg
DSC_0127.JPG -> 2017-01-01 19.17.09.jpg

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Managing Secrets in .NET Console Apps

How to use dotnet user-secrets to store and retrieve secrets and set environment variables in .NET applications.
October 9, 2021
csharp

Sometimes your code needs to work with secrets that you don’t want to risk accidentally leaking on the internet. There are many strategies for solving this problem, and here I share my preferred approach. I see a lot of articles about how to manage user secrets in ASP.NET and other web applications, but not many focusing on console or desktop applications.

User Secrets in C# Applications

1. Create a New .NET App

dotnet new console

2. Set the Secrets ID

OR

OR

<PropertyGroup>
  <UserSecretsId>ee35bcf4-291d-11ec-9dc7-7f3593499a27</UserSecretsId>
</PropertyGroup>

3. Add Secrets Locally

dotnet user-secrets set username me@example.com
dotnet user-secrets set password mySecretPass123

4. Install the UserSecrets Package

dotnet add package Microsoft.Extensions.Configuration.UserSecrets

💡 CAREFUL: If you accidentally install Microsoft.Extensions.Configuration instead of Microsoft.Extensions.Configuration.UserSecrets you won’t have access to AddUserSecrets()

5. Access Secrets in Code

using Microsoft.Extensions.Configuration;

var config = new ConfigurationBuilder().AddUserSecrets<Program>().Build();
string username = config["username"];
string password = config["password"];

NOTE: If a key does not exist its value will be null

Environment Variables

Cloud platforms (GitHub Actions, Azure, etc.) often use environment variables to manage secrets. Using local user secrets to populate environment variables is a useful way to locally develop applications that will run in the cloud.

This example shows how to populate environment variables from user secrets:

using Microsoft.Extensions.Configuration;

class Program
{
    static void Main()
    {
        SetEnvironmentVariablesFromUserSecrets();
        string username = Environment.GetEnvironmentVariable("username");
        string password = Environment.GetEnvironmentVariable("password");
    }

    static void SetEnvironmentVariablesFromUserSecrets()
    {
        var config = new ConfigurationBuilder().AddUserSecrets<Program>().Build();
        foreach (var child in config.GetChildren())
        {
            Environment.SetEnvironmentVariable(child.Key, child.Value);
        }
    }
}

GitHub Actions

GitHub Actions makes it easy to load repository secrets into environment variables. See GitHub / Encrypted secrets for more information about how to add secrets to your repository.

This example snippet of a GitHub action loads two GitHub repository secrets (USERNAME and PASSWORD) as environment variables (username and password) that can be read by my unit tests using Environment.GetEnvironmentVariable() as shown above.

    steps:
      ...
      - name: 🧪 Run Tests
        env:
          username: ${{ secrets.USERNAME }}
          password: ${{ secrets.PASSWORD }}
        run: dotnet test ./src

Conclusions

Using these strategies I am able to write code that seamlessly accesses secrets locally on my dev machine and from environment variables when running in the cloud. Since this strategy does not store secrets inside my project folder, the chance of accidentally committing a .env or other secrets file to source control approaches zero.

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Drawing with Maui.Graphics

How to use Maui.Graphics to draw and create animations in a Windows Forms and WPF applications
September 10, 2021
csharp maui graphics

.NET MAUI (Multi-Platform Application User Interface) is a new framework for creating cross-platform apps using C#. MAUI will be released as part of .NET 6 in November 2021 and it is expected to come with Maui.Graphics, a cross-platform drawing library superior to System.Drawing in many ways. Although System.Drawing.Common currently supports rendering in Linux and MacOS, cross-platform support for System.Drawing will sunset over the next few releases and begin throwing a PlatformNotSupportedException in .NET 6.

