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Disecting the function

Now this is the first part of the function that we will disect... 

  if (w > 0) {
    thisSpr->width = w;
    if (h > 0) thisSpr->height = h;
  }

We're assigning the height and width of our sprite to the values of the w and h parameters (if they're greater than zero). Nothing tricky here.

Let's look at the next if statement testing the parameter numFrames. 

  thisSpr->imgDynamic = dynamic;
  if (numFrames > 0) {
    thisSpr->frames=(char far **)malloc(numFrames*sizeof(char far *));

If we specified for there to 1 or more images for our sprite to hold (numFrames > 0) then the first thing we need to do is create an array of numFrames elements. The malloc line accomplishes that task for us. For some of you experienced C programmers of the DOS platform, you may be wondering whether or not I should've used farmalloc instead of malloc. After all, frames is a char far ** type variable. Actually, frames is a near pointer that points to an array of far pointers. The first point (*) is far (char far **), the second pointer (*) is near because there is no far modifier. The function farmalloc would've been used only if the frames variable was declared as char far * far *frames;.

If malloc was successful in allocating the array, the number of frames will be set and each far pointer will be assigned an address to a specific image of NULL depending on whether the images are dynamic and whether both height and width are greater than zero. These are the lines that do this task: 

    if (thisSpr->frames != NULL) {
      thisSpr->noOfFrames = numFrames;
      for (i=0; i<numFrames; i++)
        if (dynamic && w > 0 && h > 0) {
          thisSpr->frames[i]=(char far *)farmalloc(w*h*sizeof(char));
          if (thisSpr->frames[i] == NULL) retVal = 0;
        } else
          thisSpr->frames[i] = NULL;
    } else
      retVal = 0;

Of course if no frames we're asked for, that is if numFrames was zero or below, this line is read: 

  } else
    thisSpr->frames = NULL;

Finally we try to reserve memory for storing a background copy if we decide to erase sprites using dirty rectangles or not. (drtyRect parameter) Then we assign default values to the rest of the variables. 

  if (drtyRect) {
    thisSpr->background = (char far *)farmalloc(w * h * sizeof(char));
    if (thisSpr->background == NULL && w && h) retVal = 0;
    thisSpr->backTransparent = 1;
  } else
    thisSpr->background = NULL;

  thisSpr->maxX = ScreenWidth - 1;
  thisSpr->maxY = ScreenHeight - 1;

  thisSpr->lenXcopy = thisSpr->width;
  thisSpr->lenYcopy = thisSpr->height;

  thisSpr->displayMem = videoMem;

  thisSpr->X = thisSpr->Y = thisSpr->lastX = thisSpr->lastY =
  thisSpr->minX = thisSpr->minY = thisSpr->startXoff = thisSpr->startYoff =
  thisSpr->currFrame = thisSpr->status = thisSpr->visible = thisSpr->clock =
  thisSpr->animThreshold = thisSpr->moveThreshold = 0;

  thisSpr->otherData = NULL;
  return retVal;
}

Send your questions, comments, or ideas to: wilkeg@gamewood.net

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