找出文件类型(GIF，JPG，PNG)-DELPHI [英] Find out the type of file ( GIF, JPG, PNG) - DELPHI

问题描述

我每个URL都带有图像，并显示TImage.我将使用JPG，GIF和PNG.但是我不知道如何找出每个文件具有什么样的扩展名，然后加以区分.如何获取标头或其他让我知道文件类型的方法:GIF或PNG或JPG?

I'm carrying images per URL and showing TImage . I will work with JPG , GIF and PNG . But I do not know how to find out what kind of extension possess each file, and then differentiate. How do I get header , or any other method for me to know what type of file: GIF or PNG or JPG?

var
  MS : TMemoryStream;
  GIf: TGIFImage;
  jpegimg: TJPEGImage;


begin
  MS := TMemoryStream.Create;
  GIf := TGIFImage.Create;
  jpegimg   := TJPEGImage.Create; ///////

  try
      try
        IdHTTP1.get('http://forum.wmonline.com.br/uploads/av-8929.jpg',MS);
        Ms.Seek(0,soFromBeginning);
        //Gif.LoadFromStream(MS);
        //Logo.Picture.Assign(GIF);
        jpegimg.LoadFromStream(MS);
        Logo.Picture.Assign(jpegimg);
      except
        ShowMessage('ERRO');
        Exit;
      end;
  finally
    FreeAndNil(GIF);
    FreeAndNil(MS);
    jpegimg.Free;  ////
  end;

推荐答案

有些机制旨在允许描述请求(或响应)的内容，但是任何外部元数据可能是不可靠的，完全取决于所涉及的元数据的准确实现和设置.在某些情况下，元数据可能不正确或完全丢失.

There are mechanisms intended to allow the description of the content of a request (or response), but any external meta-data may be unreliable, being wholly dependent upon an accurate implementation and setting of the meta-data involved. In some cases that meta-data may be incorrect or entirely missing.

幸运的是，与许多文件格式一样，您提到的图像文件类型的规范都要求使用特定的标头，以将文件(或流)标识为符合(或希望符合)相关规范.

Fortunately in common with many file formats, the specifications for the image file types you mention all mandate a specific header to identify the file (or stream) as conforming (or aspiring to conform) to the relevant specification.

GIF文件的前3个字节是:

`G` `I` `F`    (ASCII)

您可能还希望检查随后的3个字节，以获取有效的GIF版本号，该版本号也以ASCII编码:

You may also wish to check the subsequent 3 bytes for a valid GIF version number, also encoded in ASCII:

`8` `9` `a`   or `8` `7` `a`

PNG文件的前8个字节具有以下值:

137 80 78 71 13 10 26 10   (decimal)

JPEG fil的前2个字节 e是:

FF D8   (hex)

因此，要检测响应流中的数据格式，您仅需要检查流的前8个字节中是否有这些预期的标头值之一.

So to detect the format of the data in a response stream you need only inspect at most the first 8 bytes of the stream for one of these expected header values.

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