How to read bytes from data of file (passing header)

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I want pass header section and get bytes only content of a file. For example i want read only RGB bytes from a bmp file, or jpg file; or i want read only text content from a txt file. How can i get content bytes.

File.ReadAllBytes("C:\\a.jpg")

This method reading with headers. I don't want headers section.

Thanks

What I have tried:

Marsal method. But it was difficult.

Posted 21-Jun-18 3:18am

gacar

Updated 21-Jun-18 4:35am

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F-ES Sitecore 21-Jun-18 9:21am

Every file format is different, you'll simply need to know how many bytes are in the header of each file format you support and skip past them.

gacar 21-Jun-18 9:32am

If so very bad. Do you know, how can i get list of file structures. Is there a list?

Eric Lynch 21-Jun-18 9:39am

In the case of ".jpg" files: https://en.wikipedia.org/wiki/JPEG

F-ES Sitecore 21-Jun-18 9:39am

You can find the structures of popular formats by googling for them.

Richard MacCutchan 21-Jun-18 10:19am

In addition to the other comments, even if you skip the headers you still need to understand the structure of the remaining bytes.

Eric Lynch 21-Jun-18 11:06am

As a suggestion, are you open to a different approach? It is possible to let .NET do a lot of the heavy lifting for you, decoding the file format, and still have access to RGB values for the pixels that make up the image. Or, are you set on reading the image byte-by-byte from the file?

gacar 21-Jun-18 11:42am

It can posible for image files with Marshal class in .Net

Eric Lynch 21-Jun-18 16:50pm

Doesn't necessarily require a Marshal. The following is a little bit slow, but works.

int x = 0;
int y = 0;
var bitmap = Image.FromFile(@"pic.jpg") as Bitmap;
Color color = bitmap.GetPixel(x, y);
byte red = color.R;
byte green = color.G;
byte blue = color.B;

A bit faster (which does require a Marshal) is described here:

https://stackoverflow.com/questions/7373703/c-sharp-getting-the-pixel-data-efficiently-from-system-drawing-bitmap

WPF also has some equivalents to System.Drawing that may be better.

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OriginalGriff · Accepted Answer · 2018-06-21T04:15:00

You can't read the RGB format directly from a JPG file - it uses a lossy compression technique which means that every pixel value in the image isn't directly represented by a value in the JPG file.

And as stated, all file formats are different, and so any "header" will be a different length or non-existent. For example, a text file would not have a header at all, while a bitmap could have multiple headers if you also consider colour tables and so forth - which are not of a fixed length anyway.

Dave Kreskowiak · Accepted Answer · 2018-06-21T04:36:00

Just to add to what's already been said, there are file formats where each "record" in the file has a header.

Your description of what you want is a very naive view of how applications and files work.

Not all files have a header, nor is any header information the same format, nor the same length, nor has the same purposes.

You actually have to know exactly how the file is laid out, which bytes mean what to the application that uses those files, what byte offset these fields are at in the file, what type they are, field and block lengths, ...

You can even run into situations where one field specifies an offset for another field or block in the file because some fields or blocks don't have specific offsets in the file.

How to read bytes from data of file (passing header)

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