QL-Win/QuickLook · error · NotSupportedException

Unsupported bit depth {imageType.BitsPerPixel}

Error message

Unsupported bit depth {imageType.BitsPerPixel}

What it means

Thrown by the ICNS image decoder after dispatching on bit depth: the supported cases are 1, 8, and 32 (plus whatever earlier cases the switch covers). If imageType.BitsPerPixel is anything else, the default arm unlocks the bitmap, disposes it, and throws NotSupportedException. This guards against ICNS type codes whose bit depth this decoder never implemented.

Source

Thrown at QuickLook.Plugin/QuickLook.Plugin.ImageViewer/AnimatedImage/Providers/IcnsProvider.cs:424

            case 4:
                Decode4BPPImage(imageType, imageData, bitmapData);
                break;

            case 8:
                Decode8BPPImage(imageType, imageData, bitmapData);
                break;

            case 32:
                if (imageType.Details == IcnsType.TypeDetails.ARGB)
                    Decode32BPPImageARGB(imageType, imageData, bitmapData);
                else
                    Decode32BPPImage(imageType, imageData, bitmapData);
                break;

            default:
                image.UnlockBits(bitmapData);
                image.Dispose();
                throw new NotSupportedException("Unsupported bit depth " + imageType.BitsPerPixel);
        }

        if (maskElement != null)
        {
            switch (maskType.BitsPerPixel)
            {
                case 1:
                    Apply1BPPMask(maskElement.data, bitmapData);
                    break;

                case 8:
                    Apply8BPPMask(maskElement.data, bitmapData);
                    break;

                default:
                    image.UnlockBits(bitmapData);
                    image.Dispose();
                    throw new NotSupportedException("Unsupport mask bit depth " + maskType.BitsPerPixel);

View on GitHub (pinned to cb5d9c429c)

Solutions

1. Pre-decode the entry as PNG/JPEG when the ICNS type is one of the compressed types instead of routing through the bit-depth decoder.
2. Extend the switch to handle the missing depth (e.g. add a 4-bit or depth-from-compressed-image case).
3. Wrap DecodeAllImages and skip icons whose depth is unsupported, returning the rest.
4. Re-encode the ICNS with iconutil from PNG sources so all entries are standard depths.

Example fix

// before
default:
    image.UnlockBits(bitmapData);
    image.Dispose();
    throw new NotSupportedException("Unsupported bit depth " + imageType.BitsPerPixel);

// after — fall back to PNG/JPEG decode for compressed types
default:
    return DecodeCompressedEntry(imageType, imageData);

static readonly HashSet<int> SupportedBpp = new(){1,8,32};
if (!SupportedBpp.Contains(imageType.BitsPerPixel)) { /* route to compressed/PNG decode path or skip */ }

static bool IsSupportedIcnsDepth(int bpp) => bpp==1 || bpp==8 || bpp==32;

try { Decode(imageType, data, bd); }
catch (NotSupportedException) { /* skip this icon, try next */ }

When it happens

Trigger: An ICNS entry whose mapped imageType.BitsPerPixel is not in the implemented set (e.g. a 4-bit or compressed/ARGB variant not handled). The switch falls through to default.

Common situations: A newer macOS icon format (compressed JPEG-2000/PNG-backed types, or HEIC-based 'ic12'/'ic13' whose decoded depth differs); an ICNS produced by an unusual exporter; misclassification of a depth during type-code resolution.

Related errors

• Unsupport mask bit depth {maskType.BitsPerPixel}
• 1 BPP mask underrun parsing ICNS file
• No icons in ICNS file
• DS_Store header too short
• DS_Store: wrong magic (expected 0x00000001)

AI-assisted analysis of QL-Win/QuickLook@cb5d9c429c (2026-08-13). Data as JSON: /api/errors/3ffa075db590ac78. Report an issue: GitHub.