What causes "Can't encode WebP" in d2phap/ImageGlass?

Clamp the quality argument to the 0-100 range before calling EncodeLossy.

d2phap/ImageGlass · error · Exception

Can't encode WebP

Error message

Can't encode WebP

What it means

Thrown by EncodeLossy(Bitmap, int) when the native call WebPEncodeBGR/WebPEncodeBGRA returns size 0, meaning libwebp failed to produce any output. The wrapper wraps it as a bare Exception with no error code, so the precise native cause is lost. It usually signals invalid input pointers, an internal allocation failure, or an internal encoder error.

Source

Thrown at v9/Components/ImageGlass.WebP/WebPWrapper.cs:413

        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);
            }
            else
            {
                size = LibWebp.WebPEncodeBGRA(bmpData.Scan0, bmp.Width, bmp.Height, bmpData.Stride, quality, out unmanagedData);
            }

            if (size == 0) throw new Exception("Can't encode WebP");

            // Copy image compress data to output array
            var rawWebP = new byte[size];
            Marshal.Copy(unmanagedData, rawWebP, 0, size);

            return rawWebP;
        }
        catch (Exception ex) { throw ex; }
        finally
        {
            // Unlock the pixels
            if (bmpData != null)
                bmp.UnlockBits(bmpData);

            // Free memory
            if (unmanagedData != IntPtr.Zero)
                LibWebp.WebPFree(unmanagedData);
        }

View on GitHub (pinned to 4a3c4fecef)

Solutions

Clamp the quality argument to the 0-100 range before calling EncodeLossy.
Reduce the bitmap dimensions to give the native allocator headroom.
Switch to the AdvancedEncode API (EncodeLossy(bmp, quality, speed)) which surfaces the WebPEncodingError code for diagnosis.
Verify you are on a 64-bit process if encoding large images.

Example fix

// before
var bytes = webp.EncodeLossy(bmp, quality); // quality may be out of range

// after
int q = Math.Clamp(quality, 0, 100);
var bytes = webp.EncodeLossy(bmp, q);
// or use advanced API to get a real error code:
// webp.EncodeLossy(bmp, q, 4);

Defensive patterns

Strategy: validation

Validate before calling

int q = Math.Clamp(quality, 0, 100);
if (bmp.Width <= 0 || bmp.Height <= 0) throw new ArgumentException("Empty bitmap");
// then call EncodeLossy(bmp, q)

Try / catch

try { var bytes = webp.EncodeLossy(bmp, Math.Clamp(quality, 0, 100)); }
catch (Exception ex) when (ex.Message == "Can't encode WebP")
{ /* switch to advanced API to read WebPEncodingError, or reduce dimensions */ }

Prevention

Always clamp quality to 0-100 before calling the simple encoder.
Prefer the advanced API when you need a real error code.
For large images, encode in a 64-bit process.

When it happens

Trigger: Calling EncodeLossy after the dimension/pixel-format checks pass, but the underlying libwebp simple encoder returns 0. Triggers: corrupted bitmap data, extremely large near-limit dimensions that exhaust memory, or a quality value outside the native library's accepted range passed through (the wrapper does not clamp quality).

Common situations: Passing quality < 0 or > 100 (the wrapper forwards it to libwebp); bitmaps at the dimension ceiling; bitmaps whose Stride is negative or mismatched; running on a 32-bit process that runs out of address space for big frames.

Related errors

AI-assisted analysis of d2phap/ImageGlass@4a3c4fecef (2026-08-13). Data as JSON: /api/errors/fc8bc3ff7700d9e8. Report an issue: GitHub.