By creating a graphics model using only Maui.Graphics dependencies, users can share drawing code across multiple rendering technologies (GDI, Skia, SharpDX, etc.), operating systems (Windows, Linux, MacOS, etc.), and application frameworks (WinForms, WPF, Maui, WinUI, etc.). This page demonstrates how to create a platform-agnostic graphics model and render it using Windows Forms and WPF. Resources in the Maui.Graphics namespace can be used by any modern .NET application (not just Maui apps).

⚠️ WARNING: Maui.Graphics is still a pre-release experimental library (as noted on their GitHub page). Although the code examples on this page work presently, the API may change between now and the official release.

1. Create a new Project

For this example I will start by creating a a .NET 5.0 WinForms project from scratch. Later we will extend the solution to include a WPF project that uses the same graphics model.

2. Add References to Maui.Graphics

We need to get the windows forms MAUI control and all its dependencies. At the time of writing (September, 2021) these packages are not yet available on NuGet, but they can be downloaded from the Microsoft.Maui.Graphics GitHub page.

💡 Tip: If you’re developing a desktop application you can improve the “rebuild all” time by editing the csproj files of your dependencies so TargetFrameworks only includes .NET Standard targets.

3. Create a Drawable Object

A drawable is a class that implements IDrawable, has a Draw() method, and can be rendered anywhere Maui.Graphics is supported. By only depending on Maui.Graphics it’s easy to create a graphics model that can be used on any operating system using any supported graphical framework.

using Microsoft.Maui.Graphics;

This drawable fills the image blue and renders 1,000 randomly-placed anti-aliased semi-transparent white lines on it. Note that the location of the lines depends on the size of the render field (passed-in as an argument).

public class RandomLines : IDrawable
{
    public void Draw(ICanvas canvas, RectangleF dirtyRect)
    {
        canvas.FillColor = Color.FromArgb("#003366");
        canvas.FillRectangle(dirtyRect);

        canvas.StrokeSize = 1;
        canvas.StrokeColor = Color.FromRgba(255, 255, 255, 100);
        Random Rand = new();
        for (int i = 0; i < 1000; i++)
        {
            canvas.DrawLine(
                x1: (float)Rand.NextDouble() * dirtyRect.Width,
                y1: (float)Rand.NextDouble() * dirtyRect.Height,
                x2: (float)Rand.NextDouble() * dirtyRect.Width,
                y2: (float)Rand.NextDouble() * dirtyRect.Height);
        }
    }
}

4. Add a GraphicsView Control

Drag/drop a GraphicsView control from the Toolbox onto your application and assign your graphics model to its Drawable field.

For animations you can use a timer to invalidate the control (forcing a redraw) automatically every 20 ms.

Windows Forms (Rendering with GDI)

Windows Forms applications may also want to intercept SizeChanged events to force redraws as the window is resized.

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        gdiGraphicsView1.Drawable = new RandomLines();
    }

    private void GdiGraphicsView1_SizeChanged(object? sender, EventArgs e) =>
        gdiGraphicsView1.Invalidate();

    private void timer1_Tick(object sender, EventArgs e) =>
        gdiGraphicsView1.Invalidate();
}

I found performance to be quite adequate. On my system 1,000 lines rendered on an 800x600 window at ~60 fps. Like System.Drawing this system slows down as a function of image size, so full-screen 1920x1080 animation was much slower (~10 fps).

WPF (Rendering with SkiaSharp)

<Skia:WDSkiaGraphicsView Name="MyGraphicsView" />

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        MyGraphicsView.Drawable = new RandomLines();

        DispatcherTimer timer = new();
        timer.Interval = TimeSpan.FromMilliseconds(20);
        timer.Tick += Timer_Tick; ;
        timer.Start();
    }

    private void Timer_Tick(object sender, EventArgs e) =>
        MyGraphicsView.Invalidate();
}

Extend the Graphics Model

Since our project is configured to display the same graphics model with both WinForms and WPF, it’s easy to edit the model in one place and it’s updated everywhere. We can replace the random lines model with one that manages randomly colored and sized balls that bounce off the edges of the window as the model advances.

