Create/Save PPM-File from unsigned char* AND unsigned short* Pointers (RGB) (C++) [closed]

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Closed 9 years ago.

I need help saving a PPM file in C/C++. I want to save an PPM image from the three basic color channels. The channels are represented as void * pointers (pRed, pGreen and pBlue). Those values are fixed so I cannot change them.

There is one more variable to decide how the void pointer has to be interpreted: this variable is:

bpp=1 the correct type is unsigned char *. Buffer elements are of type unsigned char

bpp=2 the correct type is unsigned short *. Buffer elements are of type unsigned short.

Now I would save an ppm file out of this three color channels.

The declarations regarding to the bpp variable are:

//bpp=1:
unsigned char* dRed = (unsigned char*)pImg->pRed;
unsigned char* dGreen = (unsigned char*)pImg->pGreen;
unsigned char* dBlue = (unsigned char*)pImg->pBlue;
unsigned char* dBw = (unsigned char*)pImg->pBw;

//bpp=2:
unsigned short* dRed = (unsigned short*)pImg->pRed;
unsigned short* dGreen = (unsigned short*)pImg->pGreen;
unsigned short* dBlue = (unsigned short*)pImg->pBlue;
unsigned short* dBw = (unsigned short*)pImg->pBw;

My code to write the ppm file:

ofstream output(fname, ios::binary|ios::out);

output << "P3"<< endl <<"# foreground "<<endl;
output << itoa(width, fname, 10);
output << " ";
output << itoa(height, fname, 10);
output << endl;
output << itoa(255, fname, 10) << endl;
...     
for(int i=0; i<(height*width); i++){
  if(bytesPerP==1){
    output << (unsigned char)((char*)dRed) << " ";
    output << (unsigned char)((char*)dGreen) << " ";
    output << (unsigned char)((char*)dBlue) << " ";
    dRed = dRed + (i * bytesPerP);
    dGreen = dGreen + (i * bytesPerP);
    dBlue = dBlue + (i * bytesPerP);
}else if(bytesPerP==2){
    output << ((unsigned short)((char*)dRed))%256 << " ";
    output << (unsigned short)((char*)dGreen)%256 << " ";
    output << (unsigned short)((char*)dBlue)%256 << " ";

    dRed = dRed + (i * bytesPerP);
    dGreen = dGreen + (i * bytesPerP);
    dBlue = dBlue + (i * bytesPerP);
}

Here is anywhere the error, because i get the right "structure" of the recorded image, but the colors are totally weird (psychodelic style) :) I have searched on the internet and tested many things, but none of them worked. Could someone point me in the right direction?

Answer 1

The problem is how you're outputting the bytes from your image buffers ... you need to dereference pointer to get at the actual data information, not output the actual pointer-value itself.

For instance, this:

output << (unsigned char)((char*)dRed) << " ";

outputs the 4 or 8-byte pointer value dRed cast to an unsigned char ... you want the value that the pointer dRed is pointing to, which would be done like this:

output << *dRed << " ";

Secondly, if the data for each color-channel is stored in a separate continuous linear array (i.e, red points to only red-pixels, green points to only green-pixels, vs. a buffer where it's a red-pixel, then green-pixel, then blue-pixel), then you're not incrementing the values correctly in your 16-bit-per-channel case. If the buffers are a linear array, you could simply do the following:

 output << " " << dRed[i] << " " << dGreen[i] << " " << dBlue[i] << endl;

where the extra white-space is to conform to the PPM standard where pixel value samples should be surrounded by white-space characters.

Finally, in the "plain" PPM format you're outputting, no row should be longer than 70 characters ... you're definitely going over that limit ...