How to read a 8-bit grayscale JPEG image using C++?

[Speed]

Hi,

Could you please tell me what is the simplest code to read a 8-bit
grayscale JPEG using C++?

Thanks a ton,
Speed.

[Default User]

Speed wrote:

Hi,

Could you please tell me what is the simplest code to read a 8-bit
grayscale JPEG using C++?

You asked the same basic question in comp.lang.c. Pick a language.


Brian

[Kai-Uwe Bux]

Default User wrote:

Speed wrote:

>Hi,

Could you please tell me what is the simplest code to read a 8-bit
grayscale JPEG using C++?

You asked the same basic question in comp.lang.c. Pick a language.

Are you sure it was the same question? I would expect that, over there, he
had asked

Could you please tell me what is the simplest code to read a 8-bit
grayscale JPEG using C?
In that case, maybe he would like to defer the choice of the language until
after he received answers.
To the OP: the C++ standard does not make any special provisions for JPEG
files. In as much as you might be looking for a l3rd party library
solution, please note that those are off-topic in this group.

To read a binary file completely into memory, you could use code like this
in C++:

#include <iterator>
#include <fstream>
#include <vector>
#include <iostream>

typedef std::vector< char buffer;

int main ( void ) {
// read file:
std::ifstream infile ( "speed_001.cc", std::ios::binary );
buffer the_buf ( std::istreambuf_iterator<char>( infile ),
(std::istreambuf_iterator<char>()) );
// write data:
std::cout << the_buf.size() << '\n';
std::copy( the_buf.begin(), the_buf.end(),
std::ostream_iterator<char>( std::cout, "" ) );
std::cout << '\n';
}
Best

Kai-Uwe Bux

[BobR]

Speed <lo**********@gmail.comwrote in message...

Hi,
Could you please tell me what is the simplest code to read a 8-bit
grayscale JPEG using C++?
Thanks a ton,
Speed.

#include <iostream>
#include <fstream>
#include <vector>

{
std::ifstream PicIn( "MyPic.jpg",
std::ios_base::in | std::ios_base::binary );
if( not PicIn.is_open() ){
std::cout<<"\n FAILED"<<std::endl;
return EXIT_FAILURE;
}
std::vector<unsigned charImage;

while( PicIn.peek() != EOF ){ // you didn't say 'fastest' <G>
Image.push_back( PicIn.get() );
}

std::cout<<"\n Image.size() = "
<<Image.size()<<" bytes."<<std::endl;
PicIn.close();
}

--
Bob R
POVrookie

[red floyd]

BobR wrote:

Speed <lo**********@gmail.comwrote in message...

>Hi,
Could you please tell me what is the simplest code to read a 8-bit
grayscale JPEG using C++?
Thanks a ton,
Speed.

#include <iostream>
#include <fstream>
#include <vector>

{
std::ifstream PicIn( "MyPic.jpg",
std::ios_base::in | std::ios_base::binary );
if( not PicIn.is_open() ){
std::cout<<"\n FAILED"<<std::endl;
return EXIT_FAILURE;
}
std::vector<unsigned charImage;

while( PicIn.peek() != EOF ){ // you didn't say 'fastest' <G>
Image.push_back( PicIn.get() );
}

std::cout<<"\n Image.size() = "
<<Image.size()<<" bytes."<<std::endl;
PicIn.close();
}

Oh, come on, Bob, let's do it right and really confusing! Plus, you
forgot "int main()"!!!
#include <istream>
#include <fstream>
#include <ostream>
#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
#include <cstdlib>

int main()
{
std::ifstream PicIn( "MyPic.jpg",
std::ios_base::in | std::ios_base::binary );
if( ! PicIn )
{
std::cerr<<"\n FAILED"<<std::endl;
return EXIT_FAILURE;
}
std::vector<unsigned charImage;

std::copy(
std::istreambuf_iterator<unsigned char>(PicIn.rdbuf()),
std::istreambuf_iterator<unsigned char>(),
std::back_inserter(Image));

PicIn.close();

std::cout<<"\n Image.size() = "
<<Image.size()<<" bytes."<<std::endl;

return EXIT_SUCCESS;
}

[Default User]

Kai-Uwe Bux wrote:

Default User wrote:

Speed wrote:

Hi,

Could you please tell me what is the simplest code to read a 8-bit
grayscale JPEG using C++?

You asked the same basic question in comp.lang.c. Pick a language.

Are you sure it was the same question? I would expect that, over
there, he had asked

Could you please tell me what is the simplest code to read a 8-bit
grayscale JPEG using C?

That's why I said "the same basic question" rather than "the same
question".

In that case, maybe he would like to defer the choice of the language
until after he received answers.

Seems unlikely.


Brian

[Kai-Uwe Bux]

Default User wrote:

Kai-Uwe Bux wrote:

>Default User wrote:

Speed wrote:

Hi,

Could you please tell me what is the simplest code to read a 8-bit
grayscale JPEG using C++?

