Some notes about Pixmap

Tagged: pixmap

This topic contains 7 replies, has 5 voices, and was last updated by Gwenel 5 months, 2 weeks ago.

Viewing 8 posts - 1 through 8 (of 8 total)

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December 4, 2017 at 8:10 am #12101

nerobot

Participant

Working on ImageTrimmer app I found one interesting thing:

If you use pixmap with PremultiplyAlpha=True – when loading or after making some changes with pixels, then save pixmap – and you get pixels artefacts in saved version!

I wanted to save on extra loading of pixmap and my steps were:

Load pixmap with premultiply alpha
Create image using our pixmap
Processing pixmap pixels
Save pixmap into file
(I load and save PNG format)

Then I load just saved image and see artefacts in semi-transparent pixels.

Solution is:

To load an extra pixmap for processing without premultiply alpha.

See the GIF with artefacts – bordered version is wrong (don’t pay attention on red cross).

And see on the second image with sky blue background (I can’t remove incorrect attached file).

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December 4, 2017 at 12:05 pm #12108

abakobo

Participant

I’ve had problems saving the pixmap to png too. I think it uses some png compression by default thus artifacts. It’s probably just a flag under the hood though.

December 4, 2017 at 12:58 pm #12111

nerobot

Participant

@abakobo try to just resave pixmap without touching pixelFormat and alpha:

Local pix:=Pixmap.Load( srcPath ) 'format=none, calcAlpha=false
pix.Save( destPath )

1 2	Local pix:=Pixmap.Load( srcPath ) 'format=none, calcAlpha=false pix.Save( destPath )

it is an artefacts-free for me.

December 4, 2017 at 7:57 pm #12112

Mark Sibly

Keymaster

I think it uses some png compression by default thus artifacts.

PNG uses loseless compression – if there are artifacts it’s a bug (possibly mine or STBs).

December 5, 2017 at 3:49 am #12127

nerobot

Participant

mojo.Image uses Pixmap with premultiplied alpha.

If you’ll save such pixmap then you’ll get artifacts on next loading image – alpha will be premultiplied twice here.

I think this is the reason.

PNG loader itself works well.

December 5, 2017 at 4:24 am #12128

Simon Armstrong

Participant

According to the spec https://www.w3.org/TR/PNG-Rationale.html PNG should not contain premultiplied alpha pixels. Is it likely the SavePNG function needs to un-premultiply pixel data before saving?

December 5, 2017 at 4:31 pm #12146

abakobo

Participant

Can’t reproduce the artifacts I had at a time. All working good now! (not using alpha)

November 1, 2018 at 2:57 pm #15554

Gwenel

Participant

You can unpremultiply before saving. This is a test using the basic pixellformat (RGBA8 which I think that’s Monkey2’s default format).

Sorry if it is a bit more lengthy than it needs to be at the moment.

#Import "<std>"
#Import "<mojo>"
#Import "<sdl2>"
#import "<openal>"
Using std..
Using mojo..
Using sdl2..

Const w:=800, h:=600

Function Main()
	New AppInstance
	New Myapp(w,h)
	App.Run()
End

' improve png size

Class Myapp Extends Window

	Field counter:Int = 1
	Field pixmap:Pixmap
	Field image:Image
	
	Method New( title:String="Myapp",width:Int=w,height:Int=h,flags:WindowFlags=WindowFlags.Resizable )
		Super.New(title,w,h,flags)

		App.FileDropped+=Lambda(path:String)
			Print path

			pixmap = pixmap.Load(path,,False)
			If path.Slice(path.Length-4).ToLower()=".png"
				counter = counter + 1 
				path = path.Slice(0,path.Length-4) + counter + ".png"
				Print path
			Endif
	
			' A manual premultyplying routine.
			' If you don't wannat use the Premultiply flag in Pixmap.Load or pixmap.PremultiplyAlpha().
		
	Local p:=Cast<UInt Ptr>(pixmap.Data)
	Local w:Int = pixmap.Width
	Local x:Int,y:Int
	For y=0 Until h
		For x=0 Until w

			' Read the red, green, blue, and alpha from the pixmap
			Local r:UInt = (p[y*w+x] & $ff)
			Local g:UInt = (p[y*w+x] Shr 8 & $ff) 
			Local b:UInt = (p[y*w+x] Shr 16 & $ff) 
			Local a:UInt = (p[y*w+x] Shr 24 & $ff)
		
			' The magic
		 	Local rf:Float = r / 255.0
		 	Local gf:Float = g / 255.0
		 	Local bf:Float = b / 255.0
		 	Local af:Float = a / 255.0
			rf=rf*af ; gf=gf*af ; bf=bf*af

			' Now combine everything back into the pixmap			
			r=UInt(rf*255.0)
			g=UInt(gf*255.0)
			b=UInt(bf*255.0)
			a=UInt(af*255.0)
			Local mixed:UInt = a Shl 24
			mixed=mixed + (b Shl 16)
			mixed=mixed + (g Shl 8)
			mixed=mixed + r
			p[y*w+x] = mixed
		Next
	Next
' ----------------------------------------------------------------------------------------------------------------------------
			image = New Image(pixmap)
			Local savecanvas := New Canvas(image)
			Local savepixmap := savecanvas.CopyPixmap(savecanvas.Viewport)
		
