CUFFT - padding/initializing question

I am looking at the Nvidia SDK for the convolution FFT example (for large kernels), I know the theory behind fourier transforms and their FFT implementations (the basics at least), but I can't figure out what the following code does:

const int    fftH = snapTransformSize(dataH + kernelH - 1);const int    fftW = snapTransformSize(dataW + kernelW - 1);....//gpu initialization codeprintf("...creating R2C & C2R FFT plans for %i x %i\n", fftH, fftW);        cuf ftSafeCall( cufftPlan2d(&fftPlanFwd, fftH, fftW, CUFFT_R2C) );        cufftSafeCall( cufftPlan2d(&fftPlanInv, fftH, fftW, CUFFT_C2R) );    printf("...uploading to GPU and padding convolution kernel and input data\n");        cutilSafeCall( cudaMemcpy(d_Kernel, h_Kernel, kernelH * kernelW * sizeof(float), cudaMemcpyHostToDevice) );        cutilSafeCall( cudaMemcpy(d_Data,   h_Data,   dataH   * dataW *   sizeof(float), cudaMemcpyHostToDevice) );        cutilSafeCall( cudaMemset(d_PaddedKernel, 0, fftH * fftW * sizeof(float)) );        cutilSafeCall( cudaMemset(d_PaddedData,   0, fftH * fftW * sizeof(float)) );        padKernel(            d_PaddedKernel,            d_Kernel,            fftH,            fftW,            kernelH,            kernelW,            kernelY,            kernelX        );        padDataClampToBorder(            d_PaddedData,            d_Data,            fftH,            fftW,            dataH,            dataW,            kernelH,            kernelW,            kernelY,            kernelX        );

I've never used CUFFT library before so I don't know what the snapTransformSize does

(here's the code)

int snapTransformSize(int dataSize){    int hiBit;    unsigned int lowPOT, hiPOT;    dataSize = iAlignUp(dataSize, 16);    for(hiBit = 31; hiBit >= 0; hiBit--)        if(dataSize & (1U << hiBit)) break;    lowPOT = 1U << hiBit;    if(lowPOT == dataSize)        return dataSize;    hiPOT = 1U << (hiBit + 1);    if(hiPOT <= 1024)        return hiPOT;    else         return iAlignUp(dataSize, 512);}

nor why the complex plane is such initialized.

Can you provide me explanation links or answers please?