GL Screenshot Breaks on viewport resize…sometimes - c++

Question

I’m developing a plugin for SIMDIS (basically military google earth), written in c++ using VS 2012. It’s a pretty nifty little thing to auto plot points, and one of its functions is to take a series of screenshot of the view-port and save the images off so it can be used/processed somewhere else. This works fine too… until you re-size the view-port one too many times. Re-size is done by clicking the corner of the window and dragging it bigger and smaller, and the program may launch full screen or windowed mode; either way it works fine the first few sets… or as long as the window is not re-sized.
When it breaks, the program will still march happily along, create the files, and filling them with data at what seems to be an appropriate size for whatever resolution image I’m trying to generate… but the format becomes no-good. It will still be a *.bmp, but windows stops being able to understand it. No errors are thrown though, (I think, I’m not catching any GL errors?[if that’s possible?]).
I can’t get it to consistently happen with a specific number of actions, but it seems to start failing after 3-7 view-port re-sizes. I don’t know if this is a problem with my screenshot code, an issue with the SIMDIS program or plugin, a GL issue, or what. I’ve tested it on multiple machines.
Has anyone run into this problem before? Is there something specific I should be doing that I’m not? Is this a problem native to the parent program (SIMDIS), or something I can work with/around with GL commands I don’t know about?
Screenshot code follows:
#include "TakeScreenshot.h" //has "#include <gl/GL.h>" etc...
TakeScreenshot::TakeScreenshot()
{
}
std::vector<int> * TakeScreenshot::TakeAScreenshotBMP(const char* filename)
{
//std::cout << "Screenshot! ";
std::vector<int> * returnVec = new std::vector<int>();
int VPort[4] = {0,0,0,0};
int FSize = 0;
int PackStore = 0;
//get GL viewport dimensions, x,y,w,h into vport
glGetIntegerv(GL_VIEWPORT,VPort);
//make a framebuffer, RGB
FSize = VPort[2]*VPort[3]*3;
unsigned char PStore[8294400];// 4k sized buffer
//store settings
glGetIntegerv(GL_PACK_ALIGNMENT, &PackStore);
//unpack to byte order
glPixelStorei(GL_PACK_ALIGNMENT, 1);
//read the gl buffer into our buffer
glReadPixels(VPort[0],VPort[1],VPort[2],VPort[3],GL_RGB,GL_UNSIGNED_BYTE,&PStore);
//Pass back settings
glPixelStorei(GL_PACK_ALIGNMENT, PackStore);
///
//set up file info
///
BITMAPINFOHEADER BMIH; //info header
BMIH.biSize = sizeof(BITMAPINFOHEADER);
BMIH.biSizeImage= VPort[2] * VPort[3] * 3;
BMIH.biWidth = VPort[2];
BMIH.biHeight = VPort[3];
BMIH.biPlanes = 1;
BMIH.biBitCount = 24;
BMIH.biCompression = BI_RGB;
BITMAPFILEHEADER bmfh;//file header
int nBitsOffset = sizeof(BITMAPFILEHEADER) + BMIH.biSize;
LONG lImageSize = BMIH.biSizeImage;
LONG lFileSize = nBitsOffset + lImageSize;
bmfh.bfType = 'B' + ('M'<<8);
bmfh.bfOffBits = nBitsOffset;
bmfh.bfSize = lFileSize;
bmfh.bfReserved1 = bmfh.bfReserved2 = 0;
// swap r and b values because GL has them backwards for BMP format.
unsigned char SwapByte;
for(int loop = 0; loop<FSize; loop+=3)
{
SwapByte = PStore[loop];
PStore[loop] = PStore[loop+2];
PStore[loop +2] = SwapByte;
}
///
// File writing section
///
FILE *pFile;
pFile = fopen(filename, "wb");
//if something borked
if(pFile == NULL)
{
std::cout << "TakeScreenshot::TakeAScreenshotBMP>> Error; was not able to create file (Permisions?)" << std::endl;
returnVec->push_back(-1);
returnVec->push_back(-1);
return returnVec; //exit
}
UINT nWrittenFileHeaderSize = fwrite(&bmfh,1,sizeof(BITMAPFILEHEADER), pFile);
UINT nWrittenInfoHeaderSize = fwrite(&BMIH,1,sizeof(BITMAPINFOHEADER), pFile);
UINT nWrittenDIBDataSize = fwrite(&PStore, 1, lImageSize, pFile);
fclose(pFile);
//some return data for processing later
returnVec->push_back(VPort[2]);
returnVec->push_back(VPort[3]);
return returnVec;
}
TakeScreenshot::~TakeScreenshot(void)
{
}