inserting zeros between the elements of vector with high performance and speed ( preferred to use STL) - visual-studio-2010

Question

I have extracted raster data of a geotiff image using RasterIO of the GDAL library. Since the image shown by OpenGL needs to have width and height both a multiple of 4, I have used this code after extracting the data.
the first switch block evaluates the rest of RasterXSize(width) divided by 4 and for example if it is 1, it means that we should add 3 columns meaning that we should add 3 zeros at the end of each row. This is done by the code:
for ( int i = 1; i <= RasterYSize; i++)
pRasterData.insert(pRasterData.begin()+i*RasterXSize*depthOfPixel+(i-1)*3,3,0);
and the second switch block evaluates the rest of RasterYSize(height) divided by 4 and for example if it is 1, it means that we should easily add 3 rows to the end of the data which is done by this code:
pRasterData.insert(pRasterData.end(),3*RasterXSize,0);
This is the whole code that I have used for extracting the data and preparing it to be displayed by OpenGL:
void FilesWorkFlow::ReadRasterData(GDALDataset* poDataset)
{
RasterXSize = poDataset -> GetRasterXSize();
RasterYSize = poDataset -> GetRasterYSize();
RasterCount = poDataset -> GetRasterCount();
CPLErr error = CE_None;
GDALRasterBand *poRasterBand;
poRasterBand = poDataset -> GetRasterBand(1);
eType = poRasterBand -> GetRasterDataType();
BytesPerPixel = GDALGetDataTypeSize(eType) / 8;
depthOfPixel = RasterCount * BytesPerPixel;
pRasterData.resize(RasterXSize * RasterYSize * RasterCount * BytesPerPixel);
error = poDataset -> RasterIO(GF_Read,0,0,RasterXSize,RasterYSize,&pRasterData[0],RasterXSize,RasterYSize,eType,RasterCount,0,0,0,0);
int modRasterXSize = RasterXSize % 4;
switch (modRasterXSize)
{
case 1:
{
for ( int i = 1; i <= RasterYSize; i++)
pRasterData.insert(pRasterData.begin()+i*RasterXSize*depthOfPixel+(i-1)*3,3,0);
RasterXSize = RasterXSize+3;
break;
}
case 2:
{
for ( int i = 1; i <= RasterYSize; i++)
pRasterData.insert(pRasterData.begin()+i*RasterXSize*depthOfPixel+(i-1)*2,2,0);
RasterXSize = RasterXSize+2;
break;
}
case 3:
{
for ( int i = 1; i <= RasterYSize; i++)
pRasterData.insert(pRasterData.begin()+i*RasterXSize*depthOfPixel+(i-1)*1,1,0);
RasterXSize = RasterXSize+1;
break;
}
}
int modRasterYSize = RasterYSize % 4;
switch (modRasterYSize)
{
case 1:
{
pRasterData.insert(pRasterData.end(),3*RasterXSize,0);
RasterYSize = RasterYSize+3;
break;
}
case 2:
{
pRasterData.insert(pRasterData.end(),2*RasterXSize,0);
RasterYSize = RasterYSize+2;
break;
}
case 3:
{
pRasterData.insert(pRasterData.end(),1*RasterXSize,0);
RasterYSize = RasterYSize+1;
break;
}
}
}
the first switch block is where my code gets slow and because I am working with a 16997*15931 image it takes a lot of time for the program to run through the for loop.
Note that pRasterData is a member variable of the class FilesWorkFlow and because of the problems I had in sending this variable to the COpenGLControl class written by Brett Fowle in codeguru and used by me in the project with some slight changes, decided to use vector<unsigned char> instead of unsighned char*.
Now I am wondering is there anyway to implement these part of code faster using vectors?
Is there anyway to insert zero in certain parts of vector without using for loops and wasting too much time?
something like std::transform? I don't know!
Remember that I'm using MFC in Visual Studio 2010 and it's better for me to use STL but if you have another suggestions besides using vectors or STL, I'd be glad to hear that?

Answer 1

The reason it is slow is because the members of the vector are getting moved multiple times. Think about the members in the last row of your image. They all have to be moved once for every row of the image. It would be faster to create a whole new image, copying just the pixels you need from the original image and adding zeros where appropriate.
Here's an example:
void
padColumns(
std::vector<unsigned char> &old_image,
size_t old_width,
size_t new_width
)
{
size_t height = image.size() / old_width;
assert(image.size() == old_width*height);
std::vector<unsigned char> new_image(new_width * height);
for (size_t row=0; row!=height; ++row) {
std::copy(
old_image.begin() + row*old_width,
old_image.begin() + row*old_width + old_width,
new_image.begin() + row*new_width
);
std::fill(
new_image.begin() + row*new_width + old_width,
new_image.begin() + row*new_width + new_width,
0
);
}
old_image = new_image;
}