BallField.cs

public class BallField : IDrawable
{
    private readonly Ball[] Balls;

    public BallField(int ballCount)
    {
        Balls = new Ball[ballCount];
    }

    public void Draw(ICanvas canvas, RectangleF dirtyRect)
    {
        canvas.FillColor = Colors.Navy;
        canvas.FillRectangle(dirtyRect);

        foreach (Ball ball in Balls)
        {
            ball?.Draw(canvas);
        }
    }

    public void Randomize(double width, double height)
    {
        Random rand = new();
        for (int i = 0; i < Balls.Length; i++)
        {
            Balls[i] = new()
            {
                X = rand.NextDouble() * width,
                Y = rand.NextDouble() * height,
                Radius = rand.NextDouble() * 5 + 5,
                XVel = rand.NextDouble() - .5,
                YVel = rand.NextDouble() - .5,
                R = (byte)rand.Next(50, 255),
                G = (byte)rand.Next(50, 255),
                B = (byte)rand.Next(50, 255),
            };
        }
    }

    public void Advance(double timeDelta, double width, double height)
    {
        foreach (Ball ball in Balls)
        {
            ball?.Advance(timeDelta, width, height);
        }
    }
}

Ball.cs

public class Ball
{
    public double X;
    public double Y;
    public double Radius = 5;
    public double XVel;
    public double YVel;
    public byte R, G, B;

    public void Draw(ICanvas canvas)
    {
        canvas.FillColor = Color.FromRgb(R, G, B);
        canvas.FillCircle((float)X, (float)Y, (float)Radius);
    }

    public void Advance(double timeDelta, double width, double height)
    {
        MoveForward(timeDelta);
        Bounce(width, height);
    }

    private void MoveForward(double timeDelta)
    {
        X += XVel * timeDelta;
        Y += YVel * timeDelta;
    }

    private void Bounce(double width, double height)
    {
        double minX = Radius;
        double minY = Radius;
        double maxX = width - Radius;
        double maxY = height - Radius;

        if (X < minX)
        {
            X = minX + (minX - X);
            XVel = -XVel;
        }
        else if (X > maxX)
        {
            X = maxX - (X - maxX);
            XVel = -XVel;
        }

        if (Y < minY)
        {
            Y = minY + (minY - Y);
            YVel = -YVel;
        }
        else if (Y > maxY)
        {
            Y = maxY - (Y - maxY);
            YVel = -YVel;
        }
    }
}

Download This Project

To build this project from source code you currently have to download Maui.Graphics source from GitHub and edit the solution file to point to the correct directory containing these projects. This will get a lot easier after Microsoft puts their WinForms and WPF controls on NuGet.

Coding Challenge

Can you recreate this classic screensaver using Maui.Graphics? Bonus points if the user can customize the number of shapes, the number of corners each shape has, and the number of lines drawn in each shape’s history. It’s a fun problem and I encourage you to give it a go! Here’s how I did it: mystify-maui.zip

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Draw with Maui.Graphics and Skia in a C# Console Application

This page describes how to draw graphics in a console application with Maui Graphics and Skia
August 1, 2021
csharp maui

Microsoft’s System.Drawing.Common package is commonly used for cross-platform graphics in .NET Framework and .NET Core applications, but according to the dotnet roadmap System.Drawing will soon only support Windows. As Microsoft sunsets cross-platform support for System.Drawing they will be simultaneously developing Microsoft.Maui.Graphics, a cross-platform graphics library for iOS, Android, Windows, macOS, Tizen and Linux completely in C#.

The Maui.Graphics library can be used in any .NET application (not just MAUI applications). This page documents how I used the Maui.Drawing package to render graphics in memory (using a Skia back-end) and save them as static images from a console application.

