Large memory reductions when working on multiple images simultaneously

When loading a new image, PD will now automatically suspend inactive images to compressed buffers.  This is a very fast operation (lz4 is used) and for "normal" image sizes - e.g. < 20 megapixels - any delays caused by the suspend process are basically unnoticeable.  On larger images there may be a brief stutter when switching rapidly between loaded images, but the given the huge memory reductions, I think this is a worthwhile modification.

In the future it may be nice to have an option to go even further and suspend-to-disk if memory is constrained, but for now, compressed in-memory buffers provide a fast, simple way to cut memory usage when working with multiple side-by-side images.  (I may look at extending this auto-suspend feature to other places, for example prior to initializing demanding tools.)

Classes/pdDIB.cls

@@ -1128,7 +1128,7 @@ Private Sub UnsuspendDIB()
                 'A lot of different strategies are possible here, but for now, let's keep the suspend
                 ' buffer intact for small images.  These are likely to suspend/unsuspend many times
                 ' as part of UI rendering, and the cost of keeping the suspend buffer intact is less
-                ' than the cost of constant small re-allocations
+                ' than the cost of constant small re-allocations.
                 If (m_SizeSuspended > 16384) Then Erase m_SuspensionBuffer
 
                 m_SizeSuspended = 0

Classes/pdImage.cls

@@ -15,8 +15,8 @@ Attribute VB_Exposed = False
 'PhotoDemon Image class
 'Copyright 2006-2021 by Tanner Helland
 'Created: sometime 2006
-'Last updated: 11/January/21
-'Last update: harden against potential errors when rapidly creating/deleting layers on-the-fly
+'Last updated: 24/October/21
+'Last update: new Suspend option, for reducing memory usage when a pdImage isn't active
 '
 'The pdImage class is used to store information on each image loaded by the user.
 ' One instance of this class exists for each loaded image.
@@ -739,6 +739,22 @@ Friend Sub ReverseLayerOrder()
 
 End Sub
 
+'Non-destructively suspend as many image resources as we can to either disk or memory.
+Friend Sub SuspendImage()
+
+    'Start by suspending all layers
+    Dim i As Long
+    For i = 0 To UBound(imgLayers)
+        If (Not imgLayers(i) Is Nothing) Then imgLayers(i).SuspendLayer True
+    Next i
+
+    'Suspend any other internal DIBs
+    If (Not m_ImageThumbnail Is Nothing) Then m_ImageThumbnail.SuspendDIB cf_Lz4, False
+    If (Not CompositeBuffer Is Nothing) Then CompositeBuffer.SuspendDIB cf_Lz4, False
+    If (Not CanvasBuffer Is Nothing) Then CanvasBuffer.SuspendDIB cf_Lz4, False
+
+End Sub
+
 'Used by various layer movement functions.  Given two layer indices, swap them.
 Friend Sub SwapTwoLayers(ByVal srcLayerIndex_1 As Long, ByVal srcLayerIndex_2 As Long)
 
@@ -1166,14 +1182,21 @@ End Function
 'When the attached image object is deactivated (e.g. the user leaves it loaded, but switches to a
 ' different image), you can call this function to free up some internal memory caches and other objects.
 ' This can free a non-trivial amount of resources when system memory is tight.
-Friend Sub DeactivateImage()
+'
+'For even more resource reduction, ask for a full layer suspension too - this will ask all child layers
+' to suspend their DIBs to compressed buffers.
+Friend Sub DeactivateImage(Optional ByVal suspendLayersToo As Boolean = True)
 
     'Scratch layers aren't needed unless the image is actively being edited
     Set ScratchLayer = Nothing
 
     'Some internal selection caches are free-able
     If (Not MainSelection Is Nothing) Then MainSelection.FreeNonEssentialResources
-
+
+    'Layer suspension is the biggest gain, but it also has a perf penalty so the caller may
+    ' choose to avoid this.
+    If suspendLayersToo Then Me.SuspendImage
+
 End Sub
 
 'When the attached image object is closed, we can deactivate a ton of items to save on resources.

