View from the Peak

World’s Smallest h.264 Encoder

March 19th, 2010 by Ben Mesander

Recently I have been studying the h.264 video codec and reading the ISO spec. h.264 a much more sophisticated codec than MPEG-2, which means that a well-implemented h.264 encoder has more compression tools at its disposal than the equivalent MPEG-2 encoder. But all that sophistication comes at a price: h.264 also has a big, complicated specification with a plethora of options, many of which are not commonly used, and it takes expertise to understand which parts are important to solve a given problem.

As a bit of a parlor trick, I decided to write the simplest possible h.264 encoder. I was able to do it in about 30 lines of code—although truth in advertising compels me to admit that it doesn’t actually compress the video at all!

While I don’t want to balloon this blog post with a detailed description of h.264, a little background is in order. An h.264 stream contains the encoded video data along with various parameters needed by a decoder in order to decode the video data. To structure this data, the bitstream consists of a sequence of Network Abstraction Layer (NAL) units.

Previous MPEG specifications allowed pictures to be coded as I-frames, P-frames, or B-frames. h.264 is more complex and wonderful. It allows individual frames to be coded as multiple slices, each of which can be of type I, P, or B, or even more esoteric types. This feature can be used in creative ways to achieve different video coding goals. In our encoder we will use one slice per frame for simplicity, and we will use all I-frames.

As with previous MPEG specifications, in h.264 each slice consists of one or more 16×16 macroblocks. Each macroblock in our 4:2:0 sampling scheme contains 16×16 luma samples, and two 8×8 blocks of chroma samples. For this simple encoder, I won’t be compressing the video data at all, so the samples will be directly copied into the h.264 output.

With that background in mind, for our simplest possible encoder, there are three NALs we have to emit:

  1. Sequence Parameter Set (SPS): Once per stream
  2. Picture Parameter Set (PPS): Once per stream
  3. Slice Header: Once per video frame
    1. Slice Header information
    2. Macroblock Header: Once per macroblock
    3. Coded Macroblock Data: The actual coded video for the macroblock

Since the SPS, the PPS, and the slice header are static for this application, I was able to hand-code them and include them in my encoder as a sequence of magic bits.

Putting it all together, I came up with the following code for what I call “hello264”:

#include <stdio.h>
#include <stdlib.h>
/* SQCIF */
#define LUMA_WIDTH 128
#define LUMA_HEIGHT 96
/* YUV planar data, as written by ffmpeg */
typedef struct
} __attribute__((__packed__)) frame_t;
frame_t frame;
/* H.264 bitstreams */
const uint8_t sps[] = { 0x00, 0x00, 0x00, 0x01, 0x67, 0x42, 0x00,
0x0a, 0xf8, 0x41, 0xa2 };
const uint8_t pps[] = { 0x00, 0x00, 0x00, 0x01, 0x68, 0xce,
0x38, 0x80 };
const uint8_t slice_header[] = { 0x00, 0x00, 0x00, 0x01, 0x05, 0x88,
0x84, 0x21, 0xa0 };
const uint8_t macroblock_header[] = { 0x0d, 0x00 };
/* Write a macroblock's worth of YUV data in I_PCM mode */
void macroblock(const int i, const int j)
int x, y;
if (! ((i == 0) && (j == 0)))
fwrite(&macroblock_header, 1, sizeof(macroblock_header),
for(x = i*16; x < (i+1)*16; x++)
for (y = j*16; y < (j+1)*16; y++)
fwrite(&frame.Y[x][y], 1, 1, stdout);
for (x = i*8; x < (i+1)*8; x++)
for (y = j*8; y < (j+1)*8; y++)
fwrite(&frame.Cb[x][y], 1, 1, stdout);
for (x = i*8; x < (i+1)*8; x++)
for (y = j*8; y < (j+1)*8; y++)
fwrite(&frame.Cr[x][y], 1, 1, stdout);
/* Write out PPS, SPS, and loop over input, writing out I slices */
int main(int argc, char **argv)
int i, j;
fwrite(sps, 1, sizeof(sps), stdout);
fwrite(pps, 1, sizeof(pps), stdout);
while (! feof(stdin))
fread(&frame, 1, sizeof(frame), stdin);
fwrite(slice_header, 1, sizeof(slice_header), stdout);
for (i = 0; i < LUMA_HEIGHT/16 ; i++)
for (j = 0; j < LUMA_WIDTH/16; j++)
macroblock(i, j);
fputc(0x80, stdout); /* slice stop bit */
return 0;

(This source code is available as a single file here.)