I predict Maui.Graphics will eventually evolve to overtake System.Drawing in utilization. It has many advantages for performance and memory management (discussed extensively elsewhere on the internet), but it is still early in development. As of today (July 2021) the Maui.Graphics GitHub page warns “This is an experimental library … There is no official support. Use at your own Risk.”

Maui Graphics Skia Console Quickstart

This program will create an image, fill it with blue, add 1,000 random lines, then draw some text. It is written as a .NET 5 top-level console application and requires the Microsoft.Maui.Graphics and Microsoft.Maui.Graphics.Skia NuGet packages (both are currently in preview).

We use SkiaSharp to create a canvas, but importantly that canvas implements Microsoft.Maui.Graphics.ICanvas (it’s not Skia-specific) so all the methods that draw on it can be agnostic to which rendering system was used. This makes it easy to write generic rendering methods now and have the option to switch the rendering system later.

Program.cs

using System;
using System.IO;
using Microsoft.Maui.Graphics;
using Microsoft.Maui.Graphics.Skia;

// Use Skia to create a Maui graphics context and canvas
BitmapExportContext bmpContext = SkiaGraphicsService.Instance.CreateBitmapExportContext(600, 400);
SizeF bmpSize = new(bmpContext.Width, bmpContext.Height);
ICanvas canvas = bmpContext.Canvas;

// Draw on the canvas with abstract methods that are agnostic to the renderer
ClearBackground(canvas, bmpSize, Colors.Navy);
DrawRandomLines(canvas, bmpSize, 1000);
DrawBigTextWithShadow(canvas, "This is Maui.Graphics with Skia");
SaveFig(bmpContext, Path.GetFullPath("quickstart.jpg"));

static void ClearBackground(ICanvas canvas, SizeF bmpSize, Color bgColor)
{
    canvas.FillColor = Colors.Navy;
    canvas.FillRectangle(0, 0, bmpSize.Width, bmpSize.Height);
}

static void DrawRandomLines(ICanvas canvas, SizeF bmpSize, int count = 1000)
{
    Random rand = new();
    for (int i = 0; i < count; i++)
    {
        canvas.StrokeSize = (float)rand.NextDouble() * 10;

        canvas.StrokeColor = new Color(
            red: (float)rand.NextDouble(),
            green: (float)rand.NextDouble(),
            blue: (float)rand.NextDouble(),
            alpha: .2f);

        canvas.DrawLine(
            x1: (float)rand.NextDouble() * bmpSize.Width,
            y1: (float)rand.NextDouble() * bmpSize.Height,
            x2: (float)rand.NextDouble() * bmpSize.Width,
            y2: (float)rand.NextDouble() * bmpSize.Height);
    }
}

static void DrawBigTextWithShadow(ICanvas canvas, string text)
{
    canvas.FontSize = 36;
    canvas.FontColor = Colors.White;
    canvas.SetShadow(offset: new SizeF(2, 2), blur: 1, color: Colors.Black);
    canvas.DrawString(text, 20, 50, HorizontalAlignment.Left);
}

static void SaveFig(BitmapExportContext bmp, string filePath)
{
    bmp.WriteToFile(filePath);
    Console.WriteLine($"WROTE: {filePath}");
}

MauiGraphicsDemo.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net5.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Maui.Graphics" Version="6.0.100-preview.6.299" />
    <PackageReference Include="Microsoft.Maui.Graphics.Skia" Version="6.0.100-preview.6.299" />
  </ItemGroup>

</Project>

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C# Microphone Level Monitor

How to continuously monitor the level of an audio input (mono or stereo) with C#
July 3, 2021
csharp

This page demonstrates how to continuously monitor microphone input using C#. Code here may be a helpful reference for developers interested in working with mono or stereo data captured from an audio device in real time. This project uses NAudio to provide simple access to the microphone on Windows platforms.

Mono Stereo

Full source code is available on GitHub (Program.cs)

Configure the Audio Input Device

This program starts by creating a WaveInEvent with a WaveFormat that specifies the sample rate, bit depth, and number of channels (1 for mono, 2 for stereo).

