使用ffmpeg进行webp图片压缩，ffmpeg的帮助信息查看方法

01.ffmpeg中帮助信息的查看

​ ffmpeg集成了很多编解码器，使用ffmpeg很多时候需要了解这些编解码器的参数。

​ 最近在看小泉同学的bd，去网络上搜寻op的cd资源的时候，下载下来发现一张扫图有80mb大小。音乐本身是hires压制的flac，大小是qq音乐上无损flac的3倍。一首歌占100mb的空间，这应该是hires的音乐的正常大小了。

​ 因为hi-res是96kHz是通常采样的两倍，采样位数也更高，通常的cd音质是 44.1kHz 16bit的音源，而hi-res通常是 96kHz 24bit，如果音源一样的话 应该大约是 （96/44.1） * （24/16）=3.27倍大小。这是算是验证了，一般一首正常长度的歌，一般的flac是30多mb，而hi-res 是100mb左右。

​ 但是扫图的大小感觉过大了，图像的分辨率是7623*3763，已经超过屏幕的大小，主要是cd封面大小的一张图占用80mb实在是浪费，于是想用目前流行webp格式进行压缩。

​ 执行ffmpeg.exe -i .\01.png out.webp

​ 给定后缀名，ffmpeg会自动寻找编码器进行处理

ffmpeg.exe -i .\01.png  out.webp
ffmpeg version N-93851-gdcc999819d Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 8.3.1 (GCC) 20190414
  configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libdav1d --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth --enable-libopenmpt
  libavutil      56. 27.100 / 56. 27.100
  libavcodec     58. 52.101 / 58. 52.101
  libavformat    58. 27.103 / 58. 27.103
  libavdevice    58.  7.100 / 58.  7.100
  libavfilter     7. 51.100 /  7. 51.100
  libswscale      5.  4.101 /  5.  4.101
  libswresample   3.  4.100 /  3.  4.100
  libpostproc    55.  4.100 / 55.  4.100
[png_pipe @ 000001b495fa8f40] Stream #0: not enough frames to estimate rate; consider increasing probesize
Input #0, png_pipe, from '.\01.png':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: png, rgb24(pc), 7623x3763 [SAR 31496:31496 DAR 7623:3763], 25 tbr, 25 tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (png (native) -> webp (libwebp))
Press [q] to stop, [?] for help
Output #0, webp, to 'out.webp':
  Metadata:
    encoder         : Lavf58.27.103
    Stream #0:0: Video: webp (libwebp), bgra, 7623x3763 [SAR 1:1 DAR 7623:3763], q=2-31, 200 kb/s, 25 fps, 1k tbn, 25 tbc
    Metadata:
      encoder         : Lavc58.52.101 libwebp
[libwebp @ 000001b495fbcdc0] Using libwebp for RGB-to-YUV conversion. You may want to consider passing in YUV instead for lossy encoding.
frame=    1 fps=0.1 q=-0.0 Lsize=    9820kB time=00:00:00.00 bitrate=80448240.0kbits/s speed=7.85e-05x
video:9820kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.000000%

看到这一行， Stream #0:0 -> #0:0 (png (native) -> webp (libwebp))，ffmpeg使用了一个libwebp的编码器进行压缩。

所以这里我们就要查一下libwebp这个编码器具体有哪些参数。

​

下面是ffmpeg查看帮助文档的方法

这里我们执行以下命令即可

ffmpeg -h encoder=libwebp

02.libwebp的使用

查看libwebp的帮助如下

ffmpeg -h encoder=libwebp
ffmpeg version N-93851-gdcc999819d Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 8.3.1 (GCC) 20190414
  configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libdav1d --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth --enable-libopenmpt
  libavutil      56. 27.100 / 56. 27.100
  libavcodec     58. 52.101 / 58. 52.101
  libavformat    58. 27.103 / 58. 27.103
  libavdevice    58.  7.100 / 58.  7.100
  libavfilter     7. 51.100 /  7. 51.100
  libswscale      5.  4.101 /  5.  4.101
  libswresample   3.  4.100 /  3.  4.100
  libpostproc    55.  4.100 / 55.  4.100
Encoder libwebp [libwebp WebP image]:
    General capabilities: none
    Threading capabilities: none
    Supported pixel formats: bgra yuv420p yuva420p
libwebp AVOptions:
  -lossless          <int>        E..V..... Use lossless mode (from 0 to 1) (default 0)
  -preset            <int>        E..V..... Configuration preset (from -1 to 5) (default none)
     none                         E..V..... do not use a preset
     default                      E..V..... default preset
     picture                      E..V..... digital picture, like portrait, inner shot
     photo                        E..V..... outdoor photograph, with natural lighting
     drawing                      E..V..... hand or line drawing, with high-contrast details
     icon                         E..V..... small-sized colorful images
     text                         E..V..... text-like
  -cr_threshold      <int>        E..V..... Conditional replenishment threshold (from 0 to INT_MAX) (default 0)
  -cr_size           <int>        E..V..... Conditional replenishment block size (from 0 to 256) (default 16)
  -quality           <float>      E..V..... Quality (from 0 to 100) (default 75)

我们发现有以下的选项

-lossless 是设置无损压缩

-preset 是几个预设的参数

-quality 这个就是主要的参数了，控制压缩率

-cr_size 这个参数试了一下 200和16各压了一遍，没看出什么区别，大小也一样

不设置参数默认压制出来 的图片 80mb 变成10mb左右，这已经是接近无损的质量了。

测试了一下 quality 0的时候也有500多kb大小，但是图片就有点糊了。

去网上查了一下libwebp参数的具体设置，有人得出了下面的结论
https://www.cnblogs.com/yanguhung/p/10784598.html