Classes/pdLayer.cls

@@ -2297,6 +2297,26 @@ Friend Sub ResetLayerParameters()
 
 End Sub
 
+'Non-destructively suspend as many layer resources as we can to either disk or memory.
+Friend Sub SuspendLayer(Optional ByVal suspendViewportToo As Boolean = False)
+
+    'Layer DIB is the most obvious target for memory reduction
+    If (Not layerDIB Is Nothing) Then layerDIB.SuspendDIB cf_Lz4, False
+
+    'Thumbnail is an easy target too (since it's small, suspending is roughly instantaneous)
+    If (Not m_LayerThumbnail Is Nothing) Then m_LayerThumbnail.SuspendDIB cf_Lz4, False
+
+    'Finally, lodDIBs can be suspended too, but be careful with these because there are meaningful
+    ' perf implications for viewport rendering if these have to be swapped back in frequently.
+    If suspendViewportToo Then
+        Dim i As Long
+        For i = LBound(lodDIB) To UBound(lodDIB)
+            If (Not lodDIB(i) Is Nothing) Then lodDIB(i).SuspendDIB cf_Lz4, False
+        Next i
+    End If
+
+End Sub
+
 'If something needs to access the layer DIB directly (like PD's central compositor), this function should first be called for
 ' any vector layers.  If the DIB is already synched, there is no penalty to calling this function, but if it is *not* synched,
 ' this will guarantee the DIB is ready for access.

Modules/CanvasManager.bas

@@ -459,7 +459,8 @@ Public Sub ActivatePDImage(ByVal imageID As Long, Optional ByRef reasonForActiva
     If activeImageChanging Then
 
         'Release some temporary resources on the old image, if we can
-        PDImages.GetActiveImage.DeactivateImage
+        PDImages.GetActiveImage.DeactivateImage True
+        UIImages.FreeSharedCompressBuffer
 
         'Update the current form variable
         PDImages.SetActiveImageID imageID

Modules/PDImages.bas

@@ -287,3 +287,13 @@ End Sub
 Public Sub SetActiveImageID(ByVal newID As Long)
     m_CurrentImageID = newID
 End Sub
+
+'Given an ID in the pdImages collection, suspend all layers (and other high-memory objects) inside that image.
+' This is a non-destructive process.  Necessary resources will be automatically unsuspended when those resources
+' are requested.  Note, however, that like any large-scale data transfer, there is a perf cost associated with
+' doing this.  Typically, suspending an image should be saved for things like switching to a new active image
+' (and the old image gets suspended), or opening a high-resource-usage tool and the background image won't be
+' accessible for awhile.
+Public Sub SuspendImage(ByVal imgID As Long)
+    'if pdimages.IsImageNonNull(imgid) then
+End Sub

Modules/UIImages.bas

@@ -63,6 +63,14 @@ Public Function AddImage(ByRef srcDIB As pdDIB, ByRef uniqueImageName As String)
 
 End Function
 
+'Free the shared compression buffer for UI images.  This carries a ripple effect for PD's internal
+' suspend-to-memory operations, so do this only if the memory savings are large.
+Public Sub FreeSharedCompressBuffer()
+    PDDebug.LogAction "Freeing shared memory buffer (size " & Files.GetFormattedFileSize(m_CompressBufferSize) & ")"
+    Erase m_TempCompressBuffer
+    m_CompressBufferSize = 0
+End Sub
+
 'Return a standalone DIB of a given sprite.  Do *not* use this more than absolutely necessary,
 ' as it is expensive to initialize sprites (and it sort of defeats the purpose of using a
 ' sprite sheet in the first place!)

PhotoDemon.vbp

@@ -490,7 +490,7 @@ Description="PhotoDemon Photo Editor"
 CompatibleMode="0"
 MajorVer=8
 MinorVer=9
-RevisionVer=959
+RevisionVer=962
 AutoIncrementVer=1
 ServerSupportFiles=0
 VersionComments="Copyright 2000-2021 Tanner Helland - photodemon.org"