Reading C++ code CreateFrame function (from C# prespective)

// Create test video frame
void CreateFrame(char * buffer, int w, int h, int bytespan)
{
  int wxh = w * h;
  static float seed = 1.0;
  for (int i = 0; i < h; i ++)
  {
    char* line = buffer + i * bytespan;
    for (int j = 0; j < w; j ++)
    {
      // RGB
      line[0] = 255 * sin(((float)i / wxh * seed) * 3.14);
      line[1] = 255 * cos(((float)j / wxh * seed) * 3.14);
      line[2] = 255 * sin(((float)(i + j) / wxh * seed) * 3.14);
      line += 3;
    }
  }
  seed = seed + 2.2开发者_如何学Go;
}

can any one please tall me what is line += 3; for?

and how to create such function analog in C#?

line += 3 increments the pointer line by 3 bytes, so that it points to the next pixel. line here is a pointer to a 3-byte pixel, so it really should be called something else, like pPixel.

Line is a pointer to a position within buffer. Incrementing line advances the processing down the buffer.

A C# analog might be:

static float seed = 1.0f;
static void CreateFrame(byte[] buffer, int w, int h, int bytespan)
{
  int wxh = w * h;
  for (int i = 0; i < h; i ++)
  {
    int line = i * bytespan;
    for (int j = 0; j < w; j ++)
    {
      // RGB
      buffer[line + 0] = (byte)(255 * Math.Sin(((float)i / wxh * seed) * 3.14));
      buffer[line + 1] = (byte)(255 * Math.Cos(((float)j / wxh * seed) * 3.14));
      buffer[line + 2] = (byte)(255 * Math.Sin(((float)(i + j) / wxh * seed) * 3.14));
      line += 3;
    }
  }
  seed = seed + 2.2f;
}

In C/C++, the value line in line is actually a memory address of an array, and line[1] actually represents the value at the address of the variable line plus a 1 item offset. (If the type of the items in line is an int, then it means the address of line plus four bytes; since it is a char, it means the address of line plus one byte.)

So, line += 3 means that line[1] is now equivalent to [old "line" value][4]. The coder could have written the code as:

for (int j = 0; j < w; j ++)
{
  // RGB
  line[(3 * j)] = 255 * sin(((float)i / wxh * seed) * 3.14);
  line[(3 * j) + 1] = 255 * cos(((float)j / wxh * seed) * 3.14);
  line[(3 * j) + 2] = 255 * sin(((float)(i + j) / wxh * seed) * 3.14);
}

This is pointer arithmetic. Since you are dealing with 3 elements of the array in one go you will need to update the pointer suitably otherwise you will be reading the same location twice and of course, erroneously.

You would replace the pointer by a byte array and index into it by an integer as follows:

// Create test video frame
void CreateFrame(byte[] buffer, int w, int h, int bytespan)
{
  int wxh = w * h;
  static float seed = 1.0;
  for (int i = 0; i < h; i ++)
  {
    int line = i * bytespan;
    for (int j = 0; j < w; j ++)
    {
      // RGB
      buffer[line + 0] = 255 * sin(((float)i / wxh * seed) * 3.14);
      buffer[line + 1] = 255 * cos(((float)j / wxh * seed) * 3.14);
      buffer[line + 2] = 255 * sin(((float)(i + j) / wxh * seed) * 3.14);
      line += 3;
    }
  }
  seed = seed + 2.2;
}

I just left the variable name as line, even if from what I understand, it is not really a line.