Re: Read width & Height of jpeg file - C# Discussion Boards

30-Jun-07 20:52

Hi Guys,
I'm trying to get into XNA v1.0 (..last time I touched it was Beta 1.) ...So, I'm building myself a terrain loader. Trying to get my Terrain class set up so that it can be given a heightmap file and it can initialize the size of the data array based upon the width & height of the file that is passed to it. Does anyone know of a way(preferrably a simple way) of accepting the path of a jpeg and getting the width and height out of the FileInfo?(I'm making an assumption that is where it is located, but I do not know for a fact)

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Re: Read width &amp Height of jpeg file

Mazdak30-Jun-07 20:59

Mazdak

30-Jun-07 20:59

System.Drawing.Image.FromFile(jpegpath).Size

Mazy

"This chancy chancy chancy world."

Alaric_30-Jun-07 21:11

30-Jun-07 21:11

Thanks for your help, but there is no System.Drawing namespace in the XNA Framework(System.Drawing is the wrapper around the GDI). Does anybody know of a method that doesn't use the GDI+?

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Luc Pattyn1-Jul-07 4:23

1-Jul-07 4:23

Hi, the only way to get JPEG size without using Image class is by reading (part of) the
JPEG file yourself, as in the following code:

public static Size GetJpegImageSize(string filename) {
	FileStream stream=null;
	BinaryReader rdr=null;
	try {
		stream=File.OpenRead(filename);
		rdr=new BinaryReader(stream);
		// keep reading packets until we find one that contains Size info
		for(; ; ) {
			byte code=rdr.ReadByte();
			if(code!=0xFF) throw new ApplicationException(
					"Unexpected value in file "+filename);
			code=rdr.ReadByte();
			switch(code) {
				// filler byte
				case 0xFF:
					stream.Position--;
					break;
				// packets without data
				case 0xD0: case 0xD1: case 0xD2: case 0xD3: case 0xD4: 
				case 0xD5: case 0xD6: case 0xD7: case 0xD8: case 0xD9:
					break;
				// packets with size information
				case 0xC0: case 0xC1: case 0xC2: case 0xC3:
				case 0xC4: case 0xC5: case 0xC6: case 0xC7:
				case 0xC8: case 0xC9: case 0xCA: case 0xCB:
				case 0xCC: case 0xCD: case 0xCE: case 0xCF:
					ReadBEUshort(rdr);
					rdr.ReadByte();
					ushort h=ReadBEUshort(rdr);
					ushort w=ReadBEUshort(rdr);
					return new Size(w, h);
				// irrelevant variable-length packets
				default:
					int len=ReadBEUshort(rdr);
					stream.Position+=len-2;
					break;
			}
		}
	} finally {
		if(rdr!=null) rdr.Close();
		if(stream!=null) stream.Close();
	}
}

private static ushort ReadBEUshort(BinaryReader rdr) {
	ushort hi=rdr.ReadByte();
	hi<<=8;
	ushort lo=rdr.ReadByte();
	return (ushort)(hi|lo);
}

A JPEG file contains a lot of packets (with Hufman tables and everything).
The above code does not know much about JPEG, it simply tries to jump from one packet
to the next until it finds one with size info.

Smile | :)

Luc Pattyn

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Alaric_1-Jul-07 6:29

1-Jul-07 6:29

Thanks a TON for that...did you just pull that off the top of your head? Or did you have reference material for that? What information did you refer to?

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Luc Pattyn1-Jul-07 6:40

1-Jul-07 6:40

Hi,

I once wrote a JPEG file viewer based on the JPEG standard[^].

And today I threw out everything that was not needed when only Size matters...

Smile | :)

Luc Pattyn

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Alaric_1-Jul-07 6:45

1-Jul-07 6:45

...how long did it take you to parse through all of the W3C info to turn it into something useful?

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Luc Pattyn1-Jul-07 6:55

1-Jul-07 6:55

Actually not very long, I did not read most of the spec, I just looked for the overall
structure (packets with 2B code and 2B length field) and a couple of specifics that did
interest me at the time. And with a simple viewer, I just looked inside a lot of JPEG files
to see what kind of information typically gets included.

BTW a lot of cameras add exposure information (make and model, datetime, image corrections,
etc) to the JPEG files they create, and the Image class allows you to retrieve these
using the getPropertyItem() method.

Smile | :)

Luc Pattyn

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Alaric_1-Jul-07 7:06

1-Jul-07 7:06

...I was thinking that I was going to have to find a way to pull the FileHeader off, store it in the .Net equivalent of a BITMAPFILEHEADER. Then, pull the InfoHeader off of the file and store it in the .NET equivalent of a BITMAPINFOHEADER...& pull the width & height from it

...or by using WMI. That was more along my line of thinking.