选择无损压缩时，“-lossless -q 100” 是最佳方案,注意:cwebp 仅仅对png格式的图片使用无损压缩时，会有较为高效的压缩率和图片质量

选择有损压缩时，“-q 75”是最佳方案（图片质量与体积大小达到均衡）建议其他格式图片使用有损压缩

无论何种压缩参数，加上“-m 6”都能使得输出的 WebP 图片进一步减少体积，量级是1%~2%，但是会增加耗时

非png格式的图片选择无损压缩时，“-lossless -q 100” 时编码时间长，图片质量反而极高可能变大，编码时间时间很长，cpu使用率飙升跑满，不建议使用

建议选择有损压缩时，“-q 75”是最佳方案（图片质量与体积大小达到均衡）建议使用

还有webp 和jpg的对比

即使jpg图片同比例压缩为jpg和webp图片 webp图片也要比jpg图片质量低百分之10~20之间

也就是说，webp的压缩率完全优于jpg，但是webp的编码时间比jpg长8倍。

所以说我们使用的时候按照默认参数来就行了。

调整图片大小用-vf scale=iw/3:ih/3，可以指定和原来的比例，也可以指定明确的像素值比如1280:720.

单纯调整图片大小，无损压缩把图片宽高缩小3倍iw/3:ih/3，大小就只有5mb了。

但是这里不缩小大小，只压缩，所以按照默认参数就行。

ffmpeg.exe -i input.png  -vf scale=iw:ih -codec libwebp -lossless 0 -quality 75 out.webp

编写一个压缩指定大小png图片的powershell脚本

<#
.synopsis
批量转换占用空间过大的图片
.example

使用方法如下
allPngtoWebp.ps1 -limitSize 10mb -paramStr '-codec libwebp -vf scale=iw/4:ih/4 -lossless 1 -quality 100 -pix_fmt yuv420p'


#>
param(
# 需要压缩的图片阈值大小，小于这个数值的图片不进行压缩
[string]$targetPath='.',
[int]$limitSize=10mb,
[string]$paramStr='-codec libwebp -pix_fmt yuv420p -vf scale=iw:ih  -lossless 0 -quality 75',
# 其实不用多设置，默认的设置就是最好的，默认压制出来的就是yuv420p
#[string]$paramStr='-codec libwebp  -vf scale=iw:ih  -lossless 0 -quality 75',
[switch]$noDelete
)

#只压缩png格式的图片，先检索目录下的png图片
$count=0
Get-ChildItem -Recurse -LiteralPath $targetPath| where{$_.Extension -eq '.png'} | foreach{
if($_.Length -gt $limitSize){
     Write-Host -ForegroundColor Green "detect large png picture : $($_.FullName) size:$($_.Length/1mb) "
     
     $newfullname=$_.FullName.Substring(0,$_.FullName.Length-4)+'.webp'
     Invoke-Expression "ffmpeg.exe -i '$($_.FullName)'  $paramStr  '$($newfullname)'"
    # ffmpeg.exe -i $_.FullName  $paramStr  ($_.FullName.Substring(0,$_.FullName.Length-4)+'.webp')

    if ((Test-Path -LiteralPath $newfullname)-and (-not $noDelete)){
        rm -LiteralPath $_.FullName
        Write-Host -ForegroundColor yellow "delete $($_.FullName) complete "
     $count++
     }
     else{
        Write-Host -ForegroundColor red "not find the source file or no delete "
        #break
     }
    }
}

Write-Host -ForegroundColor  Green   "compress $count  png files  complete"

把ffmpeg官方文档中关于libwebp的内容也贴以下，作为参考用：
ibwebp WebP Image encoder wrapper

libwebp is Google’s official encoder for WebP images. It can encode in either lossy or lossless mode. Lossy images are essentially a wrapper around a VP8 frame. Lossless images are a separate codec developed by Google.

9.9.1 Pixel Format

Currently, libwebp only supports YUV420 for lossy and RGB for lossless due to limitations of the format and libwebp. Alpha is supported for either mode. Because of API limitations, if RGB is passed in when encoding lossy or YUV is passed in for encoding lossless, the pixel format will automatically be converted using functions from libwebp. This is not ideal and is done only for convenience.

9.9.2 Options

• -lossless boolean

  Enables/Disables use of lossless mode. Default is 0.

• -compression_level integer

  For lossy, this is a quality/speed tradeoff. Higher values give better quality for a given size at the cost of increased encoding time. For lossless, this is a size/speed tradeoff. Higher values give smaller size at the cost of increased encoding time. More specifically, it controls the number of extra algorithms and compression tools used, and varies the combination of these tools. This maps to the method option in libwebp. The valid range is 0 to 6. Default is 4.

• -qscale float

  For lossy encoding, this controls image quality, 0 to 100. For lossless encoding, this controls the effort and time spent at compressing more. The default value is 75. Note that for usage via libavcodec, this option is called global_quality and must be multiplied by FF_QP2LAMBDA.

• -preset type

  Configuration preset. This does some automatic settings based on the general type of the image.noneDo not use a preset.defaultUse the encoder default.pictureDigital picture, like portrait, inner shotphotoOutdoor photograph, with natural lightingdrawingHand or line drawing, with high-contrast detailsiconSmall-sized colorful imagestextText-like