d2phap/ImageGlass · error · NotSupportedException

Bitmap's dimension is too large. Max is

Error message

Bitmap's dimension is too large. Max is {WEBP_MAX_DIMENSION}x{WEBP_MAX_DIMENSION} pixels.

What it means

Thrown by EncodeLossy(Bitmap, int) when the source bitmap's width or height exceeds WEBP_MAX_DIMENSION (16383 pixels), the maximum dimension the libwebp encoder accepts. This is a hard limit enforced by the WebP format itself (the bitstream stores dimensions in 14 bits). The check runs before any native call so it fails fast.

Solutions

Downscale the bitmap so neither dimension exceeds 16383 before calling EncodeLossy (e.g. clone into a smaller Bitmap).
Use the AdvancedEncode path with use_scaling, or pre-resize with Graphics.DrawImage.
Split oversized images into tiles and encode each separately.

Example fix

// before
var bytes = webp.EncodeLossy(hugeBmp, 80);

// after
const int Max = 16383;
int w = hugeBmp.Width, h = hugeBmp.Height;
double scale = Math.Min(1.0, (double)Max / Math.Max(w, h));
using var resized = new Bitmap((int)(w * scale), (int)(h * scale), hugeBmp.PixelFormat);
using (var g = Graphics.FromImage(resized)) { g.DrawImage(hugeBmp, 0, 0, resized.Width, resized.Height); }
var bytes = webp.EncodeLossy(resized, 80);

Defensive patterns

Strategy: validation

Validate before calling

const int WEBP_MAX_DIMENSION = 16383;
if (bmp.Width > WEBP_MAX_DIMENSION || bmp.Height > WEBP_MAX_DIMENSION)
    throw new ArgumentOutOfRangeException(nameof(bmp),
        $"Bitmap {bmp.Width}x{bmp.Height} exceeds WebP max {WEBP_MAX_DIMENSION}x{WEBP_MAX_DIMENSION}.");

Type guard

static bool IsWithinWebPDimensions(Bitmap bmp) =>
    bmp.Width > 0 && bmp.Height > 0 &&
    bmp.Width <= 16383 && bmp.Height <= 16383;

Try / catch

try { var bytes = webp.EncodeLossy(bmp, quality); }
catch (NotSupportedException ex) when (ex.Message.Contains("dimension is too large"))
{ /* downscale and retry, or report */ }

Prevention

Pre-validate every bitmap's dimensions against 16383 before any WebP encode.
Centralize a 'prepare for WebP' helper that resizes and converts pixel format.
Treat dimension/pixel-format validation as a pipeline stage, not an afterthought.

When it happens

Trigger: Calling EncodeLossy(bmp, quality) or Save(bmp, path, quality) with a Bitmap whose Width or Height is greater than 16383. Most often a very large panorama, a tiled image, or an image loaded at a DPI-scaled resolution.

Common situations: Stitching photos into one huge canvas; loading a high-DPI scan; passing a 20000x10000 Bitmap; forgetting that System.Drawing can load bitmaps far larger than WebP can encode.

Related errors

AI-assisted analysis of d2phap/ImageGlass@4a3c4fecef (2026-08-13). Data as JSON: /api/errors/ff63bb3eab992e74. Report an issue: GitHub.

Appendix: source

Thrown at v9/Components/ImageGlass.WebP/WebPWrapper.cs:388

        // Encode in webP format
        var rawWebP = EncodeLossy(bmp, quality);

        // Write webP file
        File.WriteAllBytes(pathFileName, rawWebP);
    }

    /// <summary>Lossy encoding bitmap to WebP (Simple encoding API)</summary>
    /// <param name="bmp">Bitmap with the image</param>
    /// <param name="quality">Between 0 (lower quality, lowest file size) and 100 (highest quality, higher file size)</param>
    /// <returns>Compressed data</returns>
    public byte[] EncodeLossy(Bitmap bmp, int quality = 75)
    {
        // test bmp
        if (bmp.Width == 0 || bmp.Height == 0)
            throw new ArgumentException("Bitmap contains no data.", nameof(bmp));

        if (bmp.Width > WEBP_MAX_DIMENSION || bmp.Height > WEBP_MAX_DIMENSION)
            throw new NotSupportedException($"Bitmap's dimension is too large. Max is {WEBP_MAX_DIMENSION}x{WEBP_MAX_DIMENSION} pixels.");

        if (bmp.PixelFormat != PixelFormat.Format24bppRgb && bmp.PixelFormat != PixelFormat.Format32bppArgb)
            throw new NotSupportedException("Only support Format24bppRgb and Format32bppArgb pixelFormat.");

        BitmapData? bmpData = null;
        var unmanagedData = IntPtr.Zero;

        try
        {
            int size;

            // Get bmp data
            bmpData = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height), ImageLockMode.ReadOnly, bmp.PixelFormat);

            // Compress the bmp data
            if (bmp.PixelFormat == PixelFormat.Format24bppRgb)
            {
                size = LibWebp.WebPEncodeBGR(bmpData.Scan0, bmp.Width, bmp.Height, bmpData.Stride, quality, out unmanagedData);

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