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Luc Pattyn1-Jul-07 7:15

1-Jul-07 7:15

Hi,

The structs you mentioned exist in a .bmp file, not in a JPEG file.
And before you ask, no I dont have a BMP Viewer yet.

As for WMI, I dont think WMI would help looking at JPEGs; it is involved in system resources.

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Alaric_1-Jul-07 8:01

1-Jul-07 8:01

I know that WMI is for system resources, but I saw an example while I was doing online research where somebody went to the WMI classes for the owner of a specific file (I think he went to one of the Operating System's Win32_Security classes) ...I was just thinking that the information might be included with it *somewhere*

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Alaric_1-Jul-07 8:15

1-Jul-07 8:15

right...I knew that the BITMAPINFOHEADER was for a bmp & not a jpg Poke tongue | ;-P

...but I was hoping that something could be done to mimic its usage. (My bmp viewer is in C++...never ported it to .NET) ...bmp's are actually extremely simple, barely a step up from a PPM.

I'm sure you could find it yourself, but the header just looks like this:

height field
bits per pixel field
compression field
colors field
important colors field

...then you just have to remember that bitmaps are encoded as BGR values instead of RGB, so you have to map them accordingly

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Alaric_1-Jul-07 9:14

1-Jul-07 9:14

ok...one thing I absolutely hate is how .NET throws this for the code you provided:

Error	6	Control cannot fall through from one case label ('case 207:') to another	D:\XNA\Files\Projects\Terrain\TerrainWIP\Terrain\Terrain\JPEG.cs	61	58	Terrain

Was your code C#.NET code? How were you able to force it to fall through the cases?

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Luc Pattyn1-Jul-07 9:20

1-Jul-07 9:20

Hi,

I did not actually run the code I posted, but it seems OK to me.
cases should be empty (that is how you can list cases to share all their code)
or end on a change-of-flow (break, return, throw...)

Did you somehow change the code and violate the above ?
I guess you did, since 207 is 0xCF and that one ended on return...

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Luc Pattyn

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Alaric_1-Jul-07 13:29

1-Jul-07 13:29

no...identical code. Cut & pasted it. .NET's saying that it won't let a case fall through to execute code from another case. ...OxCF was the case that actually had code to execute. ..the others fell through to 0xCF.

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Alaric_1-Jul-07 13:35

1-Jul-07 13:35

lol...I saw what it was. You forgot your break in the 0xCF code, so it was saying it couldn't fall from 0xCF to the default code.

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Alaric_1-Jul-07 13:50

1-Jul-07 13:50

...even if you have a return, it still wants you to have the break after it

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Luc Pattyn1-Jul-07 14:06

1-Jul-07 14:06

My Visual Studio C# 2005 Express Edition is happy without a break after a return,
and produces a warning "unreachable code" if there is such a break; both seem logical to me.

Are you using an different, maybe older, IDE ?

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Luc Pattyn1-Jul-07 14:19

1-Jul-07 14:19

I have now tried with Visual Studio 7.1 and it behaves identically:
return without break is fine, return+break gives warning.

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Alaric_1-Jul-07 14:23

1-Jul-07 14:23

I'm using VS 2005. return without break gives the error that I provided. ...break after return compiles with a warning

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Alaric_1-Jul-07 14:24

Re: Read width &amp; Height of jpeg file

1-Jul-07 14:24

...same in Visual C# 2005 Express

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Luc Pattyn1-Jul-07 14:56

Re: Read width &amp;amp;amp; Height of jpeg file [modified]

1-Jul-07 14:56

Well, that's a small mystery, one we will not solve easily, but I guess you can live with it ?

Anyway, I trust you got the code up and running...

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Luc Pattyn

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Alaric_1-Jul-07 16:03

Re: Read width and height of jpeg file

1-Jul-07 16:03

yeah, I can live with it...I'm having a new problem though:

if (code != 0xFF) throw new ApplicationException(
                               "Unexpected value in file " + filename);

throws ....code equaled 216. What exactly is that check for? Do you know what the value of 216 means in this context? The first pass through, "code" was 255, then the next value was ...[Fixed it before I finished the post]

...Since the only thing I'm wanting to do is determine the width & height of the file, I just place a check for them inside the code check so it's now looking for

if(code != 0xFF && width == 0 && height == 0)
{
     throw;
}

so now it will only throw if the width & height haven't been set yet..if they've been set, then I simply break when the code is not 0xFF. ...Is 0xFF like a key value for the header or something like that?

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Luc Pattyn1-Jul-07 22:09