Convert PNG to JPG in C# · oneimage cookbook

Load the PNG, put it on a background with Flatten (JPEG has no transparency), and save it with a .jpg extension. The extension picks the output format, so the same two lines convert between PNG, JPEG, WebP, GIF, TIFF, BMP and ICO in any direction. The program below converts a transparent PNG logo.

Terminal

dotnet new console -o ConvertImage && cd ConvertImage
dotnet add package RecursiveAI.OneImage --version 1.10.4
curl -L -o logo.png https://recursiveai.org/assets/brand/icon-512.png

OneImage needs the .NET 10 SDK. Put this in Program.cs and run dotnet run:

using OneImage;
 
Image png = Image.Load("logo.png");
 
// JPEG can't store transparency: put the logo on white, then save at quality 90.
png.Flatten(new Rgba32(255, 255, 255, 255))
   .Save("logo.jpg", new SaveOptions
   {
       EncoderOptions = new JpegEncoderOptions { Quality = 90 },
   });
 
// Any other direction is a load and a save; the extension picks the encoder.
Image.Load("logo.jpg").Save("logo-from-jpg.png");
png.Save("logo.bmp");
png.Resize(width: 64).Save("favicon.ico");
 
string[] files =
    ["logo.png", "logo.jpg", "logo-from-jpg.png", "logo.bmp", "favicon.ico"];
foreach (string file in files)
{
    ImageInfo info = Image.Identify(file);
    long kb = new FileInfo(file).Length / 1024;
    string size = $"{info.Width}x{info.Height}";
    Console.WriteLine($"{file}: {info.Format.Name} {size}, {kb} KB");
}

Text

logo.png: png 512x512, 7 KB
logo.jpg: jpeg 512x512, 8 KB
logo-from-jpg.png: png 512x512, 17 KB
logo.bmp: bmp 512x512, 1024 KB
favicon.ico: ico 64x64, 16 KB

The input format is detected from the file’s content, not its name, so a PNG saved as photo.jpg still loads. Without Flatten, the JPEG encoder writes whatever color values the transparent pixels store, which is often black.

Convert to a stream

A stream has no extension, so name the format with its Instance:

using OneImage;
 
using var output = new MemoryStream();
Image.Load("logo.png")
     .Flatten(new Rgba32(255, 255, 255, 255))
     .Save(output, JpegFormat.Instance);
Console.WriteLine($"{output.Length} bytes of JPEG in memory");

Text

6858 bytes of JPEG in memory

The format instances are PngFormat, JpegFormat, WebPFormat, GifFormat, TiffFormat, BmpFormat and IcoFormat. SaveAsync does the same for ASP.NET Core responses.

Limits

JPEG is lossy and has no alpha channel. Converting JPEG to PNG makes a larger file, not a better image.
JPEG and BMP hold one image. Saving an animated GIF or multipage TIFF as JPEG writes the first frame; see Split a multipage TIFF.
ICO entries are at most 256×256, so resize first.
Metadata is kept by a plain load and save, including any EXIF GPS position. See Remove EXIF metadata.
Not supported: HEIC and AVIF. SVG, PSD and PDF need their add-on packages (SVG to PNG, PSD to PNG, PDF to image).

For web output, WebP is usually smaller than JPEG. Every encoder option is in the guide.