如何把一张大分辨率的JPEG图存为小分辨率的JPEG图

[zkhraul]

如何把一张大分辨率的JPEG图存为小分辨率的JPEG图，IIMAGE怎么使用？有例子参考下吗，急，谢谢

[YourKing]

调用IImage的GetThumbnail

[biosli]

CxImage开源工程，参考一下~~
IImage似乎不能编码，只能解码。就是说你能把图片解成Bitmap，改变大小，但是不能再打回JPEG

[zkhraul]

我用的是WIN32,GetThumbnail好像不支持吧

[zkhraul]

我找了个资料，刚好和我人需求一样，不知道WINDOWS MOBILE下支持下面所说的GDI+吗

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：
C/C++ code
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;
}