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• Of ctors and dtors

  18 février 2011, par Multimedia Mike — Programming, Sega Dreamcast

  I haven’t given up on the Sega Dreamcast programming. I was able to compile a bunch of homebrew code for the DC many years ago and I can’t make it work anymore. Again, I was working with a purpose-built, open source RTOS named KallistiOS (or KOS). I can make the programs compile but not run. I had ELF files left over from years ago which still executed. But when I tried to build new ELF files, no luck— the programs crashed before even reaching my main() function.

  I found the problem : ELF files are comprised of a number of sections and 2 of these sections are named ’.ctors’ and ’.dtors’ which stand for constructors and destructors. The KOS RTOS performs a manual traversal of .ctors section during program initialization and this is where things go bad. The traversal code doesn’t seem to account for a .ctors section that only contains a single entry. I commented out the function that does the traversal and programs started to work, at least until it was time to exit the program and return control to the program loader. That’s when the counterpart .dtors section traversal code ran and demonstrated the same problem. I’ll exhibit the problematic code at the end of this post.

  So I’m finally tinkering with Sega Dreamcast programming once again and with a slightly better grasp of software engineering than the first time I did this.

  Portable and Compatible C ?
  If nothing else, this low-level embedded stuff exposes you to some serious toolchain arcana, the likes of which you will likely never see working strictly in the desktop arena.

  Still, this exercise makes me wonder why C code from a decade ago doesn’t compile reliably now. Part of it is because gcc has gotten stricter about the syntax it will accept. In the case of this specific crashing problem, I suspect it comes down to a difference in the way the linker generates the final ELF file. I’ve written a list of items I have had to modify in the KOS codebase in order to get it to compile on more recent gcc versions. I wonder if it would be worth publishing the specifics, or if anyone would ever find the information useful ? Oh, who am I kidding ? Of course I’ll write it up, perhaps publish a new version of the code, if only because that’s the best chance I have of finding my own work again some years down the road.

  Problematic C Code
  See if this code makes any sense to you. It somehow traverse a list of 32-bit function pointers (in different directions, depending on constructors or destructors), executing each in turn. However, it appears to fall over if the list of pointers consists of a single entry.
  

  PLAIN TEXT

  C :

  1. typedef void (*fptr)(void) ;
  2.  
  3. static fptr ctor_list[1] __attribute__((section(".ctors"))) = { (fptr) -1 } ;
  4. static fptr dtor_list[1] __attribute__((section(".dtors"))) = { (fptr) -1 } ;
  5.  
  6. /* Call this to execute all ctors */
  7. void arch_ctors() {
  8.     fptr *fpp ;
  9.  
  10.     /* Run up to the end of the list (defined by crtend) */
  11.     for (fpp=ctor_list + 1 ; *fpp != 0 ; ++fpp)
  12.          ;
  13.  
  14.     /* Now run the ctors backwards */
  15.     while (—fpp> ctor_list)
  16.         (**fpp)() ;
  17. }
  18.  
  19. /* Call this to execute all dtors */
  20. void arch_dtors() {
  21.     fptr *fpp ;
  22.  
  23.     /* Do the dtors forwards */
  24.     for (fpp=dtor_list + 1 ; *fpp != 0 ; ++fpp )
  25.         (**fpp)() ;
  26. }

• Decoding VP8 On A Sega Dreamcast

  20 février 2011, par Multimedia Mike — Sega Dreamcast, VP8

  I got Google’s libvpx VP8 codec library to compile and run on the Sega Dreamcast with its Hitachi/Renesas SH-4 200 MHz CPU. So give Google/On2 their due credit for writing portable software. I’m not sure how best to illustrate this so please accept this still photo depicting my testbench Dreamcast console driving video to my monitor :

  
  
  

  Why ? Because I wanted to try my hand at porting some existing software to this console and because I tend to be most comfortable working with assorted multimedia software components. This seemed like it would be a good exercise.

