Low compression quality compared to cwebp CLI tool

Description

When using SDImageWebPCoder to encode images with lossless options, the resulting file size and compression quality appear to be worse compared to Google's cwebp command-line tool. Despite both using lossless compression settings, SDImageWebPCoder produces slightly lower quality images, with visible artifacts and banding on gradients and shadows.

I experimented with different encoding options such as: encodeCompressionQuality, encodeWebPMethod, encodeWebPPass etc. Despite these adjustments, the output quality and file size were still worse compared to cwebp -lossless. This suggests a fundamental difference in how the WebP encoding is implemented in SDImageWebPCoder versus the official WebP compression tool.

Code

SDImageWebPCoder:

if let data = SDImageWebPCoder.shared.encodedData(with: nsImage, format: .webP, options: [.encodeWebPLossless: 1]) {
  try data.write(to: destination))
}

cwebp:

cwebp -lossless input.png -o output.webp

Results

Start with the SDImageWebPCoder lossless encoding entry point shown in the report and compare its output against cwebp -lossless using the provided example. Check which encodeCompressionQuality, encodeWebPMethod, and encodeWebPPass options are forwarded, then define done as matching cwebp's lossless quality and file-size behavior or documenting the implementation difference.