我想从图像中提取缩略图，所以我尝试使用GDI +。我所做的是创建一个新的小Bitmap来保存缩略图，并使用Graphics::DrawImage()将位图绘制到其中（从而获得缩略图）。
但是当在大量图像上使用这种方法时，它变得非常慢。那么有没有办法加快速度（只使用Windows API，而不是外部库）。
注意：我知道有些图片会在其中存储缩略图，但我希望从那些图片中提取缩略图。
答案 0 :(得分：0)
您可以通过从EXIF加载缩略图来增加速度（对于JPEG和其他一些格式）。但GetThumbnailImage的结果并不好。这是另一个实现：
用法：
Gdiplus::Bitmap* thumb = GetThumbnail(filename, thumbWidth, thumbHeight);
代码：
PropertyItem* GetPropertyItemFromImage(Gdiplus::Image* bm, PROPID propId) {
UINT itemSize = bm->GetPropertyItemSize(propId);
if (!itemSize) {
return 0;
}
PropertyItem* item = reinterpret_cast<PropertyItem*>(malloc(itemSize));
if (bm->GetPropertyItem(propId, itemSize, item) != Ok) {
free(item);
return 0;
}
return item;
}
UINT VoidToInt(void* data, unsigned int size) {
switch (size) {
case 8:
return *reinterpret_cast<UINT*>(data);
case 4:
return *reinterpret_cast<DWORD*>(data);
case 2:
return *reinterpret_cast<WORD*>(data);
default:
return *reinterpret_cast<BYTE*>(data);
}
}
typedef IStream * (STDAPICALLTYPE *SHCreateMemStreamFuncType)(const BYTE *pInit, UINT cbInit);
SHCreateMemStreamFuncType SHCreateMemStreamFunc = 0;
bool IsVista() {
static int isVista = -1;
if (isVista == -1)
{
OSVERSIONINFO osver;
osver.dwOSVersionInfoSize = sizeof(OSVERSIONINFO);
isVista = (::GetVersionEx(&osver) &&
osver.dwPlatformId == VER_PLATFORM_WIN32_NT &&
(osver.dwMajorVersion >= 6));
}
return isVista != FALSE;
}
Gdiplus::Bitmap* BitmapFromMemory(BYTE* data, unsigned int imageSize) {
if (WinUtils::IsVista()) {
if (!SHCreateMemStreamFunc) {
HMODULE lib = LoadLibrary(_T("Shlwapi.dll"));
SHCreateMemStreamFunc = reinterpret_cast<SHCreateMemStreamFuncType>(GetProcAddress(lib, "SHCreateMemStream"));
if (!SHCreateMemStreamFunc) {
return 0;
}
}
Gdiplus::Bitmap * bitmap;
IStream* pStream = SHCreateMemStreamFunc(data, imageSize);
if (pStream) {
bitmap = Gdiplus::Bitmap::FromStream(pStream);
pStream->Release();
if (bitmap) {
if (bitmap->GetLastStatus() == Gdiplus::Ok) {
return bitmap;
}
delete bitmap;
}
}
} else {
HGLOBAL buffer = ::GlobalAlloc(GMEM_MOVEABLE, imageSize);
if (buffer) {
void* pBuffer = ::GlobalLock(buffer);
if (pBuffer) {
Gdiplus::Bitmap * bitmap;
CopyMemory(pBuffer, data, imageSize);
IStream* pStream = NULL;
if (::CreateStreamOnHGlobal(buffer, FALSE, &pStream) == S_OK) {
bitmap = Gdiplus::Bitmap::FromStream(pStream);
pStream->Release();
if (bitmap) {
if (bitmap->GetLastStatus() == Gdiplus::Ok) {
return bitmap;
}
delete bitmap;
}
}
::GlobalUnlock(buffer);
}
::GlobalFree(buffer);
}
}
return 0;
}
// Based on original method from http://danbystrom.se/2009/01/05/imagegetthumbnailimage-and-beyond/
Gdiplus::Bitmap* GetThumbnail(Gdiplus::Image* bm, int width, int height, Gdiplus::Size* realSize = 0) {
using namespace Gdiplus;
if (realSize) {
realSize->Width = bm->GetWidth();
realSize->Height = bm->GetHeight();
}
Size sz = AdaptProportionalSize(Size(width, height), Size(bm->GetWidth(), bm->GetHeight()));
Bitmap* res = new Bitmap(sz.Width, sz.Height);
Graphics gr(res);
gr.SetInterpolationMode(Gdiplus::InterpolationModeHighQualityBicubic);
UINT size = bm->GetPropertyItemSize(PropertyTagThumbnailData);
if (size) {
// Loading thumbnail from EXIF data (fast)
enum ThumbCompression { ThumbCompressionJPEG, ThumbCompressionRGB, ThumbCompressionYCbCr, ThumbCompressionUnknown }
compression = ThumbCompressionJPEG;
PropertyItem* thumbnailFormatItem = GetPropertyItemFromImage(bm, PropertyTagThumbnailFormat);
if (thumbnailFormatItem) {
UINT format = VoidToInt(thumbnailFormatItem->value, thumbnailFormatItem->length);
if (format == 0) {
compression = ThumbCompressionRGB;
} else if (format == 1) {
