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• Linux Media Player Survey Circa 2001

  2 septembre 2010, par Multimedia Mike — General

  Here’s a document I scavenged from my archives. It was dated September 1, 2001 and I now publish it 9 years later. It serves as sort of a time capsule for the state of media player programs at the time. Looking back on this list, I can’t understand why I couldn’t find MPlayer while I was conducting this survey, especially since MPlayer is the project I eventually started to work for a few months after writing this piece.

  For a little context, I had been studying multimedia concepts and tech for a year and was itching to get my hands dirty with practical multimedia coding. But I wanted to tackle what I perceived as unsolved problems– like playback of proprietary codecs. I didn’t want to have to build a new media playback framework just to start working on my problems. So I surveyed the players available to see which ones I could plug into and use as a testbed for implementing new decoders.

  Regarding Real Player, I wrote : “We’re trying to move away from the proprietary, closed-source “solutions”. Heh. Was I really an insufferable open source idealist back in the day ?

  Anyway, here’s the text with some Where are they now ? commentary [in brackets] :

  ---

  Towards an All-Inclusive Media Playing Solution for Linux

  I don’t feel that the media playing solutions for Linux set their sights high enough, even though they do tend to be quite ambitious.

  I want to create a media player for Linux that can open a file, figure out what type of file it is (AVI, MOV, etc.), determine the compression algorithms used to encode the audio and video chunks inside (MPEG, Cinepak, Sorenson, etc.) and replay the file using the best audio, video, and CPU facilities available on the computer.

  Video and audio playback is a solved problem on Linux ; I don’t wish to solve that problem again. The problem that isn’t solved is reliance on proprietary multimedia solutions through some kind of WINE-like layer in order to decode compressed multimedia files.

  Survey of Linux solutions for decoding proprietary multimedia
  updated 2001-09-01

  AVI Player for XMMS
  This is based on Avifile. All the same advantages and limitations apply.
  [Top Google hit is a Freshmeat page that doesn’t indicate activity since 2001-2002.]

  Avifile
  This player does a great job at taking apart AVI and ASF files and then feeding the compressed chunks of multimedia data through to the binary Win32 decoders.

  The program is written in C++ and I’m not very good at interpreting that kind of code. But I’m learning all over again. Examining the object hierarchy, it appears that the designers had the foresight to include native support for decoders that are compiled into the program from source code. However, closer examination reveals that there is support for ONE source decoder and that’s the “decoder” for uncompressed data. Still, I tried to manipulate this routine to accept and decode data from other codecs but no dice. It’s really confounding. The program always crashes when I feed non-uncompressed data through the source decoder.
  [Lives at http://avifile.sourceforge.net/ ; not updated since 2006.]

  Real Player
  There’s not much to do with this since it is closed source and proprietary. Even though there is a plugin architecture, that’s not satisfactory. We’re trying to move away from the proprietary, closed-source “solutions”.
  [Still kickin’ with version 11.]

  XAnim
  This is a well-established Unix media player. To his credit, the author does as well as he can with the resources he has. In other words, he supports the non-proprietary video codecs well, and even has support for some proprietary video codecs through binary-only decoders.

  The source code is extremely difficult to work with as the author chose to use the X coding format which I’ve never seen used anywhere else except for X header files. The infrastructure for extending the program and supporting other codecs and file formats is there, I suppose, but I would have to wrap my head around the coding style. Maybe I can learn to work past that. The other thing that bothers me about this program is the decoding approach : It seems that each video decoder includes routines to decompress the multimedia data into every conceivable RGB and YUV output format. This seems backwards to me ; it seems better to have one decoder function that decodes the data into its native format it was compressed from (e.g., YV12 for MPEG data) and then pass that data to another layer of the program that’s in charge of presenting the data and possibly converting it if necessary. This layer would encompass highly-optimized software conversion routines including special CPU-specific instructions (e.g., MMX and SSE) and eliminate the need to place those routines in lots of other routines. But I’m getting ahead of myself.
  [This one was pretty much dead before I made this survey, the most recent update being in 1999. Still, we owe it much respect as the granddaddy of Unix multimedia playback programs.]

