jo_MPEG update

Author: FrostKiwi

posts/jo-mpeg-in-c/comparison.png

@@ -2,22 +2,46 @@
 title: Jo_MPEG converted to C
 permalink: "/{{ page.fileSlug }}/"
 date: 2022-02-18
-last_modified:
+last_modified: 2024-08-15
 description: Single-header MPEG-1 Video library ported to C
 publicTags:
   - C++
   - C
   - video
 image: jo_mpeg.png
 ---
-jo_mpeg is a C++ [single header library](https://github.com/nothings/single_file_libs) written by [Jon Olick](https://www.jonolick.com/home/mpeg-video-writer), which creates MPEG-1 videos (without audio). It is [listed as a C++ only library](https://github.com/nothings/single_file_libs#video) in stb's single header library collection. However, only the & reference format is what makes this library C++ only. Replacing those with simple pointers makes this compile with both C and C++: [jo_mpeg.h](jo_mpeg.h)
-
-( I also found this: https://github.com/yui0/slibs/blob/master/jo_mpeg.h, but it is one version behind. )
+jo_mpeg is a C++ [single header library](https://github.com/nothings/single_file_libs) written by [Jon Olick](https://www.jonolick.com/home/mpeg-video-writer), which creates MPEG-1 videos (without audio). It is [listed as a C++ only library](https://github.com/nothings/single_file_libs#video) in stb's single header library collection. However, only the & reference format is what makes this library C++ only. Replacing those with simple pointers makes this compile with both C and C++.
 
 <details>
 <summary>Full source of the C compatible <a href="jo_mpeg.h">jo_mpeg.h</a></summary>
 
 ```c
 {% rawFile "posts/jo-mpeg-in-c/jo_mpeg.h" %}
 ```
-</details>
+</details>
+
+## Results
+I encoded a couple of seconds from [Big Buck Bunny](https://peach.blender.org/) as a sample: [sample.mpg](sample.mpg).
+
+<blockquote class="reaction"><div class="reaction_text">Can't show it directly in browser, as MPEG1 is not supported anymore. Also there is <a href="https://jsmpeg.com/">jsmpeg</a>, but it also doesn't support this output.</div><img class="kiwi" src="/assets/kiwis/facepalm.svg"></blockquote>
+
+<figure>
+	<img src="comparison.png" alt="Input frame vs Output frame. Side effect of conversion: Increased saturation and contrast." />
+	<figcaption>Input frame vs Output frame. Side effect of conversion: Increased saturation and contrast.</figcaption>
+</figure>
+
+Unfortunately, the output has increased saturation and contrast. This is due to RGB -> [YCbCr](https://en.wikipedia.org/wiki/YCbCr#RGB_conversion) conversion in line `230` - `232` scaling the final color vectors scaled too much. I fixed this by reverting the color space math changes that happened with the update to `v1.02`.
+
+<figure>
+	<img src="comparisonNew.png" alt="Input frame vs Output frame. Reverted to old color math." />
+	<figcaption>Input frame vs Output frame. Reverted to old color math.</figcaption>
+</figure>
+
+I'm not sure why the code change credited to `r- lyeh` happened, but I guess the used video player handled color space incorrectly. Both [VLC](https://www.videolan.org/) and [MPV](https://mpv.io/) playback the colors correctly with `v1.03`.
+
+Quality is hardcoded and results in roughly `8mbps` at a resolution of `684x385`. Quality measurements are at roughly `27db` [PSNR](https://en.wikipedia.org/wiki/Peak_signal-to-noise_ratio#Quality_estimation_with_PSNR) and `0.9` [SSIM](https://medium.com/srm-mic/all-about-structural-similarity-index-ssim-theory-code-in-pytorch-6551b455541e)...
+
+<figure>
+	<video width="684" height="342" controls><source src="not-terrible.mp4" type="video/mp4"></video>
+	<figcaption>...Or in other words</figcaption>
+</figure>

posts/jo-mpeg-in-c/comparisonNew.png

@@ -1,9 +1,10 @@
 /* public domain Simple, Minimalistic, No Allocations MPEG writer - http://jonolick.com
  *
- * "Converted" to C by Wladislav Artsimovich
- * I just added a bunch of * and removed a bunch of & ¯\_(ツ)_/¯
+ * Converted to C by Wladislav Artsimovich https://blog.frost.kiwi/jo-mpeg-in-c
  *
  * Latest revisions:
+ * 	1.03 (15-08-2024) Reverted color space change from 1.02, as it resulted in
+ *                    overscaled color vectors and thus oversaturated colors
  * 	1.02 (22-03-2017) Fixed AC encoding bug. 
  *                    Fixed color space bug (thx r- lyeh!)
  * 	1.01 (18-10-2016) warning fixes
@@ -25,7 +26,7 @@
  * 	Movie players *should* support it as the spec allows it, but ...
  *
  * 	MPEG-1/2 currently has no active patents as far as I am aware.
- * 	
+ *  
  *	http://dvd.sourceforge.net/dvdinfo/mpeghdrs.html
  *	http://www.cs.cornell.edu/dali/api/mpegvideo-c.html
  * */
@@ -227,9 +228,9 @@ void jo_write_mpeg(FILE *fp, const unsigned char *rgbx, int width, int height, i
 				y = y >= height ? height-1 : y;
 				const unsigned char *c = rgbx + y*width*4+x*4;
 				float r = c[0], g = c[1], b = c[2];
-                Y[i] = (0.299f*r + 0.587f*g + 0.114f*b) * (219.f/255) + 16;
-                CBx[i] = (-0.299f*r - 0.587f*g + 0.886f*b) * (224.f/255) + 128;
-                CRx[i] = (0.701f*r - 0.587f*g - 0.114f*b) * (224.f/255) + 128;
+				Y[i] = (0.59f*r + 0.30f*g + 0.11f*b) * (219.f/255) + 16;
+				CBx[i] = (-0.17f*r - 0.33f*g + 0.50f*b) * (224.f/255) + 128;
+				CRx[i] = (0.50f*r - 0.42f*g - 0.08f*b) * (224.f/255) + 128;
 			}
 
 			// Downsample Cb,Cr (420 format)