  You may have observed that the video is blue. Shortest, simplest answer : Pure laziness. Short, technical answer : Path of least resistance for getting through this exercise. Longer answer follows.

  Update : I did eventually realize that the Dreamcast can work with YUV textures. Read more in my followup post.

  Process and Pitfalls
  libvpx comes with a number of little utilities including decode_to_md5.c. The first order of business was porting over enough source files to make the VP8 decoder compile along with the MD5 testbench utility.

  Again, I used the KallistiOS (KOS) console RTOS (aside : I’m still working to get modern Linux kernels compiled for the Dreamcast). I started by configuring and compiling libvpx on a regular desktop Linux system. From there, I was able to modify a number of configuration options to make the build more amenable to the embedded RTOS.
  
  I had to create a few shim header files that mapped various functions related to threading and synchronization to their KOS equivalents. For example, KOS has a threading library cleverly named kthreads which is mostly compatible with the more common pthread library functions. KOS apparently also predates stdint.h, so I had to contrive a file with those basic types.

  So I got everything compiled and then uploaded the binary along with a small VP8 IVF test vector. Imagine my surprise when an MD5 sum came out of the serial console. Further, visualize my utter speechlessness when I noticed that the MD5 sum matched what my desktop platform produced. It worked !

  Almost. When I tried to decode all frames in a test vector, the program would invariably crash. The problem was that the file that manages motion compensation (reconinter.c) needs to define MUST_BE_ALIGNED which compiles byte-wise block copy functions. This is necessary for CPUs like the SH-4 which can’t load unaligned data. Apparently, even ARM CPUs these days can handle unaligned memory accesses which is why this isn’t a configure-time option.

  Showing The Work
  I completed the first testbench application which ran the MD5 test on all 17 official IVF test vectors. The SH-4/Dreamcast version aces the whole suite.

  However, this is a video game console, so I had better be able to show the decoded video. The Dreamcast is strictly RGB— forget about displaying YUV data directly. I could take the performance hit to convert YUV -> RGB. Or, I could just display the intensity information (Y plane) rendered on a random color scale (I chose blue) on an RGB565 texture (the DC’s graphics hardware can also do paletted textures but those need to be rearranged/twiddled/swizzled).

  Results
  So, can the Dreamcast decode VP8 video in realtime ? Sure ! Well, I really need to qualify. In the test depicted in the picture, it seems to be realtime (though I wasn’t enforcing proper frame timings, just decoding and displaying as quickly as possible). Obviously, I wasn’t bothering to properly convert YUV -> RGB. Plus, that Big Buck Bunny test vector clip is only 176x144. Obviously, no audio decoding either.

  So, realtime playback, with a little fine print.

  On the plus side, it’s trivial to get the Dreamcast video hardware to upscale that little blue image to fullscreen.

  I was able to tally the total milliseconds’ worth of wall clock time required to decode the 17 VP8 test vectors. As you can probably work out from this list, when I try to play a 320x240 video, things start to break down.

  1. Processed 29 176x144 frames in 987 milliseconds.
  2. Processed 49 176x144 frames in 1809 milliseconds.
  3. Processed 49 176x144 frames in 704 milliseconds.
  4. Processed 29 176x144 frames in 255 milliseconds.
  5. Processed 49 176x144 frames in 339 milliseconds.
  6. Processed 48 175x143 frames in 2446 milliseconds.
  7. Processed 29 176x144 frames in 432 milliseconds.
  8. Processed 2 1432x888 frames in 2060 milliseconds.
  9. Processed 49 176x144 frames in 1884 milliseconds.
  10. Processed 57 320x240 frames in 5792 milliseconds.
  11. Processed 29 176x144 frames in 989 milliseconds.
  12. Processed 29 176x144 frames in 740 milliseconds.
  13. Processed 29 176x144 frames in 839 milliseconds.
  14. Processed 49 175x143 frames in 2849 milliseconds.
  15. Processed 260 320x240 frames in 29719 milliseconds.
  16. Processed 29 176x144 frames in 962 milliseconds.
  17. Processed 29 176x144 frames in 933 milliseconds.