  Xine
  This seems like a promising program. It was originally designed to play MPEGs from DVDs. It can also play MPEG files on a hard drive and utilizes the Xv extensions for hardware YUV playback. It’s also supposed to play AVI files using the same technique as Avifile but I have never, ever gotten it to work. If an AVI file has both video and sound, the binary video decoder can’t decode any frames. If the AVI file has video and no sound, the program gets confused and crashes, as far as I can tell.

  Still, it’s promising, and I’ve been trying to work around these crashes. It doesn’t yet have the type of modularization I’d like to see. Right now, it tailored to suit MPEG playback and AVI playback is an afterthought. Still, it appears to have a generalized interface for dropping in new file demultiplexers.

  I tried to extend the program for supporting source decoders by rewriting w32codec.c from scratch. I’m not having a smooth time of it so far. I’m able to perform some manipulations on the output window. However, I can’t get the program to deal with an RGB image format. It has trouble allocating an RGB surface with XvShmCreateImage(). This isn’t suprising, per my limited knowledge of X which is that Xv applies to YUV images, but it could also apply to RGB images as well. Anyway, the program should be able to fall back on regular RGB pixmaps if that Xv call fails.

  Right now, this program is looking the most promising. It will take some work to extend the underlying infrastructure, but it seems doable since I know C quite well and can understand the flow of this program, as opposed to Avifile and its C++. The C code also compiles about 10 times faster.
  [My home project for many years after a brief flirtation with MPlayer. It is still alive ; its latest release was just a month ago.]

  XMovie
  This library is a Quicktime movie player. I haven’t looked at it too extensively yet, but I do remember looking at it at one point and reading the documentation that said it doesn’t support key frames. Still, I should examine it again since they released a new version recently.
  [Heroine Virtual still puts out some software but XMovie has not been updated since 2005.]

  XMPS
  This program compiles for me, but doesn’t do much else. It can play an MP3 file. I have been able to get MPEG movies to play through it, but it refuses to show the full video frame, constricting it to a small window (obviously a bug).
  [This project is hosted on SourceForge and is listed with a registration date of 2003, well after this survey was made. So the project obviously lived elsewhere in 2001. Meanwhile, it doesn’t look like any files ever made it to SF for hosting.]

  XTheater
  I can’t even get this program to compile. It’s supposed to be an MPEG player based on SMPEG. As such, it probably doesn’t hold much promise for being easily extended into a general media player.
  [Last updated in 2002.]

  GMerlin
  I can’t get this to compile yet. I have a bug report in to the dev group.
  [Updated consistently in the last 9 years. Last update was in February of this year. I can’t find any record of my bug report, though.]

• The use cases for a element in HTML

  1er janvier 2014, par silvia

  The W3C HTML WG and the WHATWG are currently discussing the introduction of a <main> element into HTML.

  The <main> element has been proposed by Steve Faulkner and is specified in a draft extension spec which is about to be accepted as a FPWD (first public working draft) by the W3C HTML WG. This implies that the W3C HTML WG will be looking for implementations and for feedback by implementers on this spec.

  I am supportive of the introduction of a <main> element into HTML. However, I believe that the current spec and use case list don’t make a good enough case for its introduction. Here are my thoughts.

  Main use case : accessibility

  In my opinion, the main use case for the introduction of <main> is accessibility.

  Like any other users, when blind users want to perceive a Web page/application, they need to have a quick means of grasping the content of a page. Since they cannot visually scan the layout and thus determine where the main content is, they use accessibility technology (AT) to find what is known as “landmarks”.

  “Landmarks” tell the user what semantic content is on a page : a header (such as a banner), a search box, a navigation menu, some asides (also called complementary content), a footer, …. and the most important part : the main content of the page. It is this main content that a blind user most often wants to skip to directly.

  In the days of HTML4, a hidden “skip to content” link at the beginning of the Web page was used as a means to help blind users access the main content.

  In the days of ARIA, the aria @role=main enables authors to avoid a hidden link and instead mark the element where the main content begins to allow direct access to the main content. This attribute is supported by AT – in particular screen readers – by making it part of the landmarks that AT can directly skip to.

