Bug? Canvas Snapshot and transparency

Jim Brown · Jun 13, 2014

Hi Erel,
I don't really know if I should class this as a bug. I want to snapshot a canvas and retain the alpha/transparent background. Unfortunately I can't seem to capture the transparency from the Snapshot option

Here's an example. The canvas on the left just demonstrates that the image (balloons) has a transparent background. If I draw the balloons (half size) to a new canvas and snapshot it, then draw the results over to the right side canvas you can seen the balloons background is now a solid white.

BEFORE AND AFTER pressing "Snapshot"

I have attached a B4J demonstration sample. I have tried filling the new canvas with a transparent rect and also using the style "-fx-background-color: transparent" but no luck.

Thanks

Daestrum · Jun 14, 2014

Not sure if this will help, but was playing around when writing animation library, and had this code.
It doesn't use the B4J node snapshot function.

B4X:

    g.InitializeNewInstance("javafx.scene.SnapshotParameters",Null) ' javaobject
    Dim p As Paint = fx.Colors.Transparent
    g.RunMethod("setFill",Array As Object(p))
    Dim jot As JavaObject
    jot = imageview1 ' contains png with trans background
    im = jot.RunMethod("snapshot",Array As Object(g,Null)) ' im = Image
    Log("wdth: "& im.Width & "ht: "& im.Height)
    canvas1.DrawImage(im,0,0,100,100) ' canvas1 = canvas

And the canvas had Blend Mode set to SRC_OVER

Just tried your code with a few modifications and it gives what you were after.
All I changed was in your button sub.

B4X:

Sub btn_Snapshot_Action
    Dim c As Canvas
    c.Initialize("")
    c.Width=Balloons.Width/2 : c.Height=Balloons.Height/2
    c.Style="-fx-background-color: transparent"
    c.DrawRect(0,0,c.Width,c.Height,fx.Colors.Transparent,True,0)
    c.DrawImage(Balloons,0,0,Balloons.Width/2,Balloons.Height/2)
'    Dim img As Image = c.Snapshot
'    can2.DrawImage(checkeredImage,0,0,pane1.PrefWidth,pane1.PrefHeight)
'    can2.DrawImage(img,20,20,img.Width,img.Height)
    Dim g As JavaObject
    g.InitializeNewInstance("javafx.scene.SnapshotParameters",Null) ' javaobject
    Dim p As Paint = fx.Colors.Transparent
    g.RunMethod("setFill",Array As Object(p))
    Dim jot As JavaObject
    jot = c ' contains png with trans background
    Dim im As Image
    im = jot.RunMethod("snapshot",Array As Object(g,Null)) ' im = Image
    Log("wdth: "& im.Width & "ht: "& im.Height)
    can2.DrawImage(im,0,0,im.Width/2,im.Height/2) ' canvas1 = canvas
End Sub

Will leave it to you to make the code nicer ;-)

Jim Brown · Jun 14, 2014

Hi Daestrum,
Thanks for taking the time out address my issue. Your solution is perfect.

hookshy · Oct 8, 2014

the below statement does not have any effect on the background transparency
just take snapshot using java object as described by Daestrum

B4X:

  c.Style="-fx-background-color: transparent"
c.DrawRect(0,0,c.Width,c.Height,fx.Colors.Transparent,True,0)

B4X:

Sub snapshot(c As Canvas) As Image

  Dim g As JavaObject
    g.InitializeNewInstance("javafx.scene.SnapshotParameters",Null) ' javaobject
    Dim p As Paint = fx.Colors.Transparent
    g.RunMethod("setFill",Array As Object(p))
   
    Dim jot As JavaObject
    jot = c ' contains png with trans background
   
    Return jot.RunMethod("snapshot",Array As Object(g,Null))

End Sub

sorex · Jul 20, 2015

I bumped at the same issue.

Thanks @Daestrum & @hookshy for the fix.

@Erel : Is this a bug in your core libs or a limitation of the default java core?

It would be nice if the above sub was added to the core libs but as it requires the javaobjects lib I guess that's hard to do.

Erel · Jul 21, 2015

The default background for snapshots is white.
I will add another method that will allow you to set a different background (like transparent).

Bug? Canvas Snapshot and transparency

Jim Brown

Active Member

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Daestrum

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Jim Brown

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hookshy

Well-Known Member

sorex

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Erel

B4X founder

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Bug? Canvas Snapshot and transparency

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