Monday, January 23, 2012

Java Script Basics

What is JavaScript ?

JavaScript is:
  • JavaScript is a lightweight, interpreted programming language
  • Designed for creating network-centric applications
  • Complementary to and integrated with Java
  • Complementary to and integrated with HTML
  • Open and cross-platform

JavaScript Syntax:

A JavaScript consists of JavaScript statements that are placed within the <script>... </script> HTML tags in a web page.
You can place the <script> tag containing your JavaScript anywhere within you web page but it is preferred way to keep it within the <head> tags.
The <script> tag alert the browser program to begin interpreting all the text between these tags as a script. So simple syntax of your JavaScript will be as follows
<script ...>
  JavaScript code
</script>

The script tag takes two important attributes:
  • language: This attribute specifies what scripting language you are using. Typically, its value will be javascript. Although recent versions of HTML (and XHTML, its successor) have phased out the use of this attribute.
  • type: This attribute is what is now recommended to indicate the scripting language in use and its value should be set to "text/javascript".
So your JavaScript segment will look like:
<script language="javascript" type="text/javascript">
  JavaScript code
</script>

Your First JavaScript Script:

Let us write our class example to print out "Hello World".
<html>
<body>
<script language="javascript" type="text/javascript">
<!--
   document.write("Hello World!")
//-->
</script>
</body>
</html>
Above code will display following result:
Hello World!

Whitespace and Line Breaks:

JavaScript ignores spaces, tabs, and newlines that appear in JavaScript programs.
Because you can use spaces, tabs, and newlines freely in your program so you are free to format and indent your programs in a neat and consistent way that makes the code easy to read and understand.

Semicolons are Optional:

Simple statements in JavaScript are generally followed by a semicolon character, just as they are in C, C++, and Java. JavaScript, however, allows you to omit this semicolon if your statements are each placed on a separate line. For example, the following code could be written without semicolons
<script language="javascript" type="text/javascript">
<!--
  var1 = 10
  var2 = 20
//-->
</script>
But when formatted in a single line as follows, the semicolons are required:
<script language="javascript" type="text/javascript">
<!--
  var1 = 10; var2 = 20;
//-->
</script>
Note: It is a good programming practice to use semicolons.

Case Sensitivity:

JavaScript is a case-sensitive language. This means that language keywords, variables, function names, and any other identifiers must always be typed with a consistent capitalization of letters.
So identifiers TimeTIme and TIME will have different meanings in JavaScript.
NOTE: Care should be taken while writing your variable and function names in JavaScript.

Comments in JavaScript:

JavaScript supports both C-style and C++-style comments, Thus:
  • Any text between a // and the end of a line is treated as a comment and is ignored by JavaScript.
  • Any text between the characters /* and */ is treated as a comment. This may span multiple lines.
  • JavaScript also recognizes the HTML comment opening sequence <!--. JavaScript treats this as a single-line comment, just as it does the // comment.
  • The HTML comment closing sequence --> is not recognized by JavaScript so it should be written as //-->.

JavaScript Placement in HTML File:

There is a flexibility given to include JavaScript code anywhere in an HTML document. But there are following most preferred ways to include JavaScript in your HTML file.
  • Script in <head>...</head> section.
  • Script in <body>...</body> section.
  • Script in <body>...</body> and <head>...</head> sections.
  • Script in and external file and then include in <head>...</head> section.

JavaScript DataTypes:

JavaScript allows you to work with three primitive data types:
  • Numbers eg. 123, 120.50 etc.
  • Strings of text e.g. "This text string" etc.
  • Boolean e.g. true or false.
JavaScript also defines two trivial data types, null and undefined, each of which defines only a single value.

JavaScript Variables:

Like many other programming languages, JavaScript has variables. Variables can be thought of as named containers. You can place data into these containers and then refer to the data simply by naming the container.
Before you use a variable in a JavaScript program, you must declare it. Variables are declared with the var keyword as follows:
<script type="text/javascript">
<!--
var money;
var name;
//-->
</script>

JavaScript Variable Scope:

The scope of a variable is the region of your program in which it is defined. JavaScript variable will have only two scopes.
  • Global Variables: A global variable has global scope which means it is defined everywhere in your JavaScript code.
  • Local Variables: A local variable will be visible only within a function where it is defined. Function parameters are always local to that function.

JavaScript Variable Names:

While naming your variables in JavaScript keep following rules in mind.
  • You should not use any of the JavaScript reserved keyword as variable name. These keywords are mentioned in the next section. For example, break or boolean variable names are not valid.
  • JavaScript variable names should not start with a numeral (0-9). They must begin with a letter or the underscore character. For example, 123test is an invalid variable name but_123test is a valid one.
  • JavaScript variable names are case sensitive. For example, Name and name are two different variables.