We can create a function to handle incoming data and add it to the DataAvailable event handler:

var waveIn = new NAudio.Wave.WaveInEvent
{
    DeviceNumber = 0, // customize this to select your microphone device
    WaveFormat = new NAudio.Wave.WaveFormat(rate: 44100, bits: 16, channels: 1),
    BufferMilliseconds = 50
};
waveIn.DataAvailable += ShowPeakMono;
waveIn.StartRecording();

Analyze Mono Audio Data

This method is called when the incoming audio buffer is filled. One of the arguments gives you access to the raw bytes in the buffer, and it’s up to you to convert them to the appropriate data format.

This example is suitable for 16-bit (two bytes per sample) mono input.

private static void ShowPeakMono(object sender, NAudio.Wave.WaveInEventArgs args)
{
    float maxValue = 32767;
    int peakValue = 0;
    int bytesPerSample = 2;
    for (int index = 0; index < args.BytesRecorded; index += bytesPerSample)
    {
        int value = BitConverter.ToInt16(args.Buffer, index);
        peakValue = Math.Max(peakValue, value);
    }

    Console.WriteLine("L=" + GetBars(peakValue / maxValue));
}

This method converts a level (fraction) into bars suitable to display in the console:

private static string GetBars(double fraction, int barCount = 35)
{
    int barsOn = (int)(barCount * fraction);
    int barsOff = barCount - barsOn;
    return new string('#', barsOn) + new string('-', barsOff);
}

Analyze Stereo Audio Data

When the WaveFormat is configured for 2 channels, bytes in the incoming audio buffer will have left and right channel values interleaved (2 bytes for left, two bytes for right, then repeat). Left and right channels must be treated separately to display independent levels for stereo audio inputs.

This example is suitable for 16-bit (two bytes per sample) stereo input.

private static void ShowPeakStereo(object sender, NAudio.Wave.WaveInEventArgs args)
{
    float maxValue = 32767;
    int peakL = 0;
    int peakR = 0;
    int bytesPerSample = 4;
    for (int index = 0; index < args.BytesRecorded; index += bytesPerSample)
    {
        int valueL = BitConverter.ToInt16(args.Buffer, index);
        peakL = Math.Max(peakL, valueL);
        int valueR = BitConverter.ToInt16(args.Buffer, index + 2);
        peakR = Math.Max(peakR, valueR);
    }

    Console.Write("L=" + GetBars(peakL / maxValue));
    Console.Write(" ");
    Console.Write("R=" + GetBars(peakR / maxValue));
    Console.Write("\n");
}

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Working with 16-bit Images in CSharp

A summary of how I work with 16-bit TIF file data in C# (using ImageMagick and LibTiff).
June 8, 2021
csharp

Scientific image analysis frequently involves working with 12-bit and 14-bit sensor data stored in 16-bit TIF files. Images commonly encountered on the internat are 24-bit or 32-bit RGB images (where each pixel is represented by 8 bits each for red, green, blue, and possibly alpha). Typical image analysis libraries and documentation often lack information about how to work with 16-bit image data.

This page summarizes how I work with 16-bit TIF file data in C#. I prefer Magick.NET (an ImageMagick wrapper) when working with many different file formats, and LibTiff.Net whenever I know my source files will all be identically-formatted TIFs or multidimensional TIFs (stacks).

ImageMagick

ImageMagick is a free and open-source cross-platform software suite for displaying, creating, converting, modifying, and editing raster images. Although ImageMagick is commonly used at the command line, .NET wrappers exist to make it easy to use ImageMagick from within C# applications.

ImageMagick has many packages on NuGet and they are described on ImageMagick’s documentation GitHub page. TLDR: Install the Q16 package (not HDRI) to allow you to work with 16-bit data without losing precision.

ImageMagick is free and distributed under the Apache 2.0 license, so it can easily be used in commercial projects.

