When painting, set the pixel color and alpha value on the layer which I assume you are doing now. To erase the drawn pixels, set the alpha value of the pixels to 0. This is essentially the same as your color painting code, except you paint the alpha value but not the color. This will allow the pixels in layer 0 to be visible.
The code for erasing a pixel in the OnMouseDown event is shown below. This code will not erase unless there is a transparent layer over the background image in layer 0.
procedure TForm1.ImageEnViewMouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
{ Respond to a mouse down. }
var
iP1: TPoint;
iTransparency: Integer;
iRGB: TRGB;
iColor: TColor;
begin
mx := X;
my := Y;
lx := X;
ly := Y;
APX := ImageEnView.Layers[ImageEnView.LayersCurrent].ConvXScr2Bmp(X);
APY := ImageEnView.Layers[ImageEnView.LayersCurrent].ConvYScr2Bmp(Y);
AStartX := ImageEnView.Layers[ImageEnView.LayersCurrent].ConvXScr2Bmp(X);
AstartY := ImageEnView.Layers[ImageEnView.LayersCurrent].ConvYScr2Bmp(Y);
if (Button = mbRight) and (ImageEnView.MouseInteract <> [miZoom]) then
dxBarPopupMenuImageEnView1.PopupFromCursorPos;
if (Paint1.Down) and (ImageEnView.MouseCapture) then
begin
{ Paint Point Color and Opacity }
with ImageEnView do
begin
Proc.SaveUndoCaptioned('Paint Color And Alpha ' +
IntToStr(Proc.UndoCount + 1));
iP1.X := Layers[LayersCurrent].ConvXScr2Bmp(X);
iP1.Y := Layers[LayersCurrent].ConvYScr2Bmp(Y);
HighlightedPixel := iP1;
iTransparency := PenAlpha1.EditValue;
IEBitmap.Canvas.Pixels[Layers[LayersCurrent].ConvXScr2Bmp(X),
ImageEnView.Layers[LayersCurrent].ConvYScr2Bmp(Y)] :=
PenColor1.EditValue;
IEBitmap.AlphaChannel.Canvas.Pen.Color := $02000000 or (iTransparency)
or (iTransparency shl 8) or (iTransparency shl 16);
IEBitmap.AlphaChannel.Canvas.Brush.Color := $02000000 or
(iTransparency) or (iTransparency shl 8) or (iTransparency shl 16);
IEBitmap.AlphaChannel.Canvas.Pixels
[Layers[LayersCurrent].ConvXScr2Bmp(X),
Layers[LayersCurrent].ConvYScr2Bmp(Y)] := PenColor1.EditValue;
IEBitmap.Alpha[Layers[LayersCurrent].ConvXScr2Bmp(X),
Layers[LayersCurrent].ConvYScr2Bmp(Y)] := iTransparency;
Update;
Bitmap.Modified := True;
end;
end
else if (PaintColor1.Down) and (ImageEnView.MouseCapture) then
begin
{ Paint Point Color }
with ImageEnView do
begin
Proc.SaveUndoCaptioned('Paint Color ' + IntToStr(Proc.UndoCount + 1));
iP1.X := Layers[LayersCurrent].ConvXScr2Bmp(X);
iP1.Y := Layers[LayersCurrent].ConvYScr2Bmp(Y);
HighlightedPixel := iP1;
IEBitmap.Canvas.Pixels[Layers[LayersCurrent].ConvXScr2Bmp(X),
ImageEnView.Layers[LayersCurrent].ConvYScr2Bmp(Y)] :=
PenColor1.EditValue;
Update;
Bitmap.Modified := True;
end;
end
else if (PaintAlpha1.Down) and (ImageEnView.MouseCapture) then
begin
{ Paint Point Opacity }
with ImageEnView do
begin
Proc.SaveUndoCaptioned('Paint Alpha ' + IntToStr(Proc.UndoCount + 1));
iP1.X := Layers[LayersCurrent].ConvXScr2Bmp(X);
iP1.Y := Layers[LayersCurrent].ConvYScr2Bmp(Y);
HighlightedPixel := iP1;
iTransparency := (IEOpacityToAlpha(PenOpacity1.EditValue));
IEBitmap.AlphaChannel.Canvas.Pen.Color := $02000000 or (iTransparency)
or (iTransparency shl 8) or (iTransparency shl 16);
IEBitmap.AlphaChannel.Canvas.Brush.Color := $02000000 or
(iTransparency) or (iTransparency shl 8) or (iTransparency shl 16);
IEBitmap.AlphaChannel.Canvas.Pixels
[Layers[LayersCurrent].ConvXScr2Bmp(X),
Layers[LayersCurrent].ConvYScr2Bmp(Y)] := PenColor1.EditValue;
IEBitmap.Alpha[Layers[LayersCurrent].ConvXScr2Bmp(X),
Layers[LayersCurrent].ConvYScr2Bmp(Y)] := iTransparency;
Update;
Bitmap.Modified := True;
end;
end
else if (Erase1.Down) and (ImageEnView.MouseCapture) then
begin
{ Paint Point Opacity as 0}
with ImageEnView do
begin
Proc.SaveUndoCaptioned('Erase ' + IntToStr(Proc.UndoCount + 1));
iP1.X := Layers[LayersCurrent].ConvXScr2Bmp(X);
iP1.Y := Layers[LayersCurrent].ConvYScr2Bmp(Y);
HighlightedPixel := iP1;
iTransparency := 0;
IEBitmap.AlphaChannel.Canvas.Pen.Color := $02000000 or (iTransparency)
or (iTransparency shl 8) or (iTransparency shl 16);
IEBitmap.AlphaChannel.Canvas.Brush.Color := $02000000 or
(iTransparency) or (iTransparency shl 8) or (iTransparency shl 16);
IEBitmap.AlphaChannel.Canvas.Pixels
[Layers[LayersCurrent].ConvXScr2Bmp(X),
Layers[LayersCurrent].ConvYScr2Bmp(Y)] := PenColor1.EditValue;
IEBitmap.Alpha[Layers[LayersCurrent].ConvXScr2Bmp(X),
Layers[LayersCurrent].ConvYScr2Bmp(Y)] := iTransparency;
Update;
Bitmap.Modified := True;
end;
end;
...The same code should also be placed in the OnMouseMove event. Similar code can be also used to draw with GDIPlus using an TIECanvas which allows using a brush to draw brush(size(> 1).
Also for those developing for touch screens. I just found out today that ImageEnView.MouseCapture handles touch so you do not have to handle the more complicated Shift parameter... ((Shift = [ssLeft]) or (Shift = [ssLeft, ssTouch]) or (Shift = [ssTouch])).
William Miller
Adirondack Software & Graphics
Email: w2m@frontiernet.net
EBook: http://www.imageen.com/ebook/
Apprehend: http://www.frontiernet.net/~w2m/index.html
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Erasing on a transparent layer
Posted - Sep 05 2013 : 04:00:56