JavaScript Reserved Words:

The following are reserved words in JavaScript. They cannot be used as JavaScript variables, functions, methods, loop labels, or any object names.
abstract
boolean
break
byte
case
catch
char
class
const
continue
debugger
default
delete
do
double
else
enum
export
extends
false
final
finally
float
for
function
goto
if
implements
import
in
instanceof
int
interface
long
native
new
null
package
private
protected
public
return
short
static
super
switch
synchronized
this
throw
throws
transient
true
try
typeof
var
void
volatile
while
with

Sunday, January 22, 2012

audio Transcoding FFMPEG Example

 /*
  * Copyright (c) 21 Fabrice Bellard
  *
  * This file is part of FFmpeg.
  *
  * FFmpeg is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
  * License as published by the Free Software Foundation; either
  * version 2.1 of the License, or (at your option) any later version.
  *
  * FFmpeg is distributed in the hope that it will be useful,
 but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 Lesser General Public License for more details.

 You should have received a copy of the GNU Lesser General Public
 License along with FFmpeg; if not, write to the Free Software
 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */

 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>

 #ifdef HAVE_AV_CONFIG_H
 #undef HAVE_AV_CONFIG_H
 #endif

 #include "libavcodec/avcodec.h"
 #include "libavutil/mathematics.h"

 #define INBUF_SIZE 4096
 #define AUDIO_INBUF_SIZE 20480
 #define AUDIO_REFILL_THRESH 4096

 /*
  * Audio encoding example
  */
 static void audio_encode_example(const char *filename)
 {
     AVCodec *codec;
     AVCodecContext *c= NULL;
     int frame_size, i, j, out_size, outbuf_size;
     FILE *f;
     short *samples;
     float t, tincr;
     uint8_t *outbuf;

     printf("Audio encoding\n");

     /* find the MP2 encoder */
     codec = avcodec_find_encoder(CODEC_ID_MP2);
     if (!codec) {
         fprintf(stderr, "codec not found\n");
         exit(1);
     }

     c= avcodec_alloc_context();

     /* put sample parameters */
     c->bit_rate = 640;
     c->sample_rate = 441;
     c->channels = 2;

     /* open it */
     if (avcodec_open(c, codec) < 0) {
         fprintf(stderr, "could not open codec\n");
         exit(1);
     }

     /* the codec gives us the frame size, in samples */
     frame_size = c->frame_size;
     samples = malloc(frame_size * 2 * c->channels);
     outbuf_size = 1;
     outbuf = malloc(outbuf_size);

     f = fopen(filename, "wb");
     if (!f) {
         fprintf(stderr, "could not open %s\n", filename);
         exit(1);
     }

     /* encode a single tone sound */
     t = 0;
     tincr = 2 * M_PI * 440.0 / c->sample_rate;
     for(i=0;i<2;i++) {
         for(j=0;j<frame_size;j++) {
             samples[2*j] = (int)(sin(t) * 1);
             samples[2*j+1] = samples[2*j];
             t += tincr;
       }
         /* encode the samples */
         out_size = avcodec_encode_audio(c, outbuf, outbuf_size, samples);
       fwrite(outbuf, 1, out_size, f);
     }
     fclose(f);
     free(outbuf);
     free(samples);

     avcodec_close(c);
     av_free(c);
 }

 /*
  * Audio decoding.
  */
 static void audio_decode_example(const char *outfilename, const char *filename)
 {
     AVCodec *codec;
     AVCodecContext *c= NULL;
     int out_size, len;
     FILE *f, *outfile;
     uint8_t *outbuf;
     uint8_t inbuf[AUDIO_INBUF_SIZE + FF_INPUT_BUFFER_PADDING_SIZE];
     AVPacket avpkt;

     av_init_packet(&avpkt);

     printf("Audio decoding\n");

     /* find the mpeg audio decoder */
     codec = avcodec_find_decoder(CODEC_ID_MP2);
     if (!codec) {
         fprintf(stderr, "codec not found\n");
         exit(1);
     }

     c= avcodec_alloc_context();

     /* open it */
     if (avcodec_open(c, codec) < 0) {
         fprintf(stderr, "could not open codec\n");
         exit(1);
     }

     outbuf = malloc(AVCODEC_MAX_AUDIO_FRAME_SIZE);

     f = fopen(filename, "rb");
     if (!f) {
         fprintf(stderr, "could not open %s\n", filename);
         exit(1);
     }
     outfile = fopen(outfilename, "wb");
     if (!outfile) {
         av_free(c);
         exit(1);
     }

