Recherche avancée

Sur d’autres sites (6461)

• RoQ on Dreamcast

  18 mars 2011, par Multimedia Mike — Sega Dreamcast

  I have been working on that challenge to play back video on the Sega Dreamcast. To review, I asserted that the RoQ format would be a good fit for the Sega Dreamcast hardware. The goal was to play 640x480 video at 30 frames/second. Short version : I have determined that it is possible to decode such video in real time. However, I ran into certain data rate caveats.

  First off : Have you ever wondered if the Dreamcast can read an 80mm optical disc ? It can ! I discovered this when I only had 60 MB of RoQ samples to burn on a disc and a spindle full of these 210MB-capacity 80mm CD-Rs that I never have occasion to use.

  
  
  

  New RoQ Library
  There are open source RoQ decoders out there but I decided to write a new one. A few reasons : 1) RoQ is so simple that I didn’t think it would take too long ; 2) it would be nice to have a RoQ library that is license-compatible (BSD-like) with the rest of the KallistiOS distribution ; 3) the idroq.tar.gz distribution, while license-compatible, has enough issues that I didn’t want to correct it.

  Thankfully, I was correct about the task not being too difficult : I put together a new RoQ decoder in short order. I’m a bit embarrassed to admit that the part I had the most trouble with was properly converting YUV -> RGB.
  
  About the approach I took : While the original idroq.tar.gz decoder maintains YUV 4:2:0 codebooks (which led to chroma bugs during motion compensation) and FFmpeg’s decoder maintains YUV 4:4:4 codebooks, this decoder is built to convert the YUV 4:2:0 vectors into RGB565 vectors during the vector unpacking phase. Thus, the entire frame is rendered in RGB565 — no lengthy YUV -> RGB conversion after decoding — and all pixels are shuffled around as 16-bit units (minor speedup vs. shuffling everything as bytes).

  I also entertained the idea of maintaining YUYV codebooks (since the DC supports that colorspace as a texture format). But I scrapped that idea when I remembered it would lead to the same chroma bleeding problem seen in the original idroq.tar.gz decoder.

  Onto The Dreamcast
  I developed the library on a Linux computer, allowing it to output a series of PNM files for visual verification and debugging. Dropping it into a basic DC/KOS-compatible program was trivial and the first order of business was profiling.

  At first, I profiled the entire decode operation : open file, then read and decode each chunk while tossing away the results. I was roundly disappointed to see that, e.g., an 8.5-second RoQ sample needed a little more than 20 seconds to complete. Not real time. I performed a series of optimizations on the decoding library that netted notable performance gains when profiling on Linux. When I brought these same optimizations over to the DC, decoding time didn’t improve at all. This was my first suspicion that perhaps my assumptions regarding the DC’s optical drive’s data rate were not correct.

  Dreamcast Data Rate Profiling
  Let’s start with some definitions : In terms of data rate, an ’X’, i.e., 1X is the minimum data rate needed to read CD quality audio from a disc. At that speed, a drive should be able to stream 75 sectors each second. When reading mode 1/form 1 CD-ROM data, each sector has 2048 bytes (2 kbytes), so a single-speed data rate should achieve 150 kbytes/sec.

  The Dreamcast is supposed to possess a 12X optical drive. This would imply a maximum data rate of 150 kbytes/sec * 12 = 1800 kbytes/sec.

  Rigging up a trivial experiment using the RoQ samples burned on a few different CD-R discs, the best data rate I can see is about 500-525 kbytes/sec, or around 3.5X.

  Where’s the discrepancy ? My first theory has to do with the fact that not all optical media is created equal. This is why optical drives often advertise a slew of numbers which refer to the best theoretical speed for reading a CD vs. writing a CD-R vs. writing a CD-RW, etc. Perhaps the DC drive can’t read CD-Rs very quickly. To test this theory, I tried streaming a large file from a conventionally mastered CD-ROM. This worked well for the closest CD-ROM I had on hand : I was able to stream data at a rate that works out to about 6.5X.

  I smell a science project for another evening : Profiling read speeds from a mastered CD-ROM, burned CD-R, and also a mastered GD-ROM, on each of the 3 Dreamcast consoles I possess (I’ve heard that there’s variance between optical drives depending on manufacturing run).

  The Good News
  I added a little finer-grained code to profile just the video decoding functions. The good news is that the decoder meets my real time goals : That 8.5-second RoQ sample encoded at 640x480x30fps makes its way through the video decoding functions on the DC in a little less than 5 seconds. If the optical drive can supply the data fast enough, the video decoder can take care of the rest.

  The RoQ encoder included with FFmpeg does not honor any bitrate parameters. Instead, I encoded the same file at 320x240. It reportedly decoded in real time and can be streamed in real time as well.

  I say "reportedly" because I’m simply working from textual output at this point ; the next phase is to hook the decoder up to the display hardware.

• Revision cb1e60fb28 : Fix bug in reference frame counting. vp8_encode_inter_macroblock() is called in

  17 juillet 2012, par Ronald S. Bultje

  Changed Paths : Modify /vp8/encoder/encodeframe.c Fix bug in reference frame counting. vp8_encode_inter_macroblock() is called in both pick_mb_modes() as well as encode_sb(), thus the number of macroblocks in the counter were twice as big as actual numbers. This doesn't affect output. (...)

• converting .mov file to .h264 file

  29 août 2011, par Robin Rye

  ok, this is the case, i actually want to parse frames from a mov file. get the encoded h264 frames. and i've managed to do so by using ffmpeg but when i try to make a movie again by using ffmpeg -i test* test.mov i get test00: Invalid data found when processing input so there is something not correct with the structure of the frames. as i understand it a frame should have the following appearance :

  00 00 00 01 XX data -------------

  where XX is say whether it is a I-,P- or B-frame. or more specifically type(XX) = 0x0F && XX says if it is I(type(XX) = 5 ?),P(type(XX) = 7 ?) or B(type(XX) = 8 ?) frame. I'm not sure about these number, i've been looking for it but not found good sources. so that's question number one, what number should the NALU be for the different frames ?

  anyway, when i use av_read_frame on the mov file, i get frame that look like this :

  4B = size, 1B = XX and then data. (at least this is what i think i get)

  the files where i store the frames are always size long when i look at them in a hexeditor(otherwise as well of course). and XX is always 65(ie. type(XX) = 5) in the first and then 61(ie. type(XX) = 1) for a couple of frames and then back to being 65 for one frame and so on.

  i guess that these are frames like : I P P P P P P I P P P P P P P I P P P P P P P .... however then my assumption about the type numbers for the different frame types are false, which is highly likely. (any suggestion on reading about this ? except the ISO/IEC 14496-10, i don't understand it really).

  I've tried to remove the size and append 00 00 00 01 before the XX byte and the data but without success. any tips on how i could modify the frames to be valid H264 encoded frames ?