how to move two bitmap-image on a canvas - delphi

Question

I am writing an animation program under Delphi 7 consisting of moving two discs on a canvas (I choose a PaintBox) with a bounce effect on the edges.
it's woks fine if I load the pictures one by one: In this case, when the two disks that arrive from time to time are superimposed, no background rectangle appears with even a rather pleasant transparency effect.
But if I try to generalize the operation with many more discs by introducing for example a Record.
The movements are ok BUT in this case, when the discs cross, a background
rectangle appears in the upper image which spoils everything!
I even tried to write the code with an Object with :
TSphere = class (TObject)
but nothing to do, the phenomenon remains ..
Do you have any idea how to remove this display defect?
and i have another question, i would like to fill the disks with textures.
the full code :
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms,
Dialogs, ExtCtrls, StdCtrls, ComCtrls;
type
TSphere = record
W, H: integer;
vx, vy: Extended;
x, y: integer;
xx, yy: extended;
ROld, RNew: TRect;
Bitm: TBitmap;
end;
type
TForm1 = class(TForm)
PaintBox1: TPaintBox;
Button1: TButton;
Timer1: TTimer;
Label1: TLabel;
Edit1: TEdit;
Label2: TLabel;
TrackBar1: TTrackBar;
procedure FormCreate(Sender: TObject);
procedure FormDestroy(Sender: TObject);
procedure FormPaint(Sender: TObject);
procedure Button1Click(Sender: TObject);
procedure Timer1Timer(Sender: TObject);
procedure TrackBar1Change(Sender: TObject);
end;
var
Form1: TForm1;
fin: boolean;
BmpBkg: Tbitmap;
BmpMoving: TBitmap;
Spheres: array of TSphere;
const
nb = 2;
ImageWidth = 32;
implementation
{$R *.DFM}
procedure PictureStorage;
var
i: integer;
begin
SetLength(Spheres, nb);
for i := 0 to (nb - 1) do
begin
with Spheres[i] do
begin
Bitm := TBitmap.Create;
case i of
0: Bitm.loadFromFile('Sphere1.bmp');
1: Bitm.loadFromFile('Sphere2.bmp');
end;
end;
end;
end;
procedure TForm1.FormCreate(Sender: TObject);
var
i: integer;
begin
DoubleBuffered := true;
randomize;
Fin := false;
BmpBkg := TBitmap.Create;
BmpMoving := TBitmap.Create;
BmpBkg .Canvas.Brush.Color := ClBtnFace;
BmpBkg .Canvas.FillRect(Rect(0, 0, PaintBox1.height,
PaintBox1.width));
BmpBkg .Width := PaintBox1.Width;
BmpBkg .Height := PaintBox1.Height;
BmpMoving .Assign(BmpBkg );
PictureStorage;
for i := 0 to (nb - 1) do
begin
with Spheres[i] do
begin
W := Bitm.Width;
H := Bitm.Height;
Bitm.Transparent := True;
Bitm.TransParentColor := Bitm.canvas.pixels[1, 1];
xx := random(400) + 1;
yy := random(200) + 1;
x := trunc(xx);
y := trunc(yy);
vx := random(3) + 1;
vy := random(4) + 1;
RNew := bounds(x, y, W, H);
ROld := RNew;
end;
end;
Timer1.interval := 1;
Timer1.enabled := true;
end;
procedure TForm1.FormDestroy(Sender: TObject);
var
i: integer;
begin
Fin := true;
BmpBkg.free;
BmpMoving.free;
for i := 0 to (nb - 1) do
Spheres[i].Bitm.Free;
end;
procedure TForm1.FormPaint(Sender: TObject);
begin
PaintBox1.Canvas.Draw(0, 0, BmpMoving);
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
close;
end;
procedure TForm1.Timer1Timer(Sender: TObject);
var
n, i: integer;
Runion: Trect;
begin
for n := 1 to trackbar1.position do
begin
if fin then exit;
for i := 0 to (nb - 1) do
begin
with Spheres[i] do
begin
BmpMoving.Canvas.CopyRect(ROld, bmpBkg.canvas, ROld);
if (x < -ImageWidth) or (x > bmpBkg.width - W + ImageWidth)
then
vx := -vx;
if (y < 0) or (y > bmpBkg.height - H) then
vy := -vy;
xx := xx + vx;
yy := yy + vy;
x := trunc(xx);
y := trunc(yy);
RNew := bounds(x, y, W, H);
BmpMoving.Canvas.Draw(x, y, Bitm);
UnionRect(RUnion, ROld, RNew);
PaintBox1.Canvas.CopyRect(RUnion, BmpMoving.Canvas,
RUnion);
ROld := RNew;
end;
end;
end;
end;
procedure TForm1.TrackBar1Change(Sender: TObject);
begin
Edit1.text := inttostr(trackbar1.position);
if trackbar1.position = 1 then
label2.visible := true
else
label2.visible := false;
end;
end.
this program is just the start of another more important
thanks

Answer 1

Your code is almost OK.
As far as I can see your problem is caused by not completely restoring the background before you draw the bitmaps at their new locations. You need to restore the old rects of all spheres before you draw the new ones. Also you need to collect the complete union of all new and old rects before you update to screen.
As a matter of taste, I would avoid the global variables and make them fields of the form. If you also make PictureStorage a method of the form, everything works.
The timer interval of 1 seems a bit of an overkill. I would set it to 1000 div 120 (120 FPS).
I would set doublebuffered to false, as you are already doing your own doublebuffering. Also I would move the form's OnPaint to the paintbox's OnPaint, but that doesn't seem to work for you.
Here is the replacement of the OnTimer event which should work (I checked an analogue with Delphi 2006, I don't have Delphi7 installed anymore and I don't know what the n means).
procedure TForm1.Timer1Timer(Sender: TObject);
var
n, i: integer;
Runion: TRect;
begin
//I don't know what the n-loop is for, in my test I left it out
for n := 1 to TrackBar1.position do
begin
//prevent reentry?
if fin then
exit;
// Restore the background completely
for i := 0 to (nb - 1) do
with Spheres[i] do
begin
BmpMoving.Canvas.CopyRect(ROld, BmpBkg.Canvas, ROld);
// Collect the old rects into the update-rect
if i = 0 then
Runion := ROld
else
UnionRect(Runion, Runion, ROld);
end;
for i := 0 to (nb - 1) do
with Spheres[i] do
begin
if (x < -ImageWidth) or (x > BmpBkg.width - W + ImageWidth) then
vx := -vx;
if (y < 0) or (y > BmpBkg.height - H) then
vy := -vy;
xx := xx + vx;
yy := yy + vy;
x := trunc(xx);
y := trunc(yy);
RNew := bounds(x, y, W, H);
BmpMoving.Canvas.Draw(x, y, Bitm);
// Add RNew to RUnion
UnionRect(Runion, Runion, RNew);
// No painting yet, update the screen as few times as possible
ROld := RNew;
end;
//Now update the screen
//This is the reliable way for sherlock to update the screen:
OffsetRect(RUnion, Paintbox1.left, Paintbox1.top);
//RUnion in form's coordinates
InvalidateRect(Handle, #RUnion, false);
//The following works for me just as well:
(**************
PaintBox1.Canvas.CopyRect(Runion, BmpMoving.Canvas, Runion);
***************)
end;
end;

Answer 2

This code can be commented out.
Tt does not affect the program :
// Collect the old rects into the update-rect
{ if i = 0 then
Runion := ROld
else
UnionRect(Runion, Runion, ROld); }