Mega Code Archive
How to create JPEG image thumbnails programmatically
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Title: How to create JPEG image thumbnails programmatically
Question: This article shows how to create thumbnails of JPEG images programmatically. Scaling is not good, because it just displays stretched image, leaving actual image intact.
Answer:
Sometimes you need to create thumbnails of JPEG images. Using Scale property of TJpegImage class is not good, because it actually does not change the image itself, it just displays scaled image.
Following unit can be used to create thumbnails programmatically. it does not do any drawing, so it can be used as in GUI, as in console, CGI and ISAPI applications. It can be very useful in web applications. If you have images in the database, you can create CGI or ISAPI app, which seds to the browser thumbnail, not actual image.
Below is a source code of the unit called JpegConv.pas. Full unit, and a demo application, can be downloaded by using following direct link:
http://www.argosoft.com/files/misc/thumbs.zip
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unit JpegConv;
interface
uses Windows, Graphics, SysUtils, Classes;
procedure CreateThumbnail(InStream, OutStream: TStream;
Width, Height: Integer; FillColor: TColor=clWhite); overload;
procedure CreateThumbnail(const InFileName, OutFileName: string;
Width, Height: Integer; FillColor: TColor=clWhite); overload;
implementation
uses Jpeg;
procedure CreateThumbnail(InStream, OutStream: TStream;
Width, Height: Integer; FillColor: TColor=clWhite);
var
JpegImage: TJpegImage;
Bitmap: TBitmap;
Ratio: Double;
ARect: TRect;
AHeight, AHeightOffset: Integer;
AWidth, AWidthOffset: Integer;
begin
// Check for invalid parameters
if Width raise Exception.Create('Invalid Width');
if Height raise Exception.Create('Invalid Height');
JpegImage:=TJpegImage.Create;
try
// Load the image
JpegImage.LoadFromStream(InStream);
// Create bitmap, and calculate parameters
Bitmap:=TBitmap.Create;
try
Ratio:=JpegImage.Width/JpegImage.Height;
if Ratio1 then
begin
AHeight:=Round(Width/Ratio);
AHeightOffset:=(Height-AHeight) div 2;
AWidth:=Width;
AWidthOffset:=0;
end
else
begin
AWidth:=Round(Height*Ratio);
AWidthOffset:=(Width-AWidth) div 2;
AHeight:=Height;
AHeightOffset:=0;
end;
Bitmap.Width:=Width;
Bitmap.Height:=Height;
Bitmap.Canvas.Brush.Color:=FillColor;
Bitmap.Canvas.FillRect(Rect(0,0,Width,Height));
// StretchDraw original image
ARect:=Rect(AWidthOffset,AHeightOffset,AWidth+AWidthOffset,AHeight+AHeightOffset);
Bitmap.Canvas.StretchDraw(ARect,JpegImage);
// Assign back to the Jpeg, and save to the file
JpegImage.Assign(Bitmap);
JpegImage.SaveToStream(OutStream);
finally
Bitmap.Free;
end;
finally
JpegImage.Free;
end;
end;
procedure CreateThumbnail(const InFileName, OutFileName: string;
Width, Height: Integer; FillColor: TColor=clWhite); overload;
var
InStream, OutStream: TFileStream;
begin
InStream:=TFileStream.Create(InFileName,fmOpenRead);
try
OutStream:=TFileStream.Create(OutFileName,fmOpenWrite or fmCreate);
try
CreateThumbnail(InStream,OutStream,Width,Height,FillColor);
finally
OutStream.Free;
end;
finally
InStream.Free;
end;
end;
end.
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