From ddec70d99dce23d877837a970b08d613474e38b3 Mon Sep 17 00:00:00 2001
From: Martin Dias <tinchodias@gmail.com>
Date: Thu, 21 Sep 2023 02:39:29 -0300
Subject: [PATCH] Fix asFormARGB32, which wasn't converting from premultiplied
 alpha, as Form requires.

Fixes #35.
Includes 2 new tests
---
 .../AeCairoImageSurfaceTest.class.st          | 58 +++++++++++++++++++
 .../AeCairoImageSurface.class.st              | 28 +++++----
 2 files changed, 76 insertions(+), 10 deletions(-)

diff --git a/src/Alexandrie-Cairo-Tests/AeCairoImageSurfaceTest.class.st b/src/Alexandrie-Cairo-Tests/AeCairoImageSurfaceTest.class.st
index 4135f7a..75fff07 100644
--- a/src/Alexandrie-Cairo-Tests/AeCairoImageSurfaceTest.class.st
+++ b/src/Alexandrie-Cairo-Tests/AeCairoImageSurfaceTest.class.st
@@ -7,6 +7,64 @@ Class {
 	#category : #'Alexandrie-Cairo-Tests'
 }
 
+{ #category : #tests }
+AeCairoImageSurfaceTest >> testConvertRoundTrip2Translucent [
+	"The challenge in this test is that Cairo uses premultiplied-alpha on it's argb32 format, and Form straight-alpha."
+
+	| aColor aSurface aForm aSurfaceBytes anotherSurface anotherSurfaceBytes |
+	aColor := Color red alpha: 0.4.
+
+	aSurface := AeCairoImageSurface
+		extent: 1@1
+		format: AeCairoSurfaceFormat argb32.
+	aSurface newContext
+		sourceColor: aColor;
+		paint.
+
+	aForm := aSurface asFormARGB32.
+	self
+		assert: (aForm colorAt: 0@0)
+		equals: aColor.
+
+	"Take the byte array as a reference"
+	aSurface flush.
+	aSurfaceBytes := FFIExternalArray
+		fromHandle: aSurface data
+		type: FFIUInt8
+		size: aSurface byteSize.
+
+	anotherSurface := AeCairoImageSurface fromForm: aForm.
+	anotherSurface flush.
+	anotherSurfaceBytes := FFIExternalArray
+		fromHandle: anotherSurface data
+		type: FFIUInt8
+		size: anotherSurface byteSize.
+
+	"The bytes of the reconverted surface should match"
+	self
+		assert: anotherSurfaceBytes
+		equals: aSurfaceBytes
+]
+
+{ #category : #tests }
+AeCairoImageSurfaceTest >> testConvertRoundTripTranslucent [
+	"The challenge in this test is that Cairo uses premultiplied-alpha on it's argb32 format, and Form straight-alpha."
+
+	| aColor aSurface aForm anotherForm |
+	aColor := Color red alpha: 0.4.
+	aForm := Form extent: 100@100  depth: 32.
+	aForm fillColor: aColor.
+	self
+		assert: (aForm colorAt: 0@0)
+		equals: aColor.
+
+	aSurface := AeCairoImageSurface fromForm: aForm.
+	anotherForm := aSurface asForm.
+	self
+		assert: (anotherForm colorAt: 0@0)
+		equals: aColor
+]
+
 { #category : #tests }
 AeCairoImageSurfaceTest >> testConvertZeroWidthAsFormARGB32 [
 
diff --git a/src/Alexandrie-Cairo/AeCairoImageSurface.class.st b/src/Alexandrie-Cairo/AeCairoImageSurface.class.st
index d077dc3..b0ddf9a 100644
--- a/src/Alexandrie-Cairo/AeCairoImageSurface.class.st
+++ b/src/Alexandrie-Cairo/AeCairoImageSurface.class.st
@@ -218,32 +218,40 @@ AeCairoImageSurface >> asFormA8 [
 
 { #category : #converting }
 AeCairoImageSurface >> asFormARGB32 [
-	"Answer myself converted as a 32-bits Form. Loseless conversion.
+	"Answer myself converted as a 32-bits Form, which means converting pixels from premultiplied-alpha to straight-alpha.
 
 	Assumes:
 		self format = AeCairoSurfaceFormat CAIRO_FORMAT_ARGB32
 	"
 
-	| uint32Size theData |
-	self flush.
-	
-	uint32Size := self byteSize.
+	| aSizeInWords theData |
+	aSizeInWords := self byteSize >> 2.
 	
-	uint32Size = 0 ifTrue: [ 
+	aSizeInWords = 0 ifTrue: [ 
 		^ Form
 			extent: self extent
 			depth: 32 ].
 	
+	self flush.
+
 	theData := FFIExternalArray
-		fromHandle: self data getHandle
+		fromHandle: self data
 		type: FFIUInt32
-		size: uint32Size / 4.
+		size: aSizeInWords.
 
 	^ Form
 		extent: self extent
 		depth: 32
-		bits: (Bitmap newFrom: theData)
-
+		bits: (Bitmap new: aSizeInWords streamContents: [ :stream |
+			theData do: [ :pixel |
+				| alphaByte alpha a r g b |
+				alphaByte := pixel >> 24.
+				alpha := alphaByte / 255.0.
+				a := pixel bitAnd: 16rFF000000.
+				r := ( (pixel       bitAnd: 255) / alpha ) asInteger.
+				g := ( (pixel >> 8  bitAnd: 255) / alpha ) asInteger << 8.
+				b := ( (pixel >> 16 bitAnd: 255) / alpha ) asInteger << 16.
+				stream nextPut: a + r + g + b ] ])
 ]
 
 { #category : #API }