Loading bitmaps in a thread-safe manner

[chrisdawlud]

I have a function which I use for generating thumbnails for the UI, but when I call it from multiple threads I get memory access errors in TheManager->Load() . I tried locking it with both Win32 critical sections and Qmutex, but with no success. I guess TheManager->Load() itself is not thread-safe. Is there another way? 

Bitmap* Loader::loadHDR(const QString & path) {
	Bitmap* bmap;
	BitmapInfo bi;

	QFile file(path);
	if (file.exists() && file.fileName().endsWith(".exr") || file.fileName().endsWith(".hdr"))
	{
		bi.SetName(TSTR(path));
		bmap = TheManager->Load(&bi);
		if (!bmap)
		{
			return nullptr;
		}
		return bmap;
	}
	return nullptr;
}

QIcon Loader::getIcon(const QString & path, QSize size)
{
	Bitmap* bmap = loadHDR(path);
	if (bmap) {
		BitmapInfo bi_icon;
		bi_icon.SetWidth(size.width());
		bi_icon.SetHeight(size.height());
		bi_icon.SetType(BMM_TRUE_32);
 
		Bitmap* icon = TheManager->Create(&bi_icon);
 
		icon->CopyImage(bmap, COPY_IMAGE_RESIZE_LO_QUALITY,
			BMM_Color_64(0, 0, 0, 0));
 
		int type;
		uchar* buffer = static_cast<uchar*>(icon->GetStoragePtr(&type));
		QImage image(buffer, icon->Width(),
			icon->Height(), QImage::Format::Format_RGB888);
 
		if (bmap) bmap->DeleteThis();
		return QIcon(QPixmap::fromImage(image));
	}
	return QIcon();
}

[drew_avis]

I'm guessing, but...

The docs say:

Note: When several plug-ins call this method to load the same image, they all receive the same pointer to one instance of the BitmapStorage. So if one plug-in manipulates the image, the changes will get reflected everywhere. A developer may use BitmapManager::Create() followed by Bitmap::CopyImage() to create a unique instance of BitmapStorage.

Does that help?

Drew

[chrisdawlud]

The reason I was getting errors in TheManager->Load() was that the code didn't handle task cancellation. It's all working fine now.