  Both the hidden link and the ARIA @role=main approaches are, however, band aids : they are being used by those of us that make “finished” Web pages accessible by adding specific extra markup.

  A world where ARIA is not necessary and where accessibility developers would be out of a job because the normal markup that everyone writes already creates accessible Web sites/applications would be much preferable over the current world of band-aids.

  Therefore, to me, the primary use case for a <main> element is to achieve exactly this better world and not require specialized markup to tell a user (or a tool) where the main content on a page starts.

  An immediate effect would be that pages that have a <main> element will expose a “main” landmark to blind and vision-impaired users that will enable them to directly access that main content on the page without having to wade through other text on the page. Without a <main> element, this functionality can currently only be provided using heuristics to skip other semantic and structural elements and is for this reason not typically implemented in AT.

  Other use cases

  The <main> element is a semantic element not unlike other new semantic elements such as <header>, <footer>, <aside>, <article>, <nav>, or <section>. Thus, it can also serve other uses where the main content on a Web page/Web application needs to be identified.

  Data mining

  For data mining of Web content, the identification of the main content is one of the key challenges. Many scholarly articles have been published on this topic. This stackoverflow article references and suggests a multitude of approaches, but the accepted answer says “there’s no way to do this that’s guaranteed to work”. This is because Web pages are inherently complex and many <div>, <p>, <iframe> and other elements are used to provide markup for styling, notifications, ads, analytics and other use cases that are necessary to make a Web page complete, but don’t contribute to what a user consumes as semantically rich content. A <main> element will allow authors to pro-actively direct data mining tools to the main content.

  Search engines

  One particularly important “data mining” tool are search engines. They, too, have a hard time to identify which sections of a Web page are more important than others and employ many heuristics to do so, see e.g. this ACM article. Yet, they still disappoint with poor results pointing to findings of keywords in little relevant sections of a page rather than ranking Web pages higher where the keywords turn up in the main content area. A <main> element would be able to help search engines give text in main content areas a higher weight and prefer them over other areas of the Web page. It would be able to rank different Web pages depending on where on the page the search words are found. The <main> element will be an additional hint that search engines will digest.

  Visual focus

  On small devices, the display of Web pages designed for Desktop often causes confusion as to where the main content can be found and read, in particular when the text ends up being too small to be readable. It would be nice if browsers on small devices had a functionality (maybe a default setting) where Web pages would start being displayed as zoomed in on the main content. This could alleviate some of the headaches of responsive Web design, where the recommendation is to show high priority content as the first content. Right now this problem is addressed through stylesheets that re-layout the page differently depending on device, but again this is a band-aid solution. Explicit semantic markup of the main content can solve this problem more elegantly.

  Styling

  Finally, naturally, <main> would also be used to style the main content differently from others. You can e.g. replace a semantically meaningless <div id=”main”> with a semantically meaningful <main> where their position is identical. My analysis below shows, that this is not always the case, since oftentimes <div id=”main”> is used to group everything together that is not the header – in particular where there are multiple columns. Thus, the ease of styling a <main> element is only a positive side effect and not actually a real use case. It does make it easier, however, to adapt the style of the main content e.g. with media queries.

  Proposed alternative solutions

  It has been proposed that existing markup serves to satisfy the use cases that <main> has been proposed for. Let’s analyse these on some of the most popular Web sites. First let’s list the propsed algorithms.

  Proposed solution No 1 : Scooby-Doo

  On Sat, Nov 17, 2012 at 11:01 AM, Ian Hickson <ian@hixie.ch> wrote :
  | The main content is whatever content isn’t
  | marked up as not being main content (anything not marked up with <header>,
  | <aside>, <nav>, etc).
  

  This implies that the first element that is not a <header>, <aside>, <nav>, or <footer> will be the element that we want to give to a blind user as the location where they should start reading. The algorithm is implemented in https://gist.github.com/4032962.

