How To Implement Zoom Out Of The Image?¶

Introduction

If you want to zoom out your image, you could do with self. It's just a ratio compute to new pixel position. Don’t worry. Your original image still exists, we will establish a new image to keep the zoom out of the image.

Equipment

Operation System: Microsoft Windows 7 (64 bit)
Development Utility: Microsoft Visual Studio 2010

Usage

// Compute a ratio of zoom out of the image about the X-axis.
    dobRX = ((double)256 / (double)512);
    // Compute a ratio of zoom out of the image about the Y-axis.
    dobRY = ((double)256 / (double)512);
    // The height of the image.
    for ( int iY = 0; iY < imageA->DibInfo->bmiHeader.biHeight; iY++ )
    {
        // The index of the Y-axis of the zoom out.
        iYY = (int)( iY * dobRY);
        // The width of the image.
        for ( int iX = 0; iX < imageA->DibInfo->bmiHeader.biWidth; iX++ )
        {
            // The index of the X-axis of the zoom out.
            iXX = (int)( iX * dobRX);
            // The index of the bit depth of the pixel, if your bit depth is not three bit depth, you should change to fit your bitmap format.
            lIDXA = ( iX * 3 ) + ( iY * imageB->DibInfo->bmiHeader.biWidth * 3 );
            // To get the pixel of the blue channel.
            byteRGB_BA = imageA->DibArry[lIDXA+0];
            // To get the pixel of the green channel.
            byteRGB_GA = imageA->DibArry[lIDXA+1];
            // To get the pixel of the red channel.
            byteRGB_RA = imageA->DibArry[lIDXA+2];
            // The index of the zoom out of the pixel. We use iXX and iYY made new pixel position.
            lIDXB = ( iXX * 3 ) + (iYY * imageA->DibInfo->bmiHeader.biWidth * 3 );
            // Set the pixel of the blue channel.
            imageB->DibArry[lIDXB+0] = byteRGB_BA;
            // Set the pixel of the green channel.
            imageB->DibArry[lIDXB+1] = byteRGB_GA;
            // Set the pixel of the red channel.
            imageB->DibArry[lIDXB+2] = byteRGB_RA;
        } // for ( int iX = 0; iX < imageA->DibInfo->bmiHeader.biWidth; iX++ )
    } // for ( int iY = 0; iY < imageA->DibInfo->bmiHeader.biHeight; iY++ )

You can download source code and binary code form this hyperlink as below.
https://drive.google.com/file/d/0BzHb_OyLyVZlSlVNSmFheUtJdFU/view?usp=sharing

Exception

1. There is a notice, if your bit depth of bitmap file are not 24 bits, you should change your bitmap files to adapt this program, or you could rewrite this source code to fit your bitmap format.

2. You have to install Microsoft SDK v7.1, because I include windowscodes.lib.
   #pragma comment(lib, "windowscodecs.lib")

Reference

[1] Gary Bradski and Adrian Kaehler, “Learning OpenCV: Computer Vision with the OpenCV Library,” O’REILLY, September 2008, ISBN:978-0-596-51613-0

Acknowledge

Thank you (Microsoft Visual Studio 2010, Lenna Sjööblom) very much for this great development utility and beautiful photo.