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Per Pixel Alpha Blend

, 20 Apr 2001
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Demonstrates how to create per pixel alpha blending windows
  • Download source code with sample project - 188 Kb
  • Download ImgDecoder project - 14 Kb
  • Download some test images - 108 Kb
  • Introduction

    In this article I will give you some source code to create per pixel alpha blending windows.

    First, some images:

    Nice, heim?

    For creating a per-pixel window we are going to use:

    • One 24bit Bitmap with alpha channel.
    • The GDI function, UpdateLayeredWindow.

    24bit Bitmap with alpha channel

    The most tricky part of creating the bitmap is finding a simple file format that has an embedded alpha channel.

    The selected file format for the first incarnation of this article was PSP (Paint Shop Pro™ from Jasc®), but now the recommended file format has been changed to PNG (Portable Network Graphics).

    For loading PNG files I use the paintlib library. This library can also load files from PCX, TGA and TIFF formats.

    This demo application does not use paintlib directly; instead it uses a paintlib wrapper dll, ImgDecoder.Dll.

    For loading PSP files I've created a function called LoadPSPFileToDIB(PSPLoader.h/cpp).


    To use this function we must have a top-level window (in this sample this is implemented by the class CDummyDialog) with the extended style WS_EX_LAYERED set.

    Before we can pass the bitmap to the UpdateLayeredWindow function, we must pre-multiply the rgb channels with the alpha channel. For this you just have to call the PreMultiplyRGBChannels function (take a look at CPerpixelalphaDlg::DoLoadFile).

    From here on read the source code in CPerpixelalphaDlg::DoUpdateDummyDialog.

    Installing & Building

    1. Make sure you have the SDK for Windows® NT 5.0.
    2. Download and extract the source code of this article.
    3. Build and enjoy!

    If you want to build the ImgDecoder.Dll you also have to,

    1. Download paintlib and all the required libraries.
    2. Download the ImgDecoder project.
    3. Extract the ImgDecoder project to the paintlib library.
    4. Build the ImgDecoder project.
    5. Copy the resulting ImgDecoder.dll and ImgDecoder.lib to the bin directory of this article's demo application.


    1. You must mention my name in your application documentation/help box, something like, Portions © 2001 by Rui Godinho Lopes <>.
    2. I would appreciate it if you send me an email letting me know that you are using this code.
    3. I cannot be held responsible of any kind of data loss caused directly or indirectly by the use of this code - use it at your own risk.
    4. You cannot sell this code - this is free software.

    Don't forget if you have any constructive thoughts just email me!


    This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

    A list of licenses authors might use can be found here


    About the Author

    Rui Lopes
    Web Developer
    Portugal Portugal
    No Biography provided

    Comments and Discussions

    QuestionHow can i do this in VB.NET ??? Pinmember3AgL3 DeeJay14-Jun-14 9:29 
    GeneralMy vote of 5 Pinmemberxueweizhong24-May-13 1:03 
    Generalbeginner code... Pinmemberkilt12-Oct-09 6:39 
    GeneralRe: beginner code... PinmemberApuhjee23-Oct-09 5:24 
    Generalgreat tutorial ! some questions about this still Pinmembernirishere5-Sep-09 1:21 
    QuestionHow about clean WinAPI? Pinmemberflaviusv12-Oct-08 11:16 
    AnswerRe: How about clean WinAPI? PinmemberDavid_LoveCpp13-May-10 17:22 
    QuestionHow can I draw text on the alpha window? PinmemberGreg Ellis24-Aug-08 11:04 
    AnswerRe: How can I draw text on the alpha window? PinmemberGreg Ellis27-Aug-08 6:41 
    QuestionHow to use control in Layered windows Pinmemberpherosiden29-Oct-07 19:29 
    QuestionUrgent (UNICODE) ! PinmemberMutant X1233-Sep-07 21:29 
    GeneralTrouble with headers PinmemberDelfistyaosani28-Jun-07 21:52 
    GeneralRe: Trouble with headers Pinmemberkr.kim10-May-09 22:35 
    GeneralRe: Trouble with headers PinmemberDelfistyaosani11-May-09 4:29 
    GeneralRe: Trouble with headers Pinmemberkr.kim11-May-09 8:29 
    Generalundeclared identifiers PinmemberDV6413-Apr-07 9:09 
    GeneralMore Test Images Pinmemberninadpachpute12-Feb-07 23:05 
    JokeGR8 STUFF!!! PinmemberDieZeL26-Jul-06 0:09 
    QuestionHow to add control to the dialog? Pinmemberblackgole20-Jul-06 16:47 
    GeneralEfficency of layered windows Pinmemberjens pedersen9-Jun-06 4:40 
    QuestionWriting Text PinmemberMrBlob31-May-06 15:29 
    AnswerRe: Writing Text PinmemberManni Singh28-Mar-07 3:02 
    GeneralRe: Writing Text PinmemberGreg Ellis27-Aug-08 10:43 
    QuestionUpdateLayeredWindow Pinmembertoto777toto9-Apr-06 17:14 
    AnswerRe: UpdateLayeredWindow Pinmemberamit dedha28-Sep-06 7:42 

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