如何使用GDI+缩小JPG图片？

[chengwei02]

我用GDI+缩小一个JPG图片，从网上down下来的这个图片原本是208×275，大小为10K，我使用GDI+将尺寸缩小至151×200后，图片的大小反而变成了22K，我的代码如下：

void compressImpl(const std::string& path, int size, UInt8 quality)
{
  HBITMAP hbmReturn = NULL;
  Bitmap* bmPhoto = NULL;

  std::wstring upath;
  UnicodeConverter::toUTF16(path, upath);

  {
    Bitmap image(upath.c_str());

    int srcWidth = image.GetWidth();
    int srcHeight = image.GetHeight();

    float percent = 0;
    int destX = 0, destY = 0;
    if (srcWidth > srcHeight)
    {
      percent = ((float)size/(float)srcWidth);
      destX  = (int)((size - (srcWidth * percent))/2);
    }
    else
    {
      percent = ((float)size/(float)srcHeight);
      destY  = (int)((size - (srcHeight * percent))/2);
    }
    
    if (percent >= 1.0f)
      return; // skip compress

    int destWidth = (int)(srcWidth * percent);
    int destHeight = (int)(srcHeight * percent);

    bmPhoto = new Bitmap(destWidth, destHeight, PixelFormat24bppRGB);
    bmPhoto->SetResolution(image.GetHorizontalResolution(), image.GetVerticalResolution());

    Graphics *grPhoto = Graphics::FromImage(bmPhoto);
    Color colorW(255, 255, 255, 255);
    grPhoto->Clear(colorW);
    grPhoto->SetInterpolationMode(InterpolationModeHighQualityBicubic);
    grPhoto->DrawImage(&image, Rect(destX, destY, destWidth, destHeight));

    bmPhoto->GetHBITMAP(colorW, &hbmReturn);
    delete grPhoto;
  }

  // find appropriate encoder, jpeg
  CLSID encoderClsid;
  getEncoderClsid(L"image/jpeg", &encoderClsid);

  // set output quality for jpeg alone
  EncoderParameters encoderParameters;
  setEncoderQuality(&encoderParameters, quality);

  // output to image file with desired quality
  bmPhoto->Save(upath.c_str(), &encoderClsid, &encoderParameters);

  // release resources
  delete bmPhoto;
  DeleteObject(hbmReturn);
}

int getEncoderClsid(const WCHAR* format, void* clsid)
{
  UINT num = 0; // number of image encoders
  UINT size = 0; // size of the image encoder array in bytes

  ImageCodecInfo* pImageCodecInfo = NULL;
  GetImageEncodersSize(&num, &size);
  if(size == 0)
    return -1; // Failure
    
  pImageCodecInfo = (ImageCodecInfo*)(malloc(size));
  if(pImageCodecInfo == NULL)
    return -1; // Failure

  GetImageEncoders(num, size, pImageCodecInfo);
  for (UINT j = 0; j < num; ++j)
  {
    if (wcscmp(pImageCodecInfo[j].MimeType, format) == 0)
    {
      *(CLSID*)clsid = pImageCodecInfo[j].Clsid;
      free(pImageCodecInfo);
      return j;  //Success
    }
  }

  free(pImageCodecInfo);
  return -1; // Failure
}

void setEncoderQuality(void* params, UInt8 quality)
{
  EncoderParameters* encoderParams = (EncoderParameters*)params;
  encoderParams->Count = 1;
  encoderParams->Parameter[0].Guid = EncoderQuality;
  encoderParams->Parameter[0].Type = EncoderParameterValueTypeLong;
  encoderParams->Parameter[0].NumberOfValues = 1;

  encoderParams->Parameter[0].Value = &quality;
}

[hastings]

保存jpg可以选择压缩等级的：

#include <stdio.h>
#include <tchar.h>
#include <windows.h>
int A2U(const char* szA,wchar_t* szU,size_t cnt)
{
	return MultiByteToWideChar (CP_ACP, 0, szA, -1, szU, cnt) ;
}
bool SaveImageToFile(Bitmap* pimage, LPCTSTR szFileName, ULONG quality)
//quality表示压缩等级，取值范围0-100，越小，压缩等级越大，图像质量越差
{
	if(NULL==pimage || NULL==szFileName)
		return false;
	CLSID   encoderClsid;
	EncoderParameters encoderParameters,*pencoderParameters=NULL;
	TCHAR*  pstr;
	TCHAR   szFtr[16];
	TCHAR   strcode[32]=TEXT("image/");
	pstr=_tcsrchr(szFileName,TEXT('.'));
	if(NULL==pstr)
		return false;
	_tcscpy(szFtr,pstr+1);
	_tcslwr(szFtr);
	if(_tcscmp(TEXT("jpeg"),szFtr)==0 || _tcscmp(TEXT("jpg"),szFtr)==0)
	{
		_tcscat(strcode,TEXT("jpeg"));
		pencoderParameters=&encoderParameters;
	}
	else
		_tcscat(strcode,szFtr);
	if(-1==GetEncoderClsid(strcode, &encoderClsid))
		return false;
	if(quality>100)
		quality=100;
	encoderParameters.Count = 1;
	encoderParameters.Parameter[0].Guid = EncoderQuality;
	encoderParameters.Parameter[0].Type = EncoderParameterValueTypeLong;
	encoderParameters.Parameter[0].NumberOfValues = 1;
	encoderParameters.Parameter[0].Value = &quality;
#ifndef UNICODE
	wchar_t wszFileName[MAX_PATH];
	A2U(szFileName,wszFileName,MAX_PATH);
	return Ok==pimage->Save(wszFileName, &encoderClsid, pencoderParameters)?true:false;
#else
	return Ok==pimage->Save(szFileName, &encoderClsid, pencoderParameters)?true:false;
#endif
}

[hastings]

上面少了一个函数：

int GetEncoderClsid(LPCTSTR szFormat, CLSID* pClsid)  // help
{
	UINT  j;
	UINT  num = 0;          // number of image encoders
	UINT  size = 0;         // size of the image encoder array in bytes
	ImageCodecInfo* pImageCodecInfo = NULL;
	GetImageEncodersSize(&num, &size);
	if(size == 0)
		return -1;  // Failure
	pImageCodecInfo = (ImageCodecInfo*)(malloc(size));
	if(pImageCodecInfo == NULL)
		return -1;  // Failure
	GetImageEncoders(num, size, pImageCodecInfo);
#ifndef UNICODE
	wchar_t  wszFormat[32];
	A2U(szFormat,wszFormat,32);
	for(j = 0; j < num; ++j)
	{
		if( wcscmp(pImageCodecInfo[j].MimeType, wszFormat) == 0 )
#else
	for(j = 0; j < num; ++j)
	{
		if( wcscmp(pImageCodecInfo[j].MimeType, szFormat) == 0 )
#endif
		{
			*pClsid = pImageCodecInfo[j].Clsid;
			free(pImageCodecInfo);
			return j;  // Success
		}
	}
	free(pImageCodecInfo);
	return -1;  // Failure
}