• FFmpeg unexpected behavior using -loop flag

  7 décembre 2020, par all jazz

  Dear hackers of the world !

  


  I've been trying to use the beloved FFmpeg library to create a video from an image loop and audio using the famous Docker FFmpeg image, but it has been driving me crazy not producing the expected results (the results that I get when I run the ffmpeg command with the equivalent version on my Macbook).

  


  Here is the command :

  


  docker run -v $(pwd):$(pwd) -w $(pwd) jrottenberg/ffmpeg:4.3-alpine \
    -y \
    -stats \
    -loop 1 -i files/image.jpg \
    -i files/a.mp3 \
    -c:v libx265 -pix_fmt yuv420p10 \
    -c:a aac \
    -movflags +faststart \
    -shortest \
    -f mp4 test.mp4


  


  It should create a test.mp4 with the provided audio and image that is ready to be uploaded to the unfortunate Youtube.

  


  When I do this, the video seems to be lacking moov atoms (if I try to analyse it). Strangely enough, if I run this two times using the Docker image (overriding the same file), the video file will magically start to work.

  


  I also tried using different ffmpeg os images and versions. It seems that ffmpeg docummentation and code repo could also benefit from some care and love.

  


  What else I could do to get this fixed ?

  


  Here is the output from the console :

  


          -y \
        -stats \
        -loop 1 -i files/image.jpg \
        -i files/a.mp3 \
        -c:v libx265 -pix_fmt yuv420p10 \
        -c:a aac \
        -movflags +faststart \
        -shortest \
        -f mp4 test30.mp4
ffmpeg version 4.3.1 Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 6.4.0 (Alpine 6.4.0)
  configuration: --disable-debug --disable-doc --disable-ffplay --enable-shared --enable-avresample --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-gpl --enable-libass --enable-fontconfig --enable-libfreetype --enable-libvidstab --enable-libmp3lame --enable-libopus --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libxcb --enable-libx265 --enable-libxvid --enable-libx264 --enable-nonfree --enable-openssl --enable-libfdk_aac --enable-postproc --enable-small --enable-version3 --enable-libbluray --enable-libzmq --extra-libs=-ldl --prefix=/opt/ffmpeg --enable-libopenjpeg --enable-libkvazaar --enable-libaom --extra-libs=-lpthread --enable-libsrt --enable-libaribb24 --extra-cflags=-I/opt/ffmpeg/include --extra-ldflags=-L/opt/ffmpeg/lib
  libavutil      56. 51.100 / 56. 51.100
  libavcodec     58. 91.100 / 58. 91.100
  libavformat    58. 45.100 / 58. 45.100
  libavdevice    58. 10.100 / 58. 10.100
  libavfilter     7. 85.100 /  7. 85.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  7.100 /  5.  7.100
  libswresample   3.  7.100 /  3.  7.100
  libpostproc    55.  7.100 / 55.  7.100
Input #0, image2, from 'files/image.jpg':
  Duration: 00:00:00.04, start: 0.000000, bitrate: 34300 kb/s
    Stream #0:0: Video: mjpeg, gray(bt470bg/unknown/unknown), 500x500 [SAR 240:240 DAR 1:1], 25 fps, 25 tbr, 25 tbn, 25 tbc
Input #1, mp3, from 'files/a.mp3':
  Metadata:
    title           : Visions
    artist          : Hattori Hanzo
    album           : Visions
    encoded_by      : Fission
    encoder         : Lavf58.45.100
    TLEN            : 16039
    track           : 1
  Duration: 00:00:16.04, start: 0.000000, bitrate: 199 kb/s