An advantage of loading images with ImageMagick is that it will work easily whether the source file is a JPG, PNG, GIF, TIF, or something different. ImageMagick supports over 100 file formats!

Load a 16-bit TIF File as a Pixel Value Array

// Load pixel values from a 16-bit TIF using ImageMagick (Q16)
MagickImage image = new MagickImage("16bit.tif");
ushort[] pixelValues = image.GetPixels().GetValues();

That’s it! The pixelValues array will contain one value per pixel from the original image. The length of this array will equal the image’s height times its width.

Load an 8-bit TIF File as a Pixel Value Array

Since the Q16 package was installed, 2 bytes will be allocated for each pixel (16-bit) even if it only requires one byte (8-bit). In this case you must collect just the bytes you are interested in:

MagickImage image = new MagickImage("8bit.tif");
ushort[] pixelValues = image.GetPixels().GetValues();
int[] values8 = Enumerable.Range(0, pixelValues.Length / 2).Select(x => (int)pixelValues[x * 2 + 1]).ToArray();

Load a 32-bit TIF File as a Pixel Value Array

For this you’ll have to install the high dynamic range (HDRI) Q16 package, then your GetValues() method will return a float[] instead of a ushort[]. Convert these values to proper pixel intensity values by dividing by 2^16.

MagickImage image = new MagickImage("32bit.tif");
float[] pixels = image.GetPixels().GetValues();
for (int i = 0; i < pixels.Length; i++)
    pixels[i] = (long)pixels[i] / 65535;

LibTiff

LibTiff is a pure C# (.NET Standard) TIF file reader. Although it doesn’t support all the image formats that ImageMagick does, it’s really good at working with TIFs. It has a more intuitive interface for working with TIF-specific features such as multi-dimensional images (color, Z position, time, etc.).

LibTiff gives you a lower-level access to the bytes that underlie image data, so it’s on you to perform the conversion from a byte array to the intended data type. Note that some TIFs are little-endian encoded and others are big-endian encoded, and endianness can be read from the header.

LibTiff is distributed under a BSD 3-clause license, so it too can be easily used in commercial projects.

Load a 16-bit TIF as a Pixel Value Array

// Load pixel values from a 16-bit TIF using LibTiff
using Tiff image = Tiff.Open("16bit.tif", "r");

// get information from the header
int width = image.GetField(TiffTag.IMAGEWIDTH)[0].ToInt();
int height = image.GetField(TiffTag.IMAGELENGTH)[0].ToInt();
int bytesPerPixel = image.GetField(TiffTag.BITSPERSAMPLE)[0].ToInt() / 8;

// read the image data bytes
int numberOfStrips = image.NumberOfStrips();
byte[] bytes = new byte[numberOfStrips * image.StripSize()];
for (int i = 0; i < numberOfStrips; ++i)
    image.ReadRawStrip(i, bytes, i * image.StripSize(), image.StripSize());

// convert the data bytes to a double array
if (bytesPerPixel != 2)
    throw new NotImplementedException("this is only for 16-bit TIFs");
double[] data = new double[bytes.Length / bytesPerPixel];
for (int i = 0; i < data.Length; i++)
{
    if (image.IsBigEndian())
        data[i] = bytes[i * 2 + 1] + (bytes[i * 2] << 8);
    else
        data[i] = bytes[i * 2] + (bytes[i * 2 + 1] << 8);
}

Routines for detecting and converting data from 8-bit, 24-bit, and 32-bit TIF files can be created by inspecting bytesPerPixel. LibTiff has documentation describing how to work with RGB TIF files and multi-frame TIFs.

Convert a Pixel Array to a 2D Array

I often prefer to work with scientific image data as a 2D arrays of double values. I write my analysis routines to pass double[,] between methods so the file I/O can be encapsulated in a static class.

