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Fast 2:1 Image Shrink (Scale Down)

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16 Nov 2022CPOL3 min read 33.4K   1.2K   16   43
Fast implementation of 2:1 image shrink (full image or part of it)
This article includes a fast implementation of 2:1 image shrink algorithm.

Image 1


This article offers a simple and fast, practical implementation of 2:1 image shrink algorithm.
2:1 image shrink is used in very specific cases, but it does a nice job.


Image processing is always an expensive job. When one needs to show a scaled picture, it is always better to use GPU for rendering (OpenGL, DirectX). But using GPU is not always possible and it is good to have an alternative.

The high-quality image resize (enlarge/shrink) is extremely CPU demanding, especially on large images. In addition, sometimes it is required to do the processing multiple times. To relieve this, there are different tricks that could be applied. If a picture has to be shrunk (scaled down), a trick that is similar to mip-maps could be applied. Such case is if we want to shrink a picture more than twice (destination size is less than half of the original), we could speed up the whole process by applying half-shrink prior to actual scale. 2:1 scale-down is very fast, due to simplest algorithm that uses less computations.

For example, if we have image 1920x1024 and need to shrink it to 500x281, we have to execute code like this:

shrink( imageDst, imageSrc )

But we could optimize this by pre-shrink 2:1 like this:

shrinkHalf( imageHalf, imageSrc );
shrink( imaheDst, imageHalf );

In one of my projects, when I had to scale the image of 3 monitors to generate a preview, the result was:

Scale: 46 ms
ShrinkHalf + Scale: 25 ms
ShrinkHalf + ShrinkHalf + Scale: 16 ms

Which means, the trick applied once, causes %36 speedup.
And the trick applied twice, causes %66 speedup.

The demo application has two buttons:

  • Shrink 2:1 - performs image shrink of the whole source image
  • Shrink 2:1 rnd - performs image shrink of random rectangle from the source image

On the left, there is the original image. On the right, there is the original image in grayscale and the shrunk image (or shrunk part) in color.

Shrinking only parts of the image is appropriate when the original was updated and there is a need to update the shrunk copy too. This could save a lot of CPU and time for processing.

2:1 of the Whole Picture

Image 2

2:1 of Random Rectangle

Image 3

Using the Code

There is a pair of files in the project: ShrinkHalf.h and ShrinkHalf.cpp
To use the code, you only need to put these files into your project and include the ShrinkHalf.h file.
These files contain the implementations of all algorithms.

All functions receive as arguments:

  • BYTE* pixels - an array of pixels in format 24 bit BGR (b,g,r, b,g,r, ..., b,g,r)
  • int width - width in pixels of the image
  • int height - height in pixels of the image

So, in order to use the functions, you should do the following:

#include "ShrinkHalf.h"


// This will convert the color image to black/white using 16x16 matrix
shrinkHalf( dstPixels, srcPixels, srcWidth, srcHeight );

Note that, the destination does not provide dimensions, as they are calculated from the source sizes.

Here are the functions available:

//    Fast and smooth 2:1 image resize

void    shrinkHalf    ( BYTE* target, const BYTE* source, int srcWidth, int srcHeight );
void    shrinkHalfPart( BYTE* target, const BYTE* source, int srcWidth, 
                        int srcHeight, int x1, int y1, int x2, int y2 );


The first function performs 2:1 shrink of the whole image.
The second function performs 2:1 shrink of the specified rectangle from the source image, calculating the respective rectangle for the destination image.
If you use this, you should pay attention that you could need correction +1/-1 of the source rectangle points, because of integer roundup.
This is very useful if there are intensive updates that should be applied (for example, video rendering).

Points of Interest

In the shrinkHalf function, there is commented code that works, but I preferred to leave the internal call to shrinkHalfPart.

void    shrinkHalf( BYTE* target, const BYTE* source, int srcWidth, int srcHeight )
    shrinkHalfPart( target, source, srcWidth, 
                    srcHeight, 0, 0, srcWidth-1, srcHeight-1 );

    int    dstWidth    = srcWidth  / 2;
    int    dstHeight   = srcHeight / 2;

    int    srcLineBytes    = srcWidth * 3;
    int    dstLineBytes    = dstWidth * 3;
    int    dstRows         = dstHeight;

    const BYTE*    sl    = source;
          BYTE*    tl    = target;
    const BYTE*    te    = target + srcHeight/2 * dstLineBytes;

    while( tl < te )
              BYTE*    pt    = tl;
        const BYTE*    p1    = sl;
        const BYTE*    p2    = p1 + srcLineBytes;
        const BYTE*    pe    = sl + srcLineBytes;

        while( p1 < pe )
            *pt++    = (p1[0] + p1[3] + p2[0] + p2[3]) >> 2;    //    / 4;    blue;
            *pt++    = (p1[1] + p1[4] + p2[1] + p2[4]) >> 2;    //    / 4;    green;
            *pt++    = (p1[2] + p1[5] + p2[2] + p2[5]) >> 2;    //    / 4;    red;

            p1    += 6;//2*3;
            p2    += 6;//2*3;

        sl    += srcLineBytes << 1;    //    Shift by 1 is equal to " * 2 "
        tl    += dstLineBytes;

So, if someone wants a clean and simpler function, they could remove the first line that calls shrinkHalfPart and uncomment the commented code.


  • 8th April, 2020: Initial version


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Written By
Technical Lead Brosix
Bulgaria Bulgaria
I am a software development engineer .