In main(), the encoder writes out the SPS and PPS. Then it reads YUV data from standard input, stores it in a frame buffer, and then writes out a h.264 slice header. It then loops over each macroblock in the frame and calls the macroblock() function to output a macroblock header indicating the macroblock is coded as I_PCM, and inserts the YUV data.

To use the code, you will need some uncompressed video. To generate this, I used the ffmpeg package to convert a QuickTime movie from my Kodak Zi8 video camera from h.264 to SQCIF (128×96) planar YUV format sampled at 4:2:0:

ffmpeg.exe -i angel.mov -s sqcif -pix_fmt yuv420p angel.yuv

I compile the h.264 encoder:

gcc –Wall –ansi hello264.c –o hello264

And run it:

hello264 <angel.yuv >angel.264

Finally, I use ffmpeg to copy the raw h.264 NAL units into an MP4 file:

ffmpeg.exe -f h264 -i angel.264 -vcodec copy angel.mp4

Here is the resulting output:

There you have it—a complete h.264 encoder that uses minimal CPU cycles, with output larger than its input!

The next thing to add to this encoder would be CAVLC coding of macroblocks and intra prediction. The encoder would still be lossless at this point, but there would start to be compression of data. After that, the next logical step would be quantization to allow lossy compression, and then I would add P slices. As a development methodology, I prefer to bring up a simplistic version of an application, get it running, and then add refinements iteratively.

UPDATE 4/20/11: I’ve written more about the Sequence Parameter Set (SPS) here.

Ben Mesander has more than 18 years of experience leading software development teams and implementing software. His strengths include Linux, C, C++, numerical methods, control systems and digital signal processing. His experience includes embedded software, scientific software and enterprise software development environments.


  1. centos ab命令安装

    yum install apr-util -ymkdir abcd abyum -y install yum-utils -yyumdownloader httpd yum localinstall ...

  2. 程序Bug---易错点


  3. 按照网上方法js删除指定cookie,却怎么也删除不了,解决如下

    网上方法: 查找原因说是没有指定Path,记得系统里以前也没指定还是可以的,就查了一下现在的系统Path,猜测是系统Path由以前的/改为/E7-Planning 就改了前端删除方法 测试一下OK了, ...

  4. java 20 - 8 字节流的文件复制以及汉字在计算机中的存储方式

    复制文本文件:把当前目录下的FileIntputStream.java文件里面的内容复制到当前目录的b.txt文件中 分析: 数据源: FileIntputStream.java -- 读取数据 -- ...

  5. SVN提交.a文件的方法

    选择菜单View-->Ignored Files,这样就会显示出ignored的文件,找到你要上传的.a文件,右键“Add”就可以了.

  6. Binding 中 Elementname,Source,RelativeSource 三种绑定的方式

    在WPF应用的开发过程中Binding是一个非常重要的部分. 在实际开发过程中Binding的不同种写法达到的效果相同但事实是存在很大区别的. 这里将实际中碰到过的问题做下汇总记录和理解. 1. so ...

  7. 敲入url到浏览器后会发生什么

    浏览器连接DNS服务器,向url服务器请求把url转换为IP地址 DNS服务区返回URL的ip地址 浏览器建立一个TCP链接到web服务器80端口 web服务器发回的html代码 浏览器的渲染器根据h ...

  8. Three.js使用局部纹理更新

    THREE.js开发的应用运行在iphone5下发现有些时候会崩溃,跟了几天发现是因为Sprite太多频繁更新纹理占用显存导致的.通常解决纹理频繁更新问题就要用到one draw all方法,放到纹理 ...

  9. alex python of day2

      模块 sys模块:sys模块是用c语言写的,所以在lib下是不会有sys.py这个文件存在 1 import sys 2 print(sys.path) #打印环境变量 3 print(sys.a ...

  10. android6.0 Activity(四) Surface创建

     原文:http://blog.csdn.net/luoshengyang/article/details/8303098.原文代码比較老了,可是核心不变.在原文基础上改动了一些代码,以及增加自己 ...