			' Demultiply before saving
	
			p=Cast<UInt Ptr>(savepixmap.Data)
	 w = savepixmap.Width
	For y=0 Until h
		For x=0 Until w
			' Read the red, green, blue, and alpha from the pixmap
			Local r:UInt = (p[y*w+x] & $ff)
			Local g:UInt = (p[y*w+x] Shr 8 & $ff) 
			Local b:UInt = (p[y*w+x] Shr 16 & $ff) 
			Local a:UInt = (p[y*w+x] Shr 24 & $ff)
		
			' The magic
		 	Local rf:Float = r / 255.0
		 	Local gf:Float = g / 255.0
		 	Local bf:Float = b / 255.0
		 	Local af:Float = a / 255.0
			If af>0 Then rf=rf/af ; gf=gf/af ; bf=bf/af

			' Combine everything into the pixmap			
			r=UInt(rf*255.0)
			g=UInt(gf*255.0)
			b=UInt(bf*255.0)
			a=UInt(af*255.0)
			Local mixed:UInt = a Shl 24
			mixed=mixed + (b Shl 16)
			mixed=mixed + (g Shl 8)
			mixed=mixed + r
			p[y*w+x] = mixed
		Next
	Next
			savepixmap.Save(path+"")	
		End
		
	End

	Method OnRender(canvas:Canvas) Override
		App.RequestRender()
		canvas.TextureFilteringEnabled = False
		If image 
		canvas.Color = Color.Blue
		canvas.DrawOval(200,200,100,100)
		canvas.Color = Color.White
			canvas.DrawImage(image,0,0)
			canvas.DrawImage(image,100,100)
			Endif
	End		
End

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#Import "<std>"

#Import "<mojo>"

#Import "<sdl2>"

#import "<openal>"

Using std..

Using mojo..

Using sdl2..

Const w:=800, h:=600

Function Main()

New AppInstance

New Myapp(w,h)

App.Run()

End

' improve png size

Class Myapp Extends Window

Field counter:Int = 1

Field pixmap:Pixmap

Field image:Image

Method New( title:String="Myapp",width:Int=w,height:Int=h,flags:WindowFlags=WindowFlags.Resizable )

Super.New(title,w,h,flags)

App.FileDropped+=Lambda(path:String)

Print path

pixmap = pixmap.Load(path,,False)

If path.Slice(path.Length-4).ToLower()=".png"

counter = counter + 1

path = path.Slice(0,path.Length-4) + counter + ".png"

Print path

Endif

' A manual premultyplying routine.

' If you don't wannat use the Premultiply flag in Pixmap.Load or pixmap.PremultiplyAlpha().

Local p:=Cast<UInt Ptr>(pixmap.Data)

Local w:Int = pixmap.Width

Local x:Int,y:Int

For y=0 Until h

For x=0 Until w

' Read the red, green, blue, and alpha from the pixmap

Local r:UInt = (p[y*w+x] & $ff)

Local g:UInt = (p[y*w+x] Shr 8 & $ff)

Local b:UInt = (p[y*w+x] Shr 16 & $ff)

Local a:UInt = (p[y*w+x] Shr 24 & $ff)

' The magic

Local rf:Float = r / 255.0

Local gf:Float = g / 255.0

Local bf:Float = b / 255.0

Local af:Float = a / 255.0

rf=rf*af ; gf=gf*af ; bf=bf*af

' Now combine everything back into the pixmap

r=UInt(rf*255.0)

g=UInt(gf*255.0)

b=UInt(bf*255.0)

a=UInt(af*255.0)

Local mixed:UInt = a Shl 24

mixed=mixed + (b Shl 16)

mixed=mixed + (g Shl 8)

mixed=mixed + r

p[y*w+x] = mixed

' ----------------------------------------------------------------------------------------------------------------------------

image = New Image(pixmap)

Local savecanvas := New Canvas(image)

Local savepixmap := savecanvas.CopyPixmap(savecanvas.Viewport)

' Demultiply before saving

p=Cast<UInt Ptr>(savepixmap.Data)

w = savepixmap.Width

For y=0 Until h

For x=0 Until w

' Read the red, green, blue, and alpha from the pixmap

Local r:UInt = (p[y*w+x] & $ff)

Local g:UInt = (p[y*w+x] Shr 8 & $ff)

Local b:UInt = (p[y*w+x] Shr 16 & $ff)

Local a:UInt = (p[y*w+x] Shr 24 & $ff)

' The magic

Local rf:Float = r / 255.0

Local gf:Float = g / 255.0

Local bf:Float = b / 255.0

Local af:Float = a / 255.0

If af>0 Then rf=rf/af ; gf=gf/af ; bf=bf/af

' Combine everything into the pixmap

r=UInt(rf*255.0)

g=UInt(gf*255.0)

b=UInt(bf*255.0)

a=UInt(af*255.0)

Local mixed:UInt = a Shl 24

mixed=mixed + (b Shl 16)

mixed=mixed + (g Shl 8)

mixed=mixed + r

p[y*w+x] = mixed

savepixmap.Save(path+"")

End

Method OnRender(canvas:Canvas) Override

App.RequestRender()

canvas.TextureFilteringEnabled = False

If image

canvas.Color = Color.Blue

canvas.DrawOval(200,200,100,100)

canvas.Color = Color.White

canvas.DrawImage(image,0,0)

canvas.DrawImage(image,100,100)

Endif

End

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Some notes about Pixmap

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