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Date Posted: 06:57:01 03/27/00 Mon
Author: Michel
Subject: "Washed out" colors with palette images

This is a problem with the TResizeTransform.ApplyOnDest method, which caused a "washed out" look when resizing palettized images on NT with the HALFTONE stretch mode.

Here's the corrected implementation (sorry about the indentation, the forum does not maintain it):

procedure TResizeTransform.ApplyOnDest(
const Source : TDibGraphic;
const Dest : TDibGraphic );
pPoint : Windows.PPoint;
Radius : Single;
Filter : TInterpolationFilterFunc;
NewFormat : TImageFormat;
NewPalettePtr : PMaxLogPalette;

FLastPercent := -1;
FLastPercent := DoProgress(Self, FOnProgress, 0, 0, 100, FLastPercent);

if FOverrideDestFormat then
NewFormat := FDestFormat
NewFormat := Source.ImageFormat;

{ only re-use the source palette on 16 and 256 colors destinations }

{ MB Nov 28, 1999. When destination format is is 16 or 256 colors, set
NewPalettePtr to nil, to use a default palette }
if NewFormat = Source.ImageFormat then
NewPalettePtr := Source.PalettePtr
NewPalettePtr := nil;

Dest.NewImage( FWidth, FHeight, NewFormat, NewPalettePtr,
Source.XDotsPerInch, Source.YDotsPerInch);

if FInterpolated then
Filter := CFilters[FFilter].Filter;
Radius := CFilters[FFilter].Radius;

Interpolate(Source, Dest, Filter, Radius, Self);

DoProgress(Self, FOnProgress, 100, 0, 100, FLastPercent);
FLastPercent := DoProgress(Self, FOnProgress, 50, 0, 100, FLastPercent);

{ using HALFTONE stretch mode, the stretch quality is much better.
The API help on SetStretchBltMode specifies that SetBrushOrgEx
must be called afterwords.

MB Dec 27, 1999. If the graphic is very small (for example, icon
size of less), it is preferable to resize without half toning.

MB Feb 23, 2000. Under NT, using the Windows.HALFTONE stretch mode
with palettized images caused a "wash out" effect
if ((Source.Width * Source.Height)
ifTrueColor) then
SetStretchBltMode( Dest.DC, Windows.COLORONCOLOR )
SetStretchBltMode( Dest.DC, Windows.HALFTONE );

pPoint := nil;

SetBrushOrgEx( Dest.DC, 0, 0, pPoint );

StretchDiBits( Dest.DC, 0, 0, FWidth, FHeight,
0, 0, Source.Width, Source.Height,
Source.Bits, Windows.PBitmapInfo(@Source.BitmapInfo)^,

FLastPercent := DoProgress(Self, FOnProgress, 100, 0, 100, FLastPercent);

Registered users should replace the current implementation with this one.

For users evaluating the library, a new class derived from TResizeTransform overriding the ApplyOnDest method can be used to test the change.

Best regards,


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