[gdal-dev] Loading from a stream/memory rather than a file

[Martin Chapman]

Cristiaan,

Here's another way to do it that is really easy too...maybe it is the same
thing?  I create a MEM dataset from a raster file but you could easily load
it with bytes from memory.



GDALDataset* pDataSource = (GDALDataset*) GDALOpen(sFileName.c_str(),
(GDALAccess) 0);
if (!pDataSource)
	return AtlReportError(GetObjectCLSID(), _T((TCHAR*)
CPLGetLastErrorMsg()));

int nBandCount = pDataSource->GetRasterCount();
int nWidth = pDataSource->GetRasterXSize();
int nHeight = pDataSource->GetRasterYSize();
GDALDataType eBandType = pDataSource->GetRasterBand(1)->GetRasterDataType();

GDALDriver* pMemDriver = GetGDALDriverManager()->GetDriverByName("MEM");
if (!pMemDriver) 
	return AtlReportError(GetObjectCLSID(), _T((TCHAR*)
CPLGetLastErrorMsg()));

GDALDataset* pMemDataset = pMemDriver->Create(sStreamId.str().c_str(),
nWidth, nHeight, nBandCount, eBandType, NULL); 
if (!pMemDataset) 
	return AtlReportError(GetObjectCLSID(), _T((TCHAR*)
CPLGetLastErrorMsg()));

for (int i = 0; i < nBandCount; i++ )
{
	GDALRasterBand* pMemBand = pMemDataset->GetRasterBand(i + 1);
	GDALRasterBand* pRasterBand = pDataSource->GetRasterBand(i + 1);
	
pMemBand->SetColorInterpretation(pRasterBand->GetColorInterpretation());
	pMemBand->SetOffset(pRasterBand->GetOffset());
	pMemBand->SetScale(pRasterBand->GetScale());
	pMemBand->SetUnitType(pRasterBand->GetUnitType());

	BOOL bSuccess = FALSE;
	double nNullValue = pRasterBand->GetNoDataValue(&bSuccess);
	if (bSuccess == TRUE)
	{
		GDALColorTable* pColorTable = pRasterBand->GetColorTable();
		if (pColorTable != NULL)
		{
			GDALColorEntry colorEntry;
			pColorTable->GetColorEntryAsRGB((int) nNullValue,
&colorEntry);
			hr = pMemBand->SetNoDataValue((double)
colorEntry.c4);
			if (FAILED(hr)) return hr;
		}
		else
		{
			hr = pMemBand->SetNoDataValue((double) nNullValue);
			if (FAILED(hr)) return hr;
		}
	}

	if (!pMemBand->GetColorTable())
	{
		GDALColorTable* pColorTable = pRasterBand->GetColorTable();
		pMemBand->SetColorTable(pColorTable != NULL ?
pColorTable->Clone() : NULL);
	}

	unsigned char* pBytes = (unsigned char*) malloc(sizeof(unsigned
char) * nWidth * nHeight);
	memset(pBytes, 0, nWidth * nHeight);
	
	CPLErr err = pRasterBand->RasterIO(GF_Read, 0, 0, nWidth, nHeight,
pBytes, nWidth, nHeight, eBandType, 0, 0);
	if (err != CE_None) 
		return AtlReportError(GetObjectCLSID(), _T((TCHAR*)
CPLGetLastErrorMsg()));

	err = pMemBand->RasterIO(GF_Write, 0, 0, nWidth, nHeight, pBytes,
nWidth, nHeight, eBandType, 0, 0); 
	if (err != CE_None) 
		return AtlReportError(GetObjectCLSID(), _T((TCHAR*)
CPLGetLastErrorMsg()));

	if (pBytes) delete [] pBytes;
}

if (pDataSource)
	GDALClose(pDataSource);
pDataSource = NULL;

if (m_pMemDataset)
	GDALClose(m_pMemDataset);
m_pMemDataset = pMemDataset;




Martin Chapman

-----Original Message-----
From: gdal-dev-bounces at lists.osgeo.org
[mailto:gdal-dev-bounces at lists.osgeo.org] On Behalf Of Frank Warmerdam
Sent: Friday, March 07, 2008 10:22 AM
To: Christiaan Janssen
Cc: gdal-dev at lists.osgeo.org
Subject: Re: [gdal-dev] Loading from a stream/memory rather than a file

Christiaan Janssen wrote:
> Does anyone know if it is possible to create/open an image with GDAL using
a
> stream or chunk of memory instead of a file? This would be useful in
writing
> a caching mechanism but also to do in memory and other non file based
> processing.

Cristiaan,

Some driver utilize the VSI*L "virtual io" layer for file access, and then
you can either right custom file handlers, or more likely use the existing
in-memory VSI*L implementation.

This is likely the best explanation of the mechanism:
   http://www.gdal.org/cpl__vsi_8h.html#66e2e6f093fd42f8a941b962d4c8a19e

gdal/frmts/wcs/httpdriver.cpp is a simple example of it's use for reading
from memory.

Best regards,
-- 
---------------------------------------+------------------------------------
--
I set the clouds in motion - turn up   | Frank Warmerdam,
warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org

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