0022245: [Patch] fcl-image, interpolation, wrong initialization of alpha channel

View Issue Details [ Jump to Notes ]

[ Issue History ] [ Print ]

Project

Category

View Status

Date Submitted

Last Update

0022245

FPC

FCL

public

2012-06-10 13:58

2012-06-18 11:02

Reporter

Bernd Kreuss

Assigned To

Marco van de Voort

Priority

normal

Severity

minor

Reproducibility

always

Status

resolved

Resolution

fixed

Platform

all

OS Version

all

Product Version

2.6.1

Product Build

Target Version

Fixed in Version

Summary

0022245: [Patch] fcl-image, interpolation, wrong initialization of alpha channel

Description

There is a small bug in packages/fcl-image/src/fpinterpolation.inc that essentially makes it impossible to use TFPImageCanvas.StretchDraw() with images that have alpha transparency.

Attached is a patch.

When it calculates the interpolated value of the new pixel it will first initialize this pixel with colBlack. This is wrong.

The intention is to initialize all 4 channels of the pixel with zero but colBlack only has the RGB values set to zero, alpha is set to $ffff. The correct value would have been colTransparent which has them all four set to zero. The attached patch will fix it.

Without the patch any attempt to use StretchDraw() on an image with alpha transparency would end up with most of the resulting alpha values in the resulting image set to $ffff.

I am also attaching a small demo program (interpoltest.lpr), there is also a small PNG file with alpha channel in this archive for convenience, running the program should scale the image 'original.png' to 64*64 and save it to 'scaled.png' where you can then examine it with gimp to observe how the alpha channel gets messeed up during this operation.

The patch was made against the current 2.6 fixes branch, I have not yet tested 2.7 trunk but most likely it is there also.

(also maybe after it is fixed it would be nice to add a small example how to scale images to the fcl-image examples, it took me a while to figure out that it actually even *has* the capability to do this)

Steps To Reproduce

To reproduce it unzip and compile the attached demo in the attached interpoltest.tar.bz2 archive. Run the program, it will create an image 'scaled.png', observe how the alpha channel in this image was corrupted during scaling.

Tags

fcl-image

FPCOldBugId

Fixed in Revision

21639

Attached Files

interpol-must-be-all-zero.patch [^] (755 bytes) 2012-06-10 13:58 [Show Content] [Hide Content]

Index: packages/fcl-image/src/fpinterpolation.inc
===================================================================
--- packages/fcl-image/src/fpinterpolation.inc	(Revision 21487)
+++ packages/fcl-image/src/fpinterpolation.inc	(Arbeitskopie)
@@ -179,7 +179,7 @@
         begin
           sx:=PInteger(xEntry)^;
           inc(xEntry,SizeOf(integer));
-          NewCol:=colBlack;
+          NewCol:= colTransparent;
           for cx:=0 to xSupport-1 do
           begin
             f:=PSingle(xEntry)^;
@@ -198,7 +198,7 @@
       for dx:=0 to w-1 do
       begin
         CurEntry:=yEntry+SizeOf(integer);
-        NewCol:=colBlack;
+        NewCol:=colTransparent;
         for sy:=0 to ySupport-1 do
         begin
           f:=PSingle(CurEntry)^;

interpoltest.tar.bz2 [^] (18,982 bytes) 2012-06-10 13:59

Relationships

Notes
(0060447) Bernd Kreuss (reporter) 2012-06-11 18:39	btw, the attached demo has a memory leak, I forgot to free the canvas object but this does not impact its function.

(0060606) Marco van de Voort (manager) 2012-06-18 11:02	Applied, committed test as examples/interpoldemo (no picture though, to keep svn lean)

(0060607) Marco van de Voort (manager) 2012-06-18 11:02 edited on: 2012-06-18 11:02	Note that I expect 0011217 to be something similar.

Issue History
Date Modified	Username	Field	Change
2012-06-10 13:58	Bernd Kreuss	New Issue
2012-06-10 13:58	Bernd Kreuss	File Added: interpol-must-be-all-zero.patch
2012-06-10 13:59	Bernd Kreuss	File Added: interpoltest.tar.bz2
2012-06-11 15:48	Marco van de Voort	Tag Attached: fcl-image
2012-06-11 18:39	Bernd Kreuss	Note Added: 0060447
2012-06-18 11:02	Marco van de Voort	Fixed in Revision	=> 21639
2012-06-18 11:02	Marco van de Voort	Status	new => resolved
2012-06-18 11:02	Marco van de Voort	Resolution	open => fixed
2012-06-18 11:02	Marco van de Voort	Assigned To	=> Marco van de Voort
2012-06-18 11:02	Marco van de Voort	Note Added: 0060606
2012-06-18 11:02	Marco van de Voort	Note Added: 0060607
2012-06-18 11:02	Marco van de Voort	Note Edited: 0060607

freepascal bugtracker

Open source compiler for Pascal and Object Pascal