gdalrescaledalphaband.cpp

/******************************************************************************
 *
 * Project:  GDAL Core
 * Purpose:  Implementation of GDALRescaledAlphaBand, a class implementing
 *           a band mask based from a non-GDT_Byte alpha band
 * Author:   Even Rouault, <even dot rouault at spatialys dot com>
 *
 ******************************************************************************
 * Copyright (c) 2014, Even Rouault <even dot rouault at spatialys dot com>
 *
 * SPDX-License-Identifier: MIT
 ****************************************************************************/

#include "cpl_port.h"
#include "gdal_priv.h"

#include <cstddef>

#include "cpl_error.h"
#include "cpl_vsi.h"
#include "gdal.h"

//! @cond Doxygen_Suppress
/************************************************************************/
/*                        GDALRescaledAlphaBand()                       */
/************************************************************************/

GDALRescaledAlphaBand::GDALRescaledAlphaBand(GDALRasterBand *poParentIn)
    : poParent(poParentIn), pTemp(nullptr)
{
    CPLAssert(poParent->GetRasterDataType() == GDT_UInt16);

    // Defined in GDALRasterBand.
    poDS = nullptr;
    nBand = 0;

    nRasterXSize = poParent->GetXSize();
    nRasterYSize = poParent->GetYSize();

    eDataType = GDT_Byte;
    poParent->GetBlockSize(&nBlockXSize, &nBlockYSize);
}

/************************************************************************/
/*                      ~GDALRescaledAlphaBand()                        */
/************************************************************************/

GDALRescaledAlphaBand::~GDALRescaledAlphaBand()
{
    VSIFree(pTemp);
}

/************************************************************************/
/*                             IReadBlock()                             */
/************************************************************************/

CPLErr GDALRescaledAlphaBand::IReadBlock(int nXBlockOff, int nYBlockOff,
                                         void *pImage)
{
    int nXSizeRequest = nBlockXSize;
    if (nXBlockOff * nBlockXSize + nBlockXSize > nRasterXSize)
        nXSizeRequest = nRasterXSize - nXBlockOff * nBlockXSize;
    int nYSizeRequest = nBlockYSize;
    if (nYBlockOff * nBlockYSize + nBlockYSize > nRasterYSize)
        nYSizeRequest = nRasterYSize - nYBlockOff * nBlockYSize;

    GDALRasterIOExtraArg sExtraArg;
    INIT_RASTERIO_EXTRA_ARG(sExtraArg);

    return IRasterIO(GF_Read, nXBlockOff * nBlockXSize,
                     nYBlockOff * nBlockYSize, nXSizeRequest, nYSizeRequest,
                     pImage, nXSizeRequest, nYSizeRequest, GDT_Byte, 1,
                     nBlockXSize, &sExtraArg);
}

/************************************************************************/
/*                             IRasterIO()                              */
/************************************************************************/

CPLErr GDALRescaledAlphaBand::IRasterIO(
    GDALRWFlag eRWFlag, int nXOff, int nYOff, int nXSize, int nYSize,
    void *pData, int nBufXSize, int nBufYSize, GDALDataType eBufType,
    GSpacing nPixelSpace, GSpacing nLineSpace, GDALRasterIOExtraArg *psExtraArg)
{
    // Optimization in common use case.
    // This avoids triggering the block cache on this band, which helps
    // reducing the global block cache consumption.
    if (eRWFlag == GF_Read && eBufType == GDT_Byte && nXSize == nBufXSize &&
        nYSize == nBufYSize && nPixelSpace == 1)
    {
        if (pTemp == nullptr)
        {
            pTemp = VSI_MALLOC2_VERBOSE(sizeof(GUInt16), nRasterXSize);
            if (pTemp == nullptr)
            {
                return CE_Failure;
            }
        }
        for (int j = 0; j < nBufYSize; j++)
        {
            const CPLErr eErr =
                poParent->RasterIO(GF_Read, nXOff, nYOff + j, nXSize, 1, pTemp,
                                   nBufXSize, 1, GDT_UInt16, 0, 0, nullptr);
            if (eErr != CE_None)
                return eErr;

            GByte *pabyImage = static_cast<GByte *>(pData) + j * nLineSpace;
            GUInt16 *pSrc = static_cast<GUInt16 *>(pTemp);

            for (int i = 0; i < nBufXSize; i++)
            {
                // In case the dynamics was actually 0-255 and not 0-65535 as
                // expected, we want to make sure non-zero alpha will still
                // be non-zero.
                if (pSrc[i] > 0 && pSrc[i] < 257)
                    pabyImage[i] = 1;
                else
                    pabyImage[i] = static_cast<GByte>((pSrc[i] * 255) / 65535);
            }
        }
        return CE_None;
    }

    return GDALRasterBand::IRasterIO(eRWFlag, nXOff, nYOff, nXSize, nYSize,
                                     pData, nBufXSize, nBufYSize, eBufType,
                                     nPixelSpace, nLineSpace, psExtraArg);
}

//! @endcond