You asked the same basic question in comp.lang.c. Pick a language.

Are you sure it was the same question? I would expect that, over
there, he had asked

Could you please tell me what is the simplest code to read a 8-bit
grayscale JPEG using C?

That's why I said "the same basic question" rather than "the same
question".

Sorry, I misunderstood. I parsed it as

same ( basic question )

not as

( basically same ) question

Best

Kai-Uwe Bux

[BobR]

red floyd <no*****@here.dudewrote in message...

>
Oh, come on, Bob, let's do it right and really confusing! Plus, you
forgot "int main()"!!!

I didn't forget "int main()", I forgot to label (and note the 'return'):
{ // main or function
}

>
#include <istream>
#include <fstream>
#include <ostream>
#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
#include <cstdlib>

int main(){
std::ifstream PicIn( "MyPic.jpg",
std::ios_base::in | std::ios_base::binary );
if( ! PicIn ){
std::cerr<<"\n FAILED"<<std::endl;
return EXIT_FAILURE;
}
std::vector<unsigned charImage;

std::copy(
std::istreambuf_iterator<unsigned char>(PicIn.rdbuf()),
std::istreambuf_iterator<unsigned char>(),
std::back_inserter(Image));

Does not work on GCC(MinGW)3.3.1. This does:

std::vector<unsigned charImage;
std::copy(
std::istreambuf_iterator<char>( PicIn.rdbuf() ),
std::istreambuf_iterator<char>(),
std::back_inserter( Image ) );

// '.rdbuf()' gets stuck in 'char' mode (no 'unsigned char'
overload/template).

>
PicIn.close();
std::cout<<"\n Image.size() = "
<<Image.size()<<" bytes."<<std::endl;
return EXIT_SUCCESS;
}

Kai-Uwe Bux: Your example does not compile for me.
It slices off the '.at()' and '.size()' of the std::vector (and who knows
what else)! Seems to store the iterators, not 'char's.

std::vector<charImage(
std::istreambuf_iterator<char>( PicIn ),
std::istreambuf_iterator<char>() ); // the ( ...() ) fails here

std::cout<<"\n Image.size() = "
<<Image.size()<<" bytes."<<std::endl; // line 1972
/*
TestBench.cpp:1972: error: request for member `size' in `
Image(std::istreambuf_iterator<char, std::char_traits<char,
std::istreambuf_iterator<char, std::char_traits<char (*)())', which is
of
non-aggregate type `std::vector<char, std::allocator<char
()(std::istreambuf_iterator<char, std::char_traits<char,
std::istreambuf_iterator<char, std::char_traits<char (*)())'
*/
[ Do note the older compiler version. Bug? ]
I tried many variations.
Which compiler did you use?

--
Bob R
POVrookie

[Kai-Uwe Bux]

BobR wrote:
[snip]

Kai-Uwe Bux: Your example does not compile for me.
It slices off the '.at()' and '.size()' of the std::vector (and who knows
what else)! Seems to store the iterators, not 'char's.

std::vector<charImage(
std::istreambuf_iterator<char>( PicIn ),
std::istreambuf_iterator<char>() ); // the ( ...() ) fails here

Are you sure, you copied the code correctly? I am pretty sure that I put the
second iterator argument within yet another pair of parentheses:

buffer the_buf ( std::istreambuf_iterator<char>( infile ),
(std::istreambuf_iterator<char>() ) );
^^^ ^^^

What you wrote is parsed as a function declaration.

std::cout<<"\n Image.size() = "
<<Image.size()<<" bytes."<<std::endl; // line 1972
/*
TestBench.cpp:1972: error: request for member `size' in `
Image(std::istreambuf_iterator<char, std::char_traits<char,
std::istreambuf_iterator<char, std::char_traits<char (*)())', which
is
of
non-aggregate type `std::vector<char, std::allocator<char
()(std::istreambuf_iterator<char, std::char_traits<char,
std::istreambuf_iterator<char, std::char_traits<char (*)())'
*/
[ Do note the older compiler version. Bug? ]
I tried many variations.

Not the right one :-)

Which compiler did you use?

g++-4.2.0
Best

Kai-Uwe Bux

[BobR]

Kai-Uwe Bux <jk********@gmx.netwrote in message...

BobR wrote:
[snip]

Kai-Uwe Bux: Your example does not compile for me.
It slices off the '.at()' and '.size()' of the std::vector (and who

knows

what else)! Seems to store the iterators, not 'char's.
std::vector<charImage(
std::istreambuf_iterator<char>( PicIn ),
std::istreambuf_iterator<char>() ); // the ( ...() ) fails here

Are you sure, you copied the code correctly? I am pretty sure that I put

the

second iterator argument within yet another pair of parentheses:

Yes, but that weren't it.

std::vector<unsigned charImage(
std::istreambuf_iterator<char>( PicIn.rdbuf() ), // 'rdbuf()' <---
std::istreambuf_iterator<char>()
);

This works as advertised.
I guess I should go, "Duh!".