  Proposed solution No 2 : First article element

  On Sat, Nov 17, 2012 at 8:01 AM, Ian Hickson  wrote :
  | On Thu, 15 Nov 2012, Ian Yang wrote :
  | >
  | > That’s a good idea. We really need an element to wrap all the <p>s,
  | > <ul>s, <ol>s, <figure>s, <table>s ... etc of a blog post.
  |
  | That’s called <article>.
  

  This approach identifies the first <article> element on the page as containing the main content. Here’s the algorithm for this approach.

  Proposed solution No 3 : An example heuristic approach

  The readability plugin has been developed to make Web pages readable by essentially removing all the non-main content from a page. An early source of readability is available. This demonstrates what a heuristic approach can perform.

  Analysing alternative solutions

  Comparison

  I’ve picked 4 typical Websites (top on Alexa) to analyse how these three different approaches fare. Ideally, I’d like to simply apply the above three scripts and compare pictures. However, since the semantic HTML5 elements <header>, <aside>, <nav>, and <footer> are not actually used by any of these Web sites, I don’t actually have this choice.

  So, instead, I decided to make some assumptions of where these semantic elements would be used and what the outcome of applying the first two algorithms would be. I can then compare it to the third, which is a product so we can take screenshots.

  Google.com

  http://google.com – search for “Scooby Doo”.

  The search results page would likely be built with :

  • a <nav> menu for the Google bar
  • a <header> for the search bar
  • another <header> for the login section
  • another <nav> menu for the search types
  • a <div> to contain the rest of the page
  • a <div> for the app bar with the search number
  • a few <aside>s for the left and right column
  • a set of <article>s for the search results

  “Scooby Doo” would find the first element after the headers as the “main content”. This is the element before the app bar in this case. Interestingly, there is a <div @id=main> already in the current Google results page, which “Scooby Doo” would likely also pick. However, there are a nav bar and two asides in this div, which clearly should not be part of the “main content”. Google actually placed a @role=main on a different element, namely the one that encapsulates all the search results.

  “First Article” would find the first search result as the “main content”. While not quite the same as what Google intended – namely all search results – it is close enough to be useful.

  The “readability” result is interesting, since it is not able to identify the main text on the page. It is actually aware of this problem and brings a warning before displaying this page :

  

  Facebook.com

  https://facebook.com

  A user page would likely be built with :

  • a <header> bar for the search and login bar
  • a <div> to contain the rest of the page
  • an <aside> for the left column
  • a <div> to contain the center and right column
  • an <aside> for the right column
  • a <header> to contain the center column “megaphone”
  • a <div> for the status posting
  • a set of <article>s for the home stream

  “Scooby Doo” would find the first element after the headers as the “main content”. This is the element that contains all three columns. It’s actually a <div @id=content> already in the current Facebook user page, which “Scooby Doo” would likely also pick. However, Facebook selected a different element to place the @role=main : the center column.

  “First Article” would find the first news item in the home stream. This is clearly not what Facebook intended, since they placed the @role=main on the center column, above the first blog post’s title. “First Article” would miss that title and the status posting.

  The “readability” result again disappoints but warns that it failed :

  

  YouTube.com

  http://youtube.com

  A video page would likely be built with :

  • a <header> bar for the search and login bar
  • a <nav> for the menu
  • a <div> to contain the rest of the page
  • a <header> for the video title and channel links
  • a <div> to contain the video with controls
  • a <div> to contain the center and right column
  • an <aside> for the right column with an <article> per related video
  • an <aside> for the information below the video
  • a <article> per comment below the video

  “Scooby Doo” would find the first element after the headers as the “main content”. This is the element that contains the rest of the page. It’s actually a <div @id=content> already in the current YouTube video page, which “Scooby Doo” would likely also pick. However, YouTube’s related videos and comments are unlikely to be what the user would regard as “main content” – it’s the video they are after, which generously has a <div id=watch-player>.

  “First Article” would find the first related video or comment in the home stream. This is clearly not what YouTube intends.

  The “readability” result is not quite as unusable, but still very bare :

  

  Wikipedia.com

  http://wikipedia.com (“Overscan” page)

  A Wikipedia page would likely be built with :

  • a <header> bar for the search, login and menu items
  • a <div> to contain the rest of the page
  • an &ls ; article> with title and lots of text
  • <article> an <aside> with the table of contents
  • several <aside>s for the left column

  Good news : “Scooby Doo” would find the first element after the headers as the “main content”. This is the element that contains the rest of the page. It’s actually a <div id=”content” role=”main”> element on Wikipedia, which “Scooby Doo” would likely also pick.