     /* decode until eof */
     avpkt.data = inbuf;
     avpkt.size = fread(inbuf, 1, AUDIO_INBUF_SIZE, f);

     while (avpkt.size > 0) {
         out_size = AVCODEC_MAX_AUDIO_FRAME_SIZE;
         len = avcodec_decode_audio3(c, (short *)outbuf, &out_size, &avpkt);
         if (len < 0) {
             fprintf(stderr, "Error while decoding\n");
             exit(1);
         }
         if (out_size > 0) {
             /* if a frame has been decoded, output it */
             fwrite(outbuf, 1, out_size, outfile);
         }
         avpkt.size -= len;
         avpkt.data += len;
         if (avpkt.size < AUDIO_REFILL_THRESH) {
             /* Refill the input buffer, to avoid trying to decode
              * incomplete frames. Instead of this, one could also use
              * a parser, or use a proper container format through
              * libavformat. */
             memmove(inbuf, avpkt.data, avpkt.size);
             avpkt.data = inbuf;
             len = fread(avpkt.data + avpkt.size, 1,
                         AUDIO_INBUF_SIZE - avpkt.size, f);
             if (len > 0)
                 avpkt.size += len;
         }
     }

     fclose(outfile);
     fclose(f);
     free(outbuf);

     avcodec_close(c);
     av_free(c);
 }

 /*
  * Video encoding example
  */
tatic void video_encode_example(const char *filename)
 {
     AVCodec *codec;
   AVCodecContext *c= NULL;
     int i, out_size, size, x, y, outbuf_size;
     FILE *f;
     AVFrame *picture;
     uint8_t *outbuf, *picture_buf;

     printf("Video encoding\n");

     /* find the mpeg1 video encoder */
     codec = avcodec_find_encoder(CODEC_ID_MPEG1VIDEO);
     if (!codec) {
         fprintf(stderr, "codec not found\n");
         exit(1);
     }

     c= avcodec_alloc_context();
     picture= avcodec_alloc_frame();

     /* put sample parameters */
     c->bit_rate = 40;
     /* resolution must be a multiple of two */
     c->width = 352;
     c->height = 288;
     /* frames per second */
     c->time_base= (AVRational){1,25};
     c->gop_size = 10; /* emit one intra frame every ten frames */
     c->max_b_frames=1;
     c->pix_fmt = PIX_FMT_YUV420P;

     /* open it */
     if (avcodec_open(c, codec) < 0) {
         fprintf(stderr, "could not open codec\n");
         exit(1);
     }

     f = fopen(filename, "wb");
     if (!f) {
         fprintf(stderr, "could not open %s\n", filename);
         exit(1);
     }

     /* alloc image and output buffer */
     outbuf_size = 10;
     outbuf = malloc(outbuf_size);
     size = c->width * c->height;
     picture_buf = malloc((size * 3) / 2); /* size for YUV 420 */

     picture->data[0] = picture_buf;
     picture->data[1] = picture->data[0] + size;
     picture->data[2] = picture->data[1] + size / 4;
     picture->linesize[0] = c->width;
     picture->linesize[1] = c->width / 2;
     picture->linesize[2] = c->width / 2;

     /* encode 1 second of video */
     for(i=0;i<25;i++) {
         fflush(stdout);
         /* prepare a dummy image */
         /* Y */
         for(y=0;y<c->height;y++) {
             for(x=0;x<c->width;x++) {
                 picture->data[0][y * picture->linesize[0] + x] = x + y + i * 3;
             }
         }

         /* Cb and Cr */
         for(y=0;y<c->height/2;y++) {
             for(x=0;x<c->width/2;x++) {
                 picture->data[1][y * picture->linesize[1] + x] = 128 + y + i * 2;
                 picture->data[2][y * picture->linesize[2] + x] = 64 + x + i * 5;
             }
         }

         /* encode the image */
         out_size = avcodec_encode_video(c, outbuf, outbuf_size, picture);
         printf("encoding frame %3d (size=%5d)\n", i, out_size);
         fwrite(outbuf, 1, out_size, f);
     }

     /* get the delayed frames */
     for(; out_size; i++) {
         fflush(stdout);

         out_size = avcodec_encode_video(c, outbuf, outbuf_size, NULL);
         printf("write frame %3d (size=%5d)\n", i, out_size);
         fwrite(outbuf, 1, out_size, f);
     }

     /* add sequence end code to have a real mpeg file */
     outbuf[0] = 0x;
     outbuf[1] = 0x;
     outbuf[2] = 0x01;
     outbuf[3] = 0xb7;
     fwrite(outbuf, 1, 4, f);
     fclose(f);
     free(picture_buf);
     free(outbuf);
301     avcodec_close(c);
     av_free(c);
   av_free(picture);
     printf("\n");
 }