// Load pixel values from a 16-bit TIF using ImageMagick (Q16)
MagickImage image = new MagickImage("16bit.tif");
ushort[] pixelValues = image.GetPixels().GetValues();

// create a 2D array of pixel values
double[,] imageData = new double[image.Height, image.Width];
for (int i = 0; i < image.Height; i++)
    for (int j = 0; j < image.Width; j++)
        imageData[i, j] = pixelValues[i * image.Width + j];

Other Libraries

ImageProcessor

According to ImageProcessor’s GitHub page, “ImageProcessor is, and will only ever be supported on the .NET Framework running on a Windows OS” … it doesn’t appear to be actively maintained and is effectively labeled as deprecated, so I won’t spend much time looking further into it.

ImageSharp

As of the time of writing, ImageSharp does not support TIF format, but it appears likely to be supported in a future release.

System.Drawing

Although this library can save images at different depths, it can only load image files with 8-bit depths. System.Drawing does not support loading 16-bit TIFs, so another library must be used to work with these file types.

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Representing Images in Memory

quick reference for programmers interested in working with image data in memory (byte arrays)
June 3, 2021
csharp

This page is a quick reference for programmers interested in working with image data in memory (byte arrays). This topic is straightforward overall, but there are a few traps that aren’t necessarily intuitive so I try my best to highlight those here.

💡 See my newer article, Creating Bitmaps from Scratch in C#

This article assumes you have some programming experience working with byte arrays in a C-type language and have an understanding of what is meant by 32-bit, 24-bit, 16-bit, and 8-bit integers.

Pixel Values

An image is composed of a 2D grid of square pixels, and the type of image greatly influences how much memory each pixel occupies and what format its data is in.

Bits per pixel (bpp) is the number of bits it takes to represent the value a single pixel. This is typically a multiple of 8 bits (1 byte).

Common Pixel Formats

There are others (e.g., 64-bit RGB images), but these are the most typically encountered pixel formats.

Endianness

Endianness describes the order of bytes in a multi-byte value uses to store its data:

Assuming array index values ascend from left to right, 32-bit (4-byte) pixel data can be represented using either of these two formats in memory:

Bitmap images use little-endian integer format! New programmers may expect the bytes that contain “RGB” values to be sequenced in the order “R, G, B”, but this is not the case.

Premultiplied Alpha

Premultiplication refers to the relationship between color (R, G, B) and transparency (alpha). In transparent images the alpha channel may be straight (unassociated) or premultiplied (associated).

With straight alpha, the RGB components represent the full-intensity color of the object or pixel, disregarding its opacity. Later R, G, and B will each be multiplied by the alpha to adjust intensity and transparency.

With premultiplied alpha, the RGB components represent the emission (color and intensity) of each pixel, and the alpha only represents transparency (occlusion of what is behind it). This reduces the computational performance for image processing if transparency isn’t actually used.

In C# using System.Drawing premultiplied alpha is not enabled by default. This must be defined when creating new Bitmap as seen here:

var bmp = new Bitmap(400, 300, PixelFormat.Format32bppPArgb);

Benchmarking reveals the performance enhancement of drawing on bitmaps in memory using premultiplied alpha pixel format. In this test I’m using .NET 5 with the System.Drawing.Common NuGet package. Anti-aliasing is off in this example, but similar results were obtained with it enabled.

Random rand = new(0);
int width = 600;
int height = 400;
var bmp = new Bitmap(600, 400, PixelFormat.Format32bppPArgb);
var gfx = Graphics.FromImage(bmp);
var pen = new Pen(Color.Black);
gfx.Clear(Color.Magenta);
var sw = Stopwatch.StartNew();
for (int i = 0; i < 1e6; i++)
{
    pen.Color = Color.FromArgb(rand.Next());
    gfx.DrawLine(pen, rand.Next(width), rand.Next(height), rand.Next(width), rand.Next(height));
}
Console.WriteLine(sw.Elapsed);
bmp.Save("benchmark.png", ImageFormat.Png);

Time to render 1 million frames:

At the end you have a beautiful figure:

Pixel Locations in Space and Memory

A 2D image is composed of pixels, but addressing them in memory isn’t as trivial as it may seem. The dimensions of bitmaps are stored in their header, and the arrangement of pixels forms rows (left-to-right) then columns (top-to-bottom).