  “First Article” would find the title and text of the main element on the page, but it would also include an <aside>.

  The “readability” result is also in agreement.

  

  Results

  In the following table we have summarised the results for the experiments :

  Site	Scooby-Doo	First article	Readability
  Google.com	FAIL	SUCCESS	FAIL
  Facebook.com	FAIL	FAIL	FAIL
  YouTube.com	FAIL	FAIL	FAIL
  Wikipedia.com	SUCCESS	SUCCESS	SUCCESS

  Clearly, Wikipedia is the prime example of a site where even the simple approaches find it easy to determine the main content on the page. WordPress blogs are similarly successful. Almost any other site, including news sites, social networks and search engine sites are petty hopeless with the proposed approaches, because there are too many elements that are used for layout or other purposes (notifications, hidden areas) such that the pre-determined list of semantic elements that are available simply don’t suffice to mark up a Web page/application completely.

  Conclusion

  It seems that in general it is impossible to determine which element(s) on a Web page should be the “main” piece of content that accessibility tools jump to when requested, that a search engine should put their focus on, or that should be highlighted to a general user to read. It would be very useful if the author of the Web page would provide a hint through a <main> element where that main content is to be found.

  I think that the <main> element becomes particularly useful when combined with a default keyboard shortcut in browsers as proposed by Steve : we may actually find that non-accessibility users will also start making use of this shortcut, e.g. to get to videos on YouTube pages directly without having to tab over search boxes and other interactive elements, etc. Worthwhile markup indeed.

• ffmpeg get RGB values ?

  13 novembre 2012, par Nav

  I've seen this, this and this but I still can't figure out how to get the RGB values from the tutorial code.

  if(avcodec_open(pCodecCtx, pCodec)<0) {_getch();return -1;}
  
  pFrame = avcodec_alloc_frame();
  
  pFrameRGB=avcodec_alloc_frame();
  
  if(pFrameRGB==NULL)    {_getch();return -1;}
  
  numBytes=avpicture_get_size(PIX_FMT_RGB24, pCodecCtx->width, pCodecCtx->height);
  
  buffer=(uint8_t *)av_malloc(numBytes*sizeof(uint8_t));
  
  // Assign appropriate parts of buffer to image planes in pFrameRGB
  
  avpicture_fill((AVPicture *)pFrameRGB, buffer, PIX_FMT_RGB24, pCodecCtx->width, pCodecCtx->height);
  
  
  
  i=0;
  
  int framecounter=0;
  
  while(av_read_frame(pFormatCtx, &packet)>=0) 
  
  {
  
    if(packet.stream_index==videoStream) 
  
      {
  
        // Decode video frame
  
        //int avcodec_decode_video2(AVCodecContext *avctx, AVFrame *picture, int *got_picture_ptr, const AVPacket *avpkt);
  
        avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);
  
  
  
        if(frameFinished) 
  
        {
  
           // Convert the image from its native format to RGB
  
       //img_convert((AVPicture *)pFrameRGB, PIX_FMT_RGB24, (AVPicture*)pFrame, pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height);
  
           if(++i<=5)  {SaveFrame(pFrameRGB, pCodecCtx->width, pCodecCtx->height, i);}
  
        }
  
      }
  
    av_free_packet(&packet);
  
  }

  Since img_convert wasn't available, I took it from here but it threw an unhandled exception error. Why is the RGB format so tough to understand ? Why assign zero to G and B of RGB, as shown here ?

  How can I get my RGB values as a simple R, G and B which spans between 0 and 255 ? Also, I don't understand the use of x and y, when video width and height are already given. y is an increment of height, but what about x ?

  Can anyone please show me a complete working code of obtaining rgb values from a video or at least explain how data and linesize help in storing RGB info ? The lack of tutorials is appalling and the complexity of the API is surprising, after having worked on Processing videos.