World's Smallest h.264 Encoder

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 -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.

Categories: Ben, Video

Cardinal Peak
Learn more about our Audio & Video capabilities.

Dive deeper into our IoT portfolio

Take a look at the clients we have helped.

We’re always looking for top talent, check out our current openings. 

Contact Us

Please fill out the contact form below and our engineering services team will be in touch soon.

We rely on Cardinal Peak for their ability to bolster our patent licensing efforts with in-depth technical guidance. They have deep expertise and they’re easy to work with.
Diego deGarrido Sr. Manager, LSI
Cardinal Peak has a strong technology portfolio that has complemented our own expertise well. They are communicative, drive toward results quickly, and understand the appropriate level of documentation it takes to effectively convey their work. In…
Jason Damori Director of Engineering, Biamp Systems
We asked Cardinal Peak to take ownership for an important subsystem, and they completed a very high quality deliverable on time.
Matt Cowan Chief Scientific Officer, RealD
Cardinal Peak’s personnel worked side-by-side with our own engineers and engineers from other companies on several of our key projects. The Cardinal Peak staff has consistently provided a level of professionalism and technical expertise that we…
Sherisse Hawkins VP Software Development, Time Warner Cable
Cardinal Peak was a natural choice for us. They were able to develop a high-quality product, based in part on open source, and in part on intellectual property they had already developed, all for a very effective price.
Bruce Webber VP Engineering, VBrick
We completely trust Cardinal Peak to advise us on technology strategy, as well as to implement it. They are a dependable partner that ultimately makes us more competitive in the marketplace.
Brian Brown President and CEO, Decatur Electronics
The Cardinal Peak team started quickly and delivered high-quality results, and they worked really well with our own engineering team.
Charles Corbalis VP Engineering, RGB Networks
We found Cardinal Peak’s team to be very knowledgeable about embedded video delivery systems. Their ability to deliver working solutions on time—combined with excellent project management skills—helped bring success not only to the product…
Ralph Schmitt VP, Product Marketing and Engineering, Kustom Signals
Cardinal Peak has provided deep technical insights, and they’ve allowed us to complete some really hard projects quickly. We are big fans of their team.
Scott Garlington VP Engineering, xG Technology
We’ve used Cardinal Peak on several projects. They have a very capable engineering team. They’re a great resource.
Greg Read Senior Program Manager, Symmetricom
Cardinal Peak has proven to be a trusted and flexible partner who has helped Harmonic to deliver reliably on our commitments to our own customers. The team at Cardinal Peak was responsive to our needs and delivered high quality results.
Alex Derecho VP Professional Services, Harmonic
Yonder Music was an excellent collaboration with Cardinal Peak. Combining our experience with the music industry and target music market, with Cardinal Peak’s technical expertise, the product has made the mobile experience of Yonder as powerful as…
Adam Kidron founder and CEO, Yonder Music
The Cardinal Peak team played an invaluable role in helping us get our first Internet of Things product to market quickly. They were up to speed in no time and provided all of the technical expertise we lacked. They interfaced seamlessly with our i…
Kevin Leadford Vice President of Innovation, Acuity Brands Lighting
We asked Cardinal Peak to help us address a number of open items related to programming our systems in production. Their engineers have a wealth of experience in IoT and embedded fields, and they helped us quickly and diligently. I’d definitely…
Ryan Margoles Founder and CTO, notion