 /*
  * Video decoding example
  */

 static void pgm_save(unsigned char *buf, int wrap, int xsize, int ysize,
                      char *filename)
 {
     FILE *f;
     int i;

     f=fopen(filename,"w");
     fprintf(f,"P5\n%d %d\n%d\n",xsize,ysize,255);
     for(i=0;i<ysize;i++)
         fwrite(buf + i * wrap,1,xsize,f);
     fclose(f);
 }

 static void video_decode_example(const char *outfilename, const char *filename)
 {
     AVCodec *codec;
     AVCodecContext *c= NULL;
     int frame, got_picture, len;
     FILE *f;
     AVFrame *picture;
     uint8_t inbuf[INBUF_SIZE + FF_INPUT_BUFFER_PADDING_SIZE];
     char buf[1024];
     AVPacket avpkt;

     av_init_packet(&avpkt);

     /* set end of buffer to 0 (this ensures that no overreading happens for damaged mpeg streams) */
     memset(inbuf + INBUF_SIZE, 0, FF_INPUT_BUFFER_PADDING_SIZE);

     printf("Video decoding\n");

     /* find the mpeg1 video decoder */
     codec = avcodec_find_decoder(CODEC_ID_MPEG1VIDEO);
     if (!codec) {
         fprintf(stderr, "codec not found\n");
         exit(1);
     }

     c= avcodec_alloc_context();
     picture= avcodec_alloc_frame();

     if(codec->capabilities&CODEC_CAP_TRUNCATED)
         c->flags|= CODEC_FLAG_TRUNCATED; /* we do not send complete frames */

     /* For some codecs, such as msmpeg4 and mpeg4, width and height
        MUST be initialized there because this information is not
        available in the bitstream. */

     /* open it */
     if (avcodec_open(c, codec) < 0) {
         fprintf(stderr, "could not open codec\n");
         exit(1);
     }

     /* the codec gives us the frame size, in samples */

     f = fopen(filename, "rb");
     if (!f) {
         fprintf(stderr, "could not open %s\n", filename);
         exit(1);
     }

     frame = 0;
     for(;;) {
         avpkt.size = fread(inbuf, 1, INBUF_SIZE, f);
         if (avpkt.size == 0)
             break;

         /* NOTE1: some codecs are stream based (mpegvideo, mpegaudio)
            and this is the only method to use them because you cannot
            know the compressed data size before analysing it.

            BUT some other codecs (msmpeg4, mpeg4) are inherently frame
            based, so you must call them with all the data for one
            frame exactly. You must also initialize 'width' and
            'height' before initializing them. */

         /* NOTE2: some codecs allow the raw parameters (frame size,
            sample rate) to be changed at any frame. We handle this, so
            you should also take care of it */

         /* here, we use a stream based decoder (mpeg1video), so we
            feed decoder and see if it could decode a frame */
         avpkt.data = inbuf;
         while (avpkt.size > 0) {
             len = avcodec_decode_video2(c, picture, &got_picture, &avpkt);
             if (len < 0) {
                 fprintf(stderr, "Error while decoding frame %d\n", frame);
                 exit(1);
           }
             if (got_picture) {
                 printf("saving frame %3d\n", frame);
               fflush(stdout);

                 /* the picture is allocated by the decoder. no need to
                    free it */
                 snprintf(buf, sizeof(buf), outfilename, frame);
                 pgm_save(picture->data[0], picture->linesize[0],
                          c->width, c->height, buf);
                 frame++;
             }
             avpkt.size -= len;
             avpkt.data += len;
         }
     }

     /* some codecs, such as MPEG, transmit the I and P frame with a
        latency of one frame. You must do the following to have a
        chance to get the last frame of the video */
     avpkt.data = NULL;
     avpkt.size = 0;
     len = avcodec_decode_video2(c, picture, &got_picture, &avpkt);
     if (got_picture) {
         printf("saving last frame %3d\n", frame);
         fflush(stdout);

         /* the picture is allocated by the decoder. no need to
            free it */
         snprintf(buf, sizeof(buf), outfilename, frame);
         pgm_save(picture->data[0], picture->linesize[0],
                  c->width, c->height, buf);
         frame++;
     }

     fclose(f);

     avcodec_close(c);
     av_free(c);
     av_free(picture);
     printf("\n");
 }

 int main(int argc, char **argv)
 {
     const char *filename;

     /* must be called before using avcodec lib */
     avcodec_init();

     /* register all the codecs */
     avcodec_register_all();

     if (argc <= 1) {
         audio_encode_example("/tmp/test.mp2");
         audio_decode_example("/tmp/test.sw", "/tmp/test.mp2");

         video_encode_example("/tmp/test.mpg");
         filename = "/tmp/test.mpg";
     } else {
         filename = argv[1];
     }

     //    audio_decode_example("/tmp/test.sw", filename);
     video_decode_example("/tmp/test%d.pgm", filename);

     return 0;
 }

*Copyriright ffmpeg