    Stream #1:0: Audio: mp3, 44100 Hz, stereo, fltp, 191 kb/s
    Stream #1:1: Video: mjpeg, yuvj444p(pc, bt470bg/unknown/unknown), 500x500 [SAR 300:300 DAR 1:1], 90k tbr, 90k tbn, 90k tbc (attached pic)
    Metadata:
      comment         : Other
Stream mapping:
  Stream #0:0 -> #0:0 (mjpeg (native) -> hevc (libx265))
  Stream #1:0 -> #0:1 (mp3 (mp3float) -> aac (native))
Press [q] to stop, [?] for help
x265 [info]: HEVC encoder version 3.1.1+1-04b37fdfd2dc
x265 [info]: build info [Linux][GCC 6.4.0][64 bit] 10bit
x265 [info]: using cpu capabilities: MMX2 SSE2Fast LZCNT SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
x265 [info]: Main 10 profile, Level-3 (Main tier)
x265 [info]: Thread pool created using 8 threads
x265 [info]: Slices                              : 1
x265 [info]: frame threads / pool features       : 3 / wpp(8 rows)
x265 [warning]: Source height < 720p; disabling lookahead-slices
x265 [info]: Coding QT: max CU size, min CU size : 64 / 8
x265 [info]: Residual QT: max TU size, max depth : 32 / 1 inter / 1 intra
x265 [info]: ME / range / subpel / merge         : hex / 57 / 2 / 3
x265 [info]: Keyframe min / max / scenecut / bias: 25 / 250 / 40 / 5.00
x265 [info]: Lookahead / bframes / badapt        : 20 / 4 / 2
x265 [info]: b-pyramid / weightp / weightb       : 1 / 1 / 0
x265 [info]: References / ref-limit  cu / depth  : 3 / off / on
x265 [info]: AQ: mode / str / qg-size / cu-tree  : 2 / 1.0 / 32 / 1
x265 [info]: Rate Control / qCompress            : CRF-28.0 / 0.60
x265 [info]: tools: rd=3 psy-rd=2.00 early-skip rskip signhide tmvp b-intra
x265 [info]: tools: strong-intra-smoothing deblock sao
Output #0, mp4, to 'test30.mp4':
  Metadata:
    encoder         : Lavf58.45.100
    Stream #0:0: Video: hevc (libx265) (hev1 / 0x31766568), yuv420p10le(progressive), 500x500 [SAR 1:1 DAR 1:1], q=-1--1, 25 fps, 12800 tbn, 25 tbc
    Metadata:
      encoder         : Lavc58.91.100 libx265
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
    Stream #0:1: Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s
    Metadata:
      encoder         : Lavc58.91.100 aac
frame=   52 fps=0.0 q=33.0 size=       0kB time=00:00:00.80 bitrate=   0.4kbits/frame=  120 fps=119 q=33.0 size=       0kB time=00:00:03.52 bitrate=   0.1kbits/frame=  190 fps=125 q=33.0 size=       0kB time=00:00:06.33 bitrate=   0.1kbits/frame=  257 fps=127 q=33.0 size=       0kB time=00:00:09.00 bitrate=   0.0kbits/frame=  303 fps=120 q=35.0 size=     256kB time=00:00:10.86 bitrate= 193.0kbits/frame=  373 fps=123 q=36.0 size=     256kB time=00:00:13.65 bitrate= 153.6kbits/[mp4 @ 0x55d481bc6980] Starting second pass: moving the moov atom to the beginning of the file
frame=  432 fps=121 q=36.0 Lsize=     379kB time=00:00:17.16 bitrate= 180.8kbits/s speed= 4.8x
video:107kB audio:255kB subtitle:0kB other streams:0kB global headers:2kB muxing overhead: 4.667185%
x265 [info]: frame I:      2, Avg QP:23.55  kb/s: 8634.80
x265 [info]: frame P:    147, Avg QP:33.00  kb/s: 13.49
x265 [info]: frame B:    283, Avg QP:35.71  kb/s: 8.06
x265 [info]: Weighted P-Frames: Y:0.0% UV:0.0%
x265 [info]: consecutive B-frames: 34.2% 10.1% 20.8% 1.3% 33.6%

encoded 432 frames in 3.56s (121.32 fps), 49.84 kb/s, Avg QP:34.73
[aac @ 0x55d481b23ac0] Qavg: 563.168```