>
buffer the_buf ( std::istreambuf_iterator<char>( infile ),
(std::istreambuf_iterator<char>() ) );
^^^ ^^^

What you wrote is parsed as a function declaration.

Not after the correct *_iterator got 'called' with '.rdbuf()'.

--
Bob R
POVrookie

[James Kanze]

On Jul 19, 11:54 pm, Kai-Uwe Bux <[e-mail]:

BobR wrote:

[snip]

Kai-Uwe Bux: Your example does not compile for me.
It slices off the '.at()' and '.size()' of the std::vector (and who knows
what else)! Seems to store the iterators, not 'char's.

std::vector<charImage(
std::istreambuf_iterator<char>( PicIn ),
std::istreambuf_iterator<char>() ); // the ( ...() ) fails here

Are you sure, you copied the code correctly? I am pretty sure
that I put the second iterator argument within yet another
pair of parentheses:

buffer the_buf ( std::istreambuf_iterator<char>( infile ),
(std::istreambuf_iterator<char>() ) );
^^^ ^^^

What you wrote is parsed as a function declaration.

There is (or was for the longest time---I haven't verified with
the most recent versions) a bug in g++, in that it "committed"
to the function declaration as soon as it saw that the first
argument could be interpreted as a declaration. So:
buffer the_buf ( (std::istreambuf_iterator<char>( infile )),
(std::istreambuf_iterator<char>() ) );
worked, but putting the parentheses only around the second
argument didn't.

I've gotten into the habit of putting it around all of them, so
I won't have noticed if they've fixed this. (Probably have. I
ran into it some time ago, and they tend to fix bugs pretty
quickly.)

--
James Kanze (GABI Software) email:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[James Kanze]

On Jul 20, 3:16 am, "BobR" <[e-mail]:

Kai-Uwe Bux <[e-mail] in message...

[...]

buffer the_buf ( std::istreambuf_iterator<char>( infile ),
(std::istreambuf_iterator<char>() ) );
^^^ ^^^

What you wrote is parsed as a function declaration.

Not after the correct *_iterator got 'called' with '.rdbuf()'.

Not a question of the "correct" iterator: istreambuf_iterator
has a constructor which takes an istream, as well as one which
takes a streambuf. The problem is that:

std::istreambuf_iterator< char >( infile )

can be interpreted as a declaration, and when something can be
interpreted as a declaration, it is. And if what looks like
arguments to us are in fact declarations, then the above
declares a function. Putting in the extra pair of parentheses
creates a context where a declaration (of a function parameter)
is not legal, so it isn't interpreted as a declaration, but as
an expression to be passed to the constructor, and we have a
data definition, rather than a function declaration.

--
James Kanze (GABI Software) email:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[BobR]

James Kanze <ja*********@gmail.comwrote in message...
On Jul 19, 11:54 pm, Kai-Uwe Bux <[e-mail]:

BobR wrote:

/* """ quote

[snip]
Are you sure, you copied the code correctly? I am pretty sure
that I put the second iterator argument within yet another
pair of parentheses:

buffer the_buf ( std::istreambuf_iterator<char>( infile ),
(std::istreambuf_iterator<char>() ) );
^^^ ^^^

What you wrote is parsed as a function declaration.

There is (or was for the longest time---I haven't verified with
the most recent versions) a bug in g++, in that it "committed"
to the function declaration as soon as it saw that the first
argument could be interpreted as a declaration. So:
buffer the_buf ( (std::istreambuf_iterator<char>( infile )),
(std::istreambuf_iterator<char>() ) );
worked, but putting the parentheses only around the second
argument didn't.

""" */

[ GCC(MinGW)3.3.1 ]
// ref: ifstream PicIn( file, in | binary)
std::vector<unsigned charImage(
( std::istreambuf_iterator<char>( PicIn ) ),
( std::istreambuf_iterator<char>() )
); // line 1946
// 1946 \TestBench.cpp syntax error before `)' token
[ Same with 'std::vector<charImage(...', and '(PicIn.rdbuf())' ]

It works on Mr. Bux's GCC 4.2, so that 'bug' must have been fixed.

std::vector<unsigned charImage(
std::istreambuf_iterator<char>( PicIn.rdbuf() ),
std::istreambuf_iterator<char>()
);

Works fine on my poor, tired old GCC.

/* """
I've gotten into the habit of putting it around all of them, so
I won't have noticed if they've fixed this. (Probably have. I
ran into it some time ago, and they tend to fix bugs pretty
quickly.)
""" */

I noticed, with or without the parentheses! <G>
At least now I know what was happening.
Thanks James.

Your other post; 'correct' was a very poor choice of wording on my part. I
was in a hurry (supper was ready!). :-}
--
Bob R
POVrookie