Creating thumbnail graphics

The following function will convert an image to a thumbnail graphic surrounded by a black frame. If the image is wider than it is high, the image will be framed with an even black border along the top and bottom edges and if it is higher than it is wide the image will be framed with an even border along the left and right edges. The function assumes two global variables, a bitmap which will become the thumbnail (the variable name 'bitmap' below) and a global trect variable (named 'rect' below) which determines the size of the thumbnail to create ... for example to create a thumbnail of size 64 by 64 rect.bottom would be set to 64, rect.right would be set to 64, with both rect.top and rect.left set to 0.

The larger bitmap (in the 'bitmap' variable) will be 'stretch' copied in the variable lframe, and then lframe will be copied into a tiny small black bitmap (named 'bit' below) to achieve the 'blackframe' effect. The height and width of lframe must be proportionally the same as the original bitmap. For example let us assume that the thumbnail will be 64 by 64 pixels. The bitmap is 1000 by 600. If the bitmap is simply 'stretch' copied into the 64 by 64 thumbnail it will look stretched out of shape since the orignal bitmap was not square. To prevent this from happening the lframe bitmap is first set to match the proportions of the original bitmap. The original bitmap can then be stretch copied into this tiny frame wihtout distortion and then the frame can be copied onto the 64 by 64 black canvas to achieve the framing effect.

Let us suppose that in the above example the bitmap was 1000 wide and 600 high. The little frame (lframe) into which we wish to copy this bitmap to make a thumbnail would be 64 by 64 but we want the proportions to match since this is not a square bitmap. We can create a mathematical ratio to find the required values for the height and width of lframe. In this case the bitmap is higher than it is wide so we will keep the height in the thumbnail (in this example 64) and then look for the correct width to avoid distortion when stretch copying. 1000 is to 600 as 64 is to the new width. If you remember some high school algebra this can be expressed as a mathematical equation as follows : 1000 / 600 = 64 / x ... where x is the appropriate width for the little thumbnail frame. Cross multiplying expresses this ration using multiplication instead of division, yielding 1000 * x = 64 * 600, and then dividing both sides by 1000 isolates the unknown variable x yielding the equation to solve for the unknown dimension (X = (64 * 600) / 1000... A similar mathematical calculation also gives the new height in the case where the width is greater than the height and we wish to keep the thumbnail 64 pixels wide but wish to find the appropriate height to avoid distortion when stretch copying the larger bitmap into the the little frame.

The two images below show the result (128 x 128 thumbnails) one wider than it is tall and thus framed horizontally, and the other higher than it is wide and thus framed left to right (the framing effect is achieved by copying the proportional lframe thumbnail bitmap onto a black canvas - the bit bitmap in the code below) ...








function CreateThumbnail:boolean;
var lframe : tbitmap; {thumbnail which will sit in frame}
    destrect : trect;
    sourcerect : trect;
    bit : tbitmap; {the black frame around the thumbnail}
begin
    Result := true;
try
 If bitmap.height <> bitmap.width then begin
    If bitmap.Height > bitmap.width then begin
       lframe.Width := (rect.right * Bitmap.width) div bitmap.height;
       lframe.height := rect.bottom;
       destrect.Left := 0;
       destrect.Top := 0;
       destrect.Right := bit64.width;
       destrect.bottom := bit64.height;
       bit.canvas.brush.Style := bsSolid;
       bit.canvas.brush.Color := clBlack;
       bit.canvas.FillRect(destrect);
       destrect.right := lframe.width;
       destrect.bottom := lframe.height;
       lframe.canvas.StretchDraw(destrect, bitmap);
       destrect.left := (bit.width ­ lframe.width) div 2;
       destrect.Top := 0;
       destrect.Right := destrect.left + lframe.width;
       destrect.bottom := bit.height;
       sourcerect.Left := 0;
       sourcerect.Top := 0;
       sourcerect.right := lframe.width;
       sourcerect.bottom := lframe.height;
       bit.canvas.CopyRect(destrect, lframe.canvas,
                   sourcerect)
    end ;
    If bitmap.Height < bitmap.width then begin
       lframe.height := (rect.bottom * Bitmap.height) div bitmap.width;
       lframe.width := rect.right;
       destrect.Left := 0;
       destrect.Top := 0;
       destrect.Right := bit64.width;
       destrect.bottom := bit64.height;
       bit.canvas.brush.Style := bsSolid;
       bit.canvas.brush.Color := clBlack;
       bit.canvas.FillRect(destrect);
       destrect.right := lframe.width;
       destrect.bottom := lframe.height;
       lframe.canvas.StretchDraw(destrect, bitmap);
       destrect.left := 0;
       destrect.Top := (bit.Height ­ lframe.height) div 2;
       destrect.Right := bit.width;
       destrect.bottom := destrect.top + lframe.height;
       sourcerect.Left := 0;
       sourcerect.Top := 0;
       sourcerect.right := lframe.width;
       sourcerect.bottom := lframe.height;
       bit.canvas.CopyRect(destrect, lframe.canvas,
                   sourcerect);
    end;
end else begin  {square picture}
       destrect.left := 0;
       destrect.top := 0;
       destrect.right := bit.width;
       destrect.bottom := bit.height;
       bit.canvas.StretchDraw(destrect, bitmap);
end;
except
   Result := false;
end;
end;