d2phap/ImageGlass · error · Exception

Failed WebPDecode with error {result}

Error message

Failed WebPDecode with error {result}

What it means

Thrown in WebPWrapper.Decode after LibWebp.WebPDecode returns a status other than VP8_STATUS_OK. By this point the ABI handshake (50), header features (51), and crop bounds (52) have all passed; the failure is in the actual bitstream decode — corrupt pixel data, an unsupported VP8L/VP8X path in the bundled libwebp, truncated stream, or a malformed animated chunk. The outer catch re-wraps the message with '\r\nIn WebP.Decode'.

Source

Thrown at v9/Components/ImageGlass.WebP/WebPWrapper.cs:208

            {
                config.output.colorspace = WEBP_CSP_MODE.MODE_BGR;
                bmp = new Bitmap(config.input.Width, config.input.Height, PixelFormat.Format24bppRgb);
            }
            bmpData = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height), ImageLockMode.WriteOnly, bmp.PixelFormat);

            // Specify the output format
            config.output.u.RGBA.rgba = bmpData.Scan0;
            config.output.u.RGBA.stride = bmpData.Stride;
            config.output.u.RGBA.size = (UIntPtr)(bmp.Height * bmpData.Stride);
            config.output.height = bmp.Height;
            config.output.width = bmp.Width;
            config.output.is_external_memory = 1;

            // Decode
            result = LibWebp.WebPDecode(ptrRawWebP, rawWebP.Length, ref config);
            if (result != VP8StatusCode.VP8_STATUS_OK)
            {
                throw new Exception("Failed WebPDecode with error " + result);
            }
            LibWebp.WebPFreeDecBuffer(ref config.output);

            return bmp;
        }
        catch (Exception ex) { throw new Exception(ex.Message + "\r\nIn WebP.Decode"); }
        finally
        {
            //Unlock the pixels
            if (bmpData != null)
                bmp.UnlockBits(bmpData);

            //Free memory
            if (pinnedWebP.IsAllocated)
                pinnedWebP.Free();
        }
    }

View on GitHub (pinned to 4a3c4fecef)

Solutions

  1. Re-acquire the file from a trusted source to rule out corruption/truncation.
  2. Update libwebp (and the matching ImageGlass.WebP wrapper) to a version that supports the WebP variant (VP8L/VP8X/animated).
  3. Try decoding without scaling/cropping first (use_scaling=0, use_cropping=0) to isolate whether the options or the data are at fault.
  4. Catch the exception and mark the file undecodable; do not retry on the same bytes.

Example fix

// before
var bmp = WebPWrapper.Decode(raw, options);

// after: isolate cause by a plain decode, then handle failure
Bitmap bmp;
try { bmp = WebPWrapper.Decode(raw, options); }
catch (Exception ex) when (ex.Message.Contains("In WebP.Decode"))
{
    Log.Warn($"WebP decode failed for {path}: {ex.Message}");
    bmp = null; // or fall back to another codec
}
Defensive patterns

Strategy: try-catch

Validate before calling

// Rule out scaling/cropping as the cause: try a plain decode first.
if (options.use_scaling == 1 || options.use_cropping == 1)
{ var plain = new WebPDecoderOptions(); WebPWrapper.Decode(rawWebP, plain); }

Try / catch

try { bmp = WebPWrapper.Decode(rawWebP, options); }
catch (Exception ex) when (ex.Message.Contains("WebPDecode") && !ex.Message.Contains("WebPGetFeatures"))
{ /* bitstream decode failure — corrupt/unsupported variant */ }

Prevention

When it happens

Trigger: A WebP whose header is valid but whose compressed payload is corrupt or truncated; a WebP using a feature (animation, ICC, alpha) the loaded libwebp cannot decode; memory exhaustion mid-decode surfacing as a non-OK status.

Common situations: Partially downloaded WebP; bit-flip corruption on disk; WebP produced by a non-standard encoder the libwebp version rejects; very large WebP combined with low memory.

Related errors

AI-assisted analysis of d2phap/ImageGlass@4a3c4fecef (2026-08-13). Data as JSON: /api/errors/ccf7f1b463376f3f. Report an issue: GitHub.