Programming Languages: C/C++, Java, Delphi, HTML, CGI, Assembly x86, CNC G-language
RDBMS Microsoft SQL Server, MySQL, SQLite, MS Access, ODBC, JDBC

Networking: TCP/IP, Winsock, HTTP, HTTPS, FTP, SMTP, POP3, TELNET, IMAP, SOCKS 4/5, RTSP, CGI, MS Internet Information Server

Projects I have worked on:
  • Brosix
  • Screen-Sharing product
  • Video chat application
  • Server software
  • Java Virtual Machine
  • Application Server + Internet Proxy Services
  • Instant Messenger - like MSN and Yahoo ones
  • Voicer - Freeware VoIP application
  • Web Server
  • SSH Proxy - SOCKS 4&5 Proxy that can relay TCP Connections through HTTPS Proxy
  • HTTP Proxy
  • SOCKS proxy
  • Battery Test Suite
  • MFC custom UI controls
  • Internet Address Book - Synchronizes local Outlook, Outlook Express, Netscape and Eudora address book with database on WEB server located in the Internet
  • Advertising Screen Saver - Screen Saver that sends e-mails and gain prizes for the computer owner
  • Proxy Send Mail - Send Mail service that can send e-mails through SOCKS 4, SOCKS 5 and HTTPS proxies
  • Proxy Hunter - Very fast, and also works as proxy checker
  • Java Disassembler
  • Java custom UI controls
  • Delphi custom UI controls
  • CNC Gravuring System
  • Font editor for DOS
  • Little DOS games
  • Graphics library for DOS (in Assembly)
  • Galaxian like game for Apple][ in assembly
  • Graphics editor for Apple][
  • Font editor for Apple][
  • More information about my current work you can find here:

    Comments and Discussions

    GeneralRe: Vectorization Pin
    YDaoust23-Nov-22 4:43
    YDaoust23-Nov-22 4:43 
    AnswerAmazing Pin
    Ruth_Haephrati25-Oct-22 1:45
    professionalRuth_Haephrati25-Oct-22 1:45 
    GeneralMy vote of 5 Pin
    Ruth_Haephrati25-Oct-22 1:44
    professionalRuth_Haephrati25-Oct-22 1:44 
    QuestionWhy not use bitblt or stetch bitblt? Pin
    Member 1507871629-Jun-22 16:35
    Member 1507871629-Jun-22 16:35 
    AnswerRe: Why not use bitblt or stetch bitblt? Pin
    Svetoslav Chekanov9-Aug-22 2:24
    Svetoslav Chekanov9-Aug-22 2:24 
    AnswerRe: Why not use bitblt or stetch bitblt? Pin
    Svetoslav Chekanov13-Dec-22 1:59
    Svetoslav Chekanov13-Dec-22 1:59 
    PraiseRe: Why not use bitblt or stetch bitblt? Pin
    Member 150787164-Jan-23 18:08
    Member 150787164-Jan-23 18:08 
    GeneralRe: Why not use bitblt or stetch bitblt? Pin
    Svetoslav Chekanov4-Jan-23 22:54
    Svetoslav Chekanov4-Jan-23 22:54 
    I don't have example, but can give you a few advices:
    1. If possible, always prefer hardware acceleration. CPU based software rendering is for any case.
    2. Prepare all frames in Device-Dependent Format at the given size. Device dependent format is blitted faster to the screen.
    3. If the size of the rendering area is changed - pre-render the frames (pt.2)
    You could prepare all the frames at once if this will not took too much time.
    Or you can invalidate all frames and prepare them on demand - when the frame has to be shown. This will spread the cpu usage uniformly. Once the first pass of the animation is done, the frames are already prepared.
    AnswerRe: Why not use bitblt or stetch bitblt? Pin
    Member 1507871611-Jan-23 10:41
    Member 1507871611-Jan-23 10:41 
    GeneralRe: Why not use bitblt or stetch bitblt? Pin
    Svetoslav Chekanov11-Jan-23 23:15
    Svetoslav Chekanov11-Jan-23 23:15 
    GeneralMy vote of 5 Pin
    tugrulGtx4-Apr-22 3:18
    tugrulGtx4-Apr-22 3:18 
    GeneralRe: My vote of 5 Pin
    Svetoslav Chekanov17-May-22 22:20
    Svetoslav Chekanov17-May-22 22:20 
    QuestionAt what threshold the quality of the image is affected substantial? Pin
    deepok13-Apr-22 0:14
    deepok13-Apr-22 0:14 
    AnswerRe: At what threshold the quality of the image is affected substantial? Pin
    Svetoslav Chekanov17-May-22 22:26
    Svetoslav Chekanov17-May-22 22:26 
    SuggestionLinux utility Pin
    Member 1514928613-Apr-21 2:03
    Member 1514928613-Apr-21 2:03 
    SuggestionMore than just half pre-scaling Pin
    Chad3F16-Apr-20 9:30
    Chad3F16-Apr-20 9:30 
    GeneralRe: More than just half pre-scaling Pin
    Svetoslav Chekanov1-Jun-20 7:03
    Svetoslav Chekanov1-Jun-20 7:03 
    GeneralRe: More than just half pre-scaling Pin
    YDaoust25-Oct-22 23:39
    YDaoust25-Oct-22 23:39 
    QuestionLevel of quality loss? Pin
    Chad3F16-Apr-20 9:18
    Chad3F16-Apr-20 9:18 
    AnswerRe: Level of quality loss? Pin
    Svetoslav Chekanov1-Jun-20 6:58
    Svetoslav Chekanov1-Jun-20 6:58 

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