Width and height are the dimensions (in pixels) of the visible image, but…

⚠️ Image size in memory is not just width * height * bytesPerPixel

Because of old hardware limitations, bitmap widths in memory (also called the stride) must be multiplies of 4 bytes. This is effortless when using ARGB formats because each pixel is already 4 bytes, but when working with RGB images it’s possible to have images with an odd number of bytes in each row, requiring data to be padded such that the stride length is a multiple of 4.

// calculate stride length of a bitmap row in memory
int stride = 4 * ((imageWidth * bytesPerPixel + 3) / 4);

Working with Bitmap Bytes in C#

This example demonstrates how to convert a 3D array (X, Y, C) into a flat byte array ready for copying into a bitmap. Notice this code adds padding to the image width to ensure the stride is a multiple of 4 bytes. Notice also the integer encoding is little endian.

public static byte[] GetBitmapBytes(byte[,,] input)
{
    int height = input.GetLength(0);
    int width = input.GetLength(1);
    int bpp = input.GetLength(2);
    int stride = 4 * ((width * bpp + 3) / 4);

    byte[] pixelsOutput = new byte[height * stride];
    byte[] output = new byte[height * stride];

    for (int y = 0; y < height; y++)
        for (int x = 0; x < width; x++)
            for (int z = 0; z < bpp; z++)
                output[y * stride + x * bpp + (bpp - z - 1)] = input[y, x, z];

    return output;
}

For completeness, here’s the complimentary code that converts a flat byte array from bitmap memory to a 3D array (assuming we know the image dimensions and bytes per pixel from reading the image header):

public static byte[,,] GetBitmapBytes3D(byte[] input, int width, int height, int bpp)
{
    int stride = 4 * ((width * bpp + 3) / 4);

    byte[,,] output = new byte[height, width, bpp];
    for (int y = 0; y < height; y++)
        for (int x = 0; x < width; x++)
            for (int z = 0; z < bpp; z++)
                output[y, x, z] = input[stride * y + x * bpp + (bpp - z - 1)];

    return output;
}

Marshalling Bytes in and out of Bitmaps

The code examples above are intentionally simple to focus on the location of pixels in memory and the endianness of their values. To actually convert between byte[] and System.Drawing.Bitmap you must use Marshall.Copy as shown:

public static byte[] BitmapToBytes(Bitmap bmp)
{
    Rectangle rect = new(0, 0, bmp.Width, bmp.Height);
    BitmapData bmpData = bmp.LockBits(rect, ImageLockMode.ReadWrite, bmp.PixelFormat);
    int byteCount = Math.Abs(bmpData.Stride) * bmp.Height;
    byte[] bytes = new byte[byteCount];
    Marshal.Copy(bmpData.Scan0, bytes, 0, byteCount);
    bmp.UnlockBits(bmpData);
    return bytes;
}

public static Bitmap BitmapFromBytes(byte[] bytes, PixelFormat bmpFormat)
{
    Bitmap bmp = new(width, height, bmpFormat);
    var rect = new Rectangle(0, 0, width, height);
    BitmapData bmpData = bmp.LockBits(rect, ImageLockMode.ReadOnly, bmpFormat);
    Marshal.Copy(bytes, 0, bmpData.Scan0, bytes.Length);
    bmp.UnlockBits(bmpData);
    return bmp;
}

How to Create a Bitmap in Memory Without a Graphics Library

The code examples above use System.Drawing.Common to create graphics, but creating bitmaps in a byte[] array is not difficult and can be done in any language. See the Creating Bitmaps from Scratch article for more information.

Reference

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