Webp Introduction

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What is Webp?

Webp是一种图像格式，支持单张图片，动图； 什么平台支持？ 如何使用？

Open-Source projects

libwebp: https://chromium.googlesource.com/webm/libwebp
mirror: https://github.com/webmproject/libwebp/

User Guide

Options

  1. q: 指定quality factor
  2. alpha_q: 指定透明通道的压缩率
  3. preset: 指定图片的type
  4. -z: 指定lossless压缩的效率
  5. m: 压缩的速度档次，0最快，6是最慢
  6. segment: 指定要使用的segments，默认是4
  7. -size: 指定目标size (in bytes)
  8. -psnr: 指定目标的PSNR
  9. -sns: spatial noise shaping, important
  10. -f: filter strength, default 60，是指loop filter的强度
  11. -sharpness: filter sharpness, 0: most sharp, 7: 最小sharp
  12. -strong: use strong filter
  13. -sharp_yuv: use sharper RGB->YUV的转换
  14. -partition_limit
  15. -pass: analysis pass number
  16. -qrange，指定最小和最大QP的范围
  17. -mt: 使用multi-threading
  18. -alpha_method: 指定透明通道压缩的方法
  19. -alpha_filter: predictive filtering for alpha plane
  20. -exact: preserve RGB value in transparent area; 也就是不修改完全透明区域的
  21. -noalpha: 丢掉alpha的特征
  22. -lossless: 无损压缩图像
  23. -near_lossless: use near-lossless image preprocessing
  24. -hint: 指定图片的特征或者提示；
  25. -af: using autoaf，指标是ssim

如何把其他图片转换成Webp?

JPG format YUVJ420: YUV420P with JPEG color-range" (i.e. pixels use 0-255 range instead of 16-235 range, where 0 instead of 16 is black, and 255 instead of 235 is white);

使用工具cwebp，最简单的使用就是: cwebp input.png -q 80 -o output.webp

重要的参数

重要的参数包括-preset -sns -f -m: -preset: 用于指定content的类型，应该最先被设置，这样后续的设置就在这个基础上来修改; 编码器内部会根据preset去调整sns_strength, filter_sharpness, filter_strength和preprocessing.
-f: 对应的是编码器内部的in-loop filter的filter strength;
-m: 用来指定编码速度，[0, 6]，0最快，6最慢，default是4
-sns: 用来打开一些visual optimization，主要用来调节bit allocation. 用来把easy part的bit分配给hard part。通常升高sns，会导致文件增大，质量变好；

sns

method >= 4: distortion会包括spectural distortion Distortion: 在method > 4的时候，会有distortion和spectral distortion 设置segment的时候，可以控制幅度。 控制uv_ac，越小的话，dq_uv_ac变大，和dq_uv_dc，越大,uv_dc boost的越多 It will set do_search_

how to calculate alpha: Do transform, collect coefficients stats, find last_none_zero and max_values; alpha = last_non_zero/max_value 1. 计算每个mb的alpha 原理： TX: 4x4 convert coefficients to bins，calculate the histogram of abs(level) max_value: 出现系数最多的系数的出现次数，越大，说明0系数的越多，说明能量越少 last_non_zero: 最后一个非零系数，越大，说明高频的能量越多， alpha = ALPHA_SCALE * last_non_+zero / max_value; best_alpha = MAX_ALPHA - (3 * best_alpha + best_uv_alpha + 2) >> 2 高能量: 

1
2
3
4
5
MBAnalyze:323 analyze mbx 0, mby 1
mode 0, last_nonzero_count 31, max_value 95
mode 1, last_nonzero_count 31, max_value 88
MBAnalyze:330 best_alpha 179
MBAnalyze:341 mbx 0, mby 1, alpha 57, uv_alpha 255
低能量： 
1
2
3
4
5
MBAnalyze:323 analyze mbx 0, mby 3
mode 0, last_nonzero_count 11, max_value 234
mode 1, last_nonzero_count 4, max_value 244
MBAnalyze:330 best_alpha 23
MBAnalyze:341 mbx 0, mby 3, alpha 234, uv_alpha 16
alpha越大，Q越小； beta

filter params

Decoder perspective: 9.4 Loop Filter Type and Levels frame-level loop filter params: 1. filter_type: normal or simple, simple filter only applied to luma edges;
2. loop_filter_level: defines the threshold, if the different is below this threshold, it should be filtered, otherwise, it should not be changed. Usually the level correlated to quantizer levels.
3. sharpness_level, constant over the frame, 如果loop_filter_level=0，那么应该跳过loop_filter;
Differences in excess of a threshold(associated to the loop_filter_level) are assumed to not being modified.

mode_ref_lf_delta_update=1, allow per-macroblock adjustment, through delta; There are two types of delta values, one for reference frame-based adjustment, and the other group is for mode-based adjustment.

Filter header to write into bitstreams: simple: level: 对应fstrength_ sharpness

sharpness设置越大，fstrength越大； Q越大，fstrength越大；

fstrength是针对segment来的；

lf_delta

process; base_strength = F(q, sharpness) strength = 5 * filter_strength * base_strength / *(256 + beta_) level0 = fstrength_

只filter 4x4的块，不filter 16x16和skip的块；

How to decide the strength for each segments?

Why VP8StoreFilterStats? 存储不同的delta_q下，do filter之后的ssim并存储起来； How to decide final strength_; VP8AdjustFilterStrength

Internal Process

Speed 0/1

Speed 2~6

Alpha Encoding

主要通过一下几个接口来处理 1. VP8EncInitAlpha 2. VP8EncStartAlpha 3. VP8EncFinishAlpha 4. VP8EncDeleteAlpha

graph TD;
    A-->B;
    A-->C;
    B-->D;
    C-->D;

主要调用EncodeAlpha进行处理 ## Feature ### Specify target_size/target_PSNR sns:

Region-based quantization

Adaptive Loop-filter

根据prediction mode或参考帧来决定loop filter strength Adjustment of loop-filter stregnth for each segment.

RDO

Preprocessing

sharp YUV

This is applied when converting RGB to YUV #### Smooth segment #### pseudo-random dithering log2 #### Alpha cleanup ### Lossless 主要通过调用WebPEncodeLosslessRGB，内部是调用EncodeLossless. VP8LEncodeImage is used to encoding lossless.
内部调用VP8LEncodeStream QuantizeLevels VP8FiltersInit ApplyFiltersAndEncode EncodeAlphaInternal ---> EncodeLossless ---> VP8LEncodeStream ### Segment 不打开Segment情况下，

rd_opt_level: 1. no_rd 2. rd_opt_basic 3. rd_opt_trellis, perform trellis-quant on the final decision only 4. rd_opt_trellis_all

if rd_opt_level < rd_opt_basic>, then use VP8EncLoop ## Reference Official-Webp-Doc
React Native using Webp
Webp Tools
VP8 Encode Parameter Guide
VP8 Tech Overview
Webp Compression Techniques