Delphi graphics32 saving layers as transparent PNG goes wrong - delphi

Question

I am having a strange problem, and I do not think I can solve it.
I have an ImgView containing layers (transparent png images) and I intend to save all the layers as png files (like a "Save Project" thing), so that later on I can re-open them and place them where I left them. (like an "Open Project" thing)
This is my problem, following steps work just fine:
I add layers (transparent PNG files)
I move them around and place them where I want them
I press save project (so here I save all layers as png image files)
It works
If I do the next following steps, something goes wrong:
I add layers (transparent PNG files)
I move them around and place them where I want them
I change the location of the layers (as in: send to back one layer for example) (so this step is different)
I press save project (so here I save all layers as png image files)
It crashes with "Access violation at address 005380FB in module 'MyApp.exe'. Read of address 000000C0"
Right now it only gives me the above error, but a few runs ago, it pointed me to this line:
procedure TCustomBitmap32.ResetAlpha(const AlphaValue: Byte);
var
I: Integer;
P: PByteArray;
begin
if not FMeasuringMode then <<<<<------ this line
So if I change the index of layers... I cannot save them anymore as PNG ?!?!?!
Here is my save procedure:
for i:=0 to mainForm.ImgView.Layers.Count-2 do
begin
mylay := TBitmapLayer(mainForm.ImgView.Layers.Items[i]);
SaveBMPAsPng(mylay.Bitmap,'C:\MyApp\tmp\'+getLayerPan(i)+'.png');
end;
// where getLayerPan is a function that retrieves a name that I gave to the layer
... and
procedure SaveBmpAsPng(bmp:TBitmap32;dest:string);
var
Y: Integer;
X: Integer;
Png: TPortableNetworkGraphic32;
function IsWhite(Color32: TColor32): Boolean;
begin
Result:= (TColor32Entry(Color32).B = 255) and
(TColor32Entry(Color32).G = 255) and
(TColor32Entry(Color32).R = 255);
end;
begin
bmp.ResetAlpha;
for Y := 0 to bmp.Height-1 do
for X := 0 to bmp.Width-1 do
begin
if IsWhite(bmp.Pixel[X, Y]) then
bmp.Pixel[X,Y]:=Color32(255,255,255,0);
end;
Png:= TPortableNetworkGraphic32.Create;
Png.Assign(bmp);
Png.SaveToFile(dest);
Png.Free;
end;
What could be wrong?
Please help...
EDIT
I think I discovered my problem...
When I move the layers around, the only way (that I know of) to do it clean, is to load all layers into an imagelist (TBitmap32List was my choice at that moment) and after that clean the layers and re-add them from the imagelist to my ImageView in the desired order.
I can only assume that this is where something goes wrong.
It must be because in the layers I have transparent PNGs, and when I load them into the Bitmap32List, I load them as BMPs.
I must look for another way of reorganizing my layers before going any further. I will update you with my solution. If any of you know of a better way of reordering layers in ImageView32, please let me know.
EDIT
So, please observe in the image bellow that the GUI is done, and working. I have the panels representing the layers, I can move them around (as you may see in the image I am dragging layer 'Elementul 0' and movin it up in the chain).
And I repeat, my logic also works when I use temporary files for moving layers up or down in the order. One of the answers suggested that I should just use the Index property to change a layers position in the layers hierarchy, and I am saying that it cannot be done without at least adding new layers to the image. So this is not a double question. It is just a response to one of the answers I received.
Thank you

Answer 1

Your problem is a lot simpler than you might think. Working with layers comes natural:
Send to back
Set the layer's index to 0 or simply call SendToBack. All layers previously before it will have their index increased by 1. All layers previously after it remain at the same position.
Send backward
Decrease the layer's index by 1. The layer previously before it will now come after it, thus has its index increased by one.
Send forward
Increase the layer's index by 1. The layer previously after it will now come before it, thus has its index decreased by one.
Send to front
Set the layer's index to the number of layers minus 1. The layers previously after it have their increased decreased by one.
Hence there is absolutely no need to touch the bitmap, save to disk it to disk, or use any kind of temporary layers to change the order. In virtually every case, the right thing happens when you just set the index of the layer to the position (counting from 0, back to front) you want it to appear at. After moving a panel in your list, you could set the corresponding layer's index to the new index of the panel in the list. However, because the panel is ordered front to back and GR32 orders back to front, you need to translate the index of the panel to the desired index of the layer.
Here's an example how to do that with a TListBox and a TButton:
procedure TForm1.SendBackwardButtonClick(Sender: TObject);
var
LNewListBoxItemIndex: Integer;
begin
// Calculate the new list index and make sure it's valid
LNewListBoxItemIndex := Max(0, Min(ListBox1.ItemIndex + 1, ListBox1.Items.Count - 1));
// Transform the current and new list indices and use them to move the layer
ImgView321.Layers[ListBox1.Items.Count - 1 - ListBox1.ItemIndex].Index :=
ListBox1.Items.Count - 1 - LNewListBoxItemIndex;
// Move the list item
ListBox1.Items.Move(ListBox1.ItemIndex, LNewListBoxItemIndex);
// Preserve the selection (if applicable)
ListBox1.ItemIndex := LNewListBoxItemIndex;
end;
You may also decide to fully synchronize the list with the layers. In that case you should associate each item (possibly TPanel) with a layer.
// Create layers from front to back
LLayer := TBitmapLayer.Create(ImgView321.Layers);
ListBox1.Items.AddObject('First layer', LLayer);
// Could use LPanel := TPanel.Create(...); LPanel.Tag := Integer(Pointer(LLayer)) instead
LLayer := TBitmapLayer.Create(ImgView321.Layers);
ListBox1.Items.AddObject('Second layer', LLayer);
// Now the list is correct but the layers are not in the right order.
// Use the code listed below whenever you need to synchronize the layers
// with the list. In theory it may be slow (O(n^2)) but practically it
// won't matter much assuming you won't have hundreds of layers.
// Don't update the screen every time we move a layer to get closer to the final result
ImgView321.BeginUpdate;
try
for LIndex := 0 to ListBox1.Items.Count - 1 do
// Get the associated layer and make it the least visible of all processed so far
TCustomLayer(ListBox1.Items.Objects[LIndex]).SendToBack;
// Could use TCustomLayer(Pointer(SomePanel.Tag)).SendToBack instead
finally
// Always do this not to have strange behavior after an error
ImgView321.EndUpdate;
end;
// When it's done, update the screen
ImgView321.Changed;

Answer 2

By your description of how you changed the order of the layers, it is most likely the reason for your problem. Since you did not post that part of the code it can not be assessed with certainty.
Anyway, to rearrange the layers, you can use the Index property of TCustomLayer (of which TBitmapLayer is a descendant)

Answer 3

So the solution to the problem is to NOT use a Bitmap32List as a temporary container for png layers when reordering layers, because something gets lost in the process.
So try other solution for reordering. My curent solution is to drop the layers as PNG files to the disk, then reload them from the disk in the desired order.
Another solution (untested yet) would be to create a number of new layers, equal to the number of existing layers, move actual layers there, then get them back one by one in the desired order, and then remove the extra layers.
Anyway. That was the question, and this is the answer so far