What causes "Pixel format {format} is not supported" in AvaloniaUI/Avalonia?

Choose a format present in the writer (Rgba8888, Bgra8888, Rgb24, Bgr24, etc.).

AvaloniaUI/Avalonia · error · NotSupportedException

Pixel format {format} is not supported

Error message

Pixel format {format} is not supported

What it means

PixelFormatWriter.Write mirrors the reader: it writes an Rgba8888Pixel span into a destination buffer for the same fixed set of formats. Any PixelFormatEnum outside that set hits the default and throws NotSupportedException. This is the software write-back half of the transcoding pipeline.

Source

Thrown at src/Avalonia.Base/Media/Imaging/PixelFormatWriter.cs:489

                Write<Rgb24PixelFormatWriter>(pixels, dest, size, stride, alphaFormat, srcAlphaFormat);
                break;
            case PixelFormatEnum.Rgb32:
                Write<Rgb32PixelFormatWriter>(pixels, dest, size, stride, alphaFormat, srcAlphaFormat);
                break;
            case PixelFormatEnum.Bgr24:
                Write<Bgr24PixelFormatWriter>(pixels, dest, size, stride, alphaFormat, srcAlphaFormat);
                break;
            case PixelFormatEnum.Bgr32:
                Write<Bgr32PixelFormatWriter>(pixels, dest, size, stride, alphaFormat, srcAlphaFormat);
                break;
            case PixelFormatEnum.Bgr555:
                Write<Bgr555PixelFormatWriter>(pixels, dest, size, stride, alphaFormat, srcAlphaFormat);
                break;
            case PixelFormatEnum.Bgr565:
                Write<Bgr565PixelFormatWriter>(pixels, dest, size, stride, alphaFormat, srcAlphaFormat);
                break;
            default:
                throw new NotSupportedException($"Pixel format {format} is not supported");
        }
    }
}

View on GitHub (pinned to 11c5427268)

Solutions

Choose a format present in the writer (Rgba8888, Bgra8888, Rgb24, Bgr24, etc.).
Ensure the chosen format is also natively supported by the render backend to skip the software writer entirely.
When extending PixelFormat, add the corresponding writer case.

Example fix

// before - format with no writer case -> throws on write-back
new WriteableBitmap(uncommonFormat, AlphaFormat.Premultiplied, size, dpi);

// after
new WriteableBitmap(PixelFormat.Rgba8888, AlphaFormat.Premultiplied, size, dpi);

Defensive patterns

Strategy: validation

Validate before calling

if (!PixelFormatReader.SupportsFormat(format))
    throw new NotSupportedException($"Format {format} has no software writer.");
PixelFormatWriter.Write(pixels, dest, size, stride, format, alphaFormat, srcAlphaFormat);

Type guard

static bool IsWritableFormat(PixelFormat f) => PixelFormatReader.SupportsFormat(f);

Prevention

Use standard formats covered by the writer switch.
Add writer cases when introducing custom formats.
Choose a backend-native format to avoid the software write path.

When it happens

Trigger: Writing converted pixels back into a buffer whose format is not in the writer's switch — happens during software transcoding when the platform can't handle the format and the writer also lacks a case for it.

Common situations: Custom or uncommon PixelFormat with no writer case; creating a WriteableBitmap in an unsupported format and triggering the BitmapMemory transcoding path which calls the writer on lock/commit.

Related errors

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/dd951ff0c32f2207. Report an issue: GitHub.