Below is my program. I am trying to apply grayscale filter using bitmapdata class in visual c++. I am getting AccessViolationException at 11, tagged by the comment. I have tried using CLR:Safe and CLR:pure but no use. In c# this would be solved by using unsafe block. Any suggestions? None of the other solutions on related questions worked.
Bitmap^ bmp = gcnew Bitmap(pictureBox1->Image);
BitmapData^ data = bmp->LockBits(Rectangle(0,0,bmp->Width,bmp->Height), ImageLockMode::ReadWrite, PixelFormat::Format24bppRgb);
int blue=0, green=0, red=0;
System::IntPtr s = data->Scan0;
int* P = (int*)(void*)s;
for (int i =0; i<bmp->Height;i++)
{
for (int j = 0; j < bmp->Width*3; j++)
{
blue = (int)P[0]; //access violation exception
green =(int )P[1];
red = (int)P[2];
int avg = (int)((blue + green + red) / 3);
P[0] = avg;
P[1] = avg;
P[2] = avg;
P +=3;
}
}
bmp->UnlockBits(data);
pictureBox1->Image = bmp;
You are using an int* when you should be using a byte*. Your pixels are three bytes each, one byte per channel. Your int is (likely) 4 bytes, so p[0] returns an entire pixel plus on byte past it. This is why you get an access violation; you are overrunning the bounds of the image buffer.
When you increment a pointer, you are adding sizeof *p bytes to it. In this case, P += 3 increments the pointer P by 12 bytes. Much too much, and you'll never be able to read a single pixel (or channel) of a 24bpp image with an int*. You are also assuming that your stride is Width * 3, which may or may not be correct (bitmaps are 4 byte aligned.)
Byte* base = (Byte*)data->Scan0;
int stride = data->Stride;
for(int y = 0; y < data->Height; ++y) {
Byte* src = base + y * stride;
for(int x = 0; x < data->Width; ++x, src += 3) {
// bitmaps are stored in BGR order (though not really important here).
// I'm assuming a 24bpp bitmap.
Byte b = src[0];
Byte g = src[1];
Byte r = src[2];
int average = (r + g + b) / 3;
src[0] = src[1] = src[2] = (Byte)average;
}
}