compression = ThumbCompressionJPEG;
} else {
compression = ThumbCompressionUnknown;
}
free(thumbnailFormatItem);
} else {
PropertyItem* compressionItem = GetPropertyItemFromImage(bm, PropertyTagThumbnailCompression);
if (compressionItem) {
WORD compressionTag = *reinterpret_cast<WORD*>(compressionItem->value);
if (compressionTag == 1) {
compression = ThumbCompressionRGB;
PropertyItem* photometricInterpretationItem = GetPropertyItemFromImage(bm, PropertyTagPhotometricInterp);
if (photometricInterpretationItem) {
WORD photoMetricInterpretationTag = VoidToInt(photometricInterpretationItem->value, photometricInterpretationItem->length);
free(photometricInterpretationItem);
if (photoMetricInterpretationTag == 6) {
compression = ThumbCompressionYCbCr;
}
}
} else if (compressionTag == 6) {
compression = ThumbCompressionJPEG;
}
free(compressionItem);
}
}
int originalThumbWidth = 0, originalThumbHeight = 0;
if (compression == ThumbCompressionJPEG || compression == ThumbCompressionRGB) {
PropertyItem* thumbDataItem = GetPropertyItemFromImage(bm, PropertyTagThumbnailData);
if (thumbDataItem) {
if (compression == ThumbCompressionJPEG) {
Bitmap* src = BitmapFromMemory(reinterpret_cast<BYTE*>(thumbDataItem->value), thumbDataItem->length);
if (src) {
gr.DrawImage(src, 0, 0, sz.Width, sz.Height);
delete src;
free(thumbDataItem);
return res;
}
} else if (compression == ThumbCompressionRGB) {
PropertyItem* widthItem = GetPropertyItemFromImage(bm, PropertyTagThumbnailImageWidth);
if (widthItem) {
originalThumbWidth = VoidToInt(widthItem->value, widthItem->length);
free(widthItem);
}
PropertyItem* heightItem = GetPropertyItemFromImage(bm, PropertyTagThumbnailImageHeight);
if (heightItem) {
originalThumbHeight = VoidToInt(heightItem->value, heightItem->length);
free(heightItem);
}
if (originalThumbWidth && originalThumbHeight) {
BITMAPINFOHEADER bih;
memset(&bih, 0, sizeof(bih));
bih.biSize = sizeof(bih);
bih.biWidth = originalThumbWidth;
bih.biHeight = -originalThumbHeight;
bih.biPlanes = 1;
bih.biBitCount = 24;
BITMAPINFO bi;
memset(&bi, 0, sizeof(bi));
bi.bmiHeader = bih;
BYTE* data = reinterpret_cast<BYTE*>(thumbDataItem->value);
BYTE temp;
// Convert RGB to BGR
for (unsigned int offset = 0; offset < thumbDataItem->length; offset += 3) {
temp = data[offset];
data[offset] = data[offset + 2];
data[offset + 2] = temp;
}
Bitmap src(&bi, thumbDataItem->value);
if (src.GetLastStatus() == Ok) {
gr.DrawImage(&src, 0, 0, sz.Width, sz.Height);
free(thumbDataItem);
return res;
}
}
} else {
// other type of compression not implemented
}
free(thumbDataItem);
}
}
}
// Fallback - Load full image and draw it (slow)
gr.DrawImage(bm, 0, 0, sz.Width, sz.Height);
return res;
}
Gdiplus::Bitmap* GetThumbnail(const CString& filename, int width, int height, Gdiplus::Size* realSize) {
using namespace Gdiplus;
Image bm(filename);
if (bm.GetLastStatus() != Ok) {
return 0;
}
return GetThumbnail(&bm, width, height, realSize);
}
Gdiplus::Size AdaptProportionalSize(const Gdiplus::Size& szMax, const Gdiplus::Size& szReal)
{
int nWidth;
int nHeight;
double sMaxRatio;
double sRealRatio;
if (szMax.Width < 1 || szMax.Height < 1 || szReal.Width < 1 || szReal.Height < 1)
return Size();
sMaxRatio = szMax.Width / static_cast<double>(szMax.Height);
sRealRatio = szReal.Width / static_cast<double>(szReal.Height);
if (sMaxRatio < sRealRatio) {
nWidth = min(szMax.Width, szReal.Width);
nHeight = static_cast<int>(round(nWidth / sRealRatio));
} else {
nHeight = min(szMax.Height, szReal.Height);
nWidth = static_cast<int>(round(nHeight * sRealRatio));
}
return Size(nWidth, nHeight);
}