d2phap/ImageGlass · error · Exception

Can't configure lossless preset

Error message

Can't configure lossless preset

What it means

Thrown by EncodeLossless(Bitmap, int) when LibWebp.WebPConfigLosslessPreset returns 0 for the requested speed level. The wrapper only takes this branch on DLL versions > 1082 (i.e. libwebp that exposes WebPConfigLosslessPreset). Returning 0 means the preset could not be applied, usually because speed is out of the valid 0-9 range or the preset table is unavailable in this build.

Source

Thrown at v9/Components/ImageGlass.WebP/WebPWrapper.cs:546

    /// <summary>Lossless encoding image in bitmap (Advanced encoding API)</summary>
    /// <param name="bmp">Bitmap with the image</param>
    /// <param name="speed">Between 0 (fastest, lowest compression) and 9 (slower, best compression)</param>
    /// <returns>Compressed data</returns>
    public byte[] EncodeLossless(Bitmap bmp, int speed)
    {
        //Initialize configuration structure
        WebPConfig config = new WebPConfig();

        //Set compression parameters
        if (LibWebp.WebPConfigInit(ref config, WebPPreset.WEBP_PRESET_DEFAULT, (speed + 1) * 10) == 0)
            throw new Exception("Can't config preset");

        //Old version of DLL does not support info and WebPConfigLosslessPreset
        if (LibWebp.WebPGetDecoderVersion() > 1082)
        {
            if (LibWebp.WebPConfigLosslessPreset(ref config, speed) == 0)
                throw new Exception("Can't configure lossless preset");
        }
        else
        {
            config.lossless = 1;
            config.method = speed;
            if (config.method > 6)
                config.method = 6;
            config.quality = (speed + 1) * 10;
        }
        config.pass = speed + 1;
        config.thread_level = 1;
        config.alpha_filtering = 2;
        config.use_sharp_yuv = 1;
        config.exact = 0;

        return AdvancedEncode(bmp, config, false);
    }

View on GitHub (pinned to 4a3c4fecef)

Solutions

1. Clamp speed to the documented 0-9 range before calling EncodeLossless(bmp, speed).
2. Verify the libwebp build exposes lossless presets (use a stock build rather than a stripped one).
3. If the DLL reports <= 1082 the code already falls back to manual config; consider downgrading or pinning a known-good libwebp.

Example fix

// before
var bytes = webp.EncodeLossless(bmp, speed);

// after
int s = Math.Clamp(speed, 0, 9);
var bytes = webp.EncodeLossless(bmp, s);

int s = Math.Clamp(speed, 0, 9);
var bytes = webp.EncodeLossless(bmp, s);

static bool IsValidLosslessSpeed(int speed) => speed >= 0 && speed <= 9;

try { var bytes = webp.EncodeLossless(bmp, speed); }
catch (Exception ex) when (ex.Message.Contains("lossless preset"))
{ var bytes = webp.EncodeLossless(bmp, Math.Clamp(speed, 0, 9)); }

When it happens

Trigger: Calling EncodeLossless(bmp, speed) on a DLL version > 1082 with a speed value outside 0-9, or with a libwebp build whose lossless preset support is broken/limited.

Common situations: Passing a negative speed or speed > 9; a custom libwebp build compiled without the lossless preset module; mismatch between the wrapper's assumed API surface and the actual DLL.

Related errors

• Can't config preset
• Bitmap's dimension is too large. Max is {WEBP_MAX_DIMENSION}
• Only support Format24bppRgb and Format32bppArgb pixelFormat.
• Can't configure preset
• This DLL version not support EncodeNearLossless

AI-assisted analysis of d2phap/ImageGlass@4a3c4fecef (2026-08-13). Data as JSON: /api/errors/b2aeedfe0cf52233. Report an issue: GitHub.