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

从报告中所示的 SDImageWebPCoder 无损编码入口开始，并使用提供的示例将其输出与 cwebp -lossless 进行比较。检查转发了哪些 encodeCompressionQuality、encodeWebPMethod 和 encodeWebPPass 选项，然后将与 cwebp 的无损质量和文件大小行为匹配，或记录实现差异，定义为完成。