A fast way to work on Windows bitmap pixel data without using the slow GetPixel and SetPixel methods

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Introduction

This article shows a fast way to view and change pixel color data (Windows bitmaps) without using the GetPixel and SetPixel methods.

Background

I spent a couple of months in trouble not knowing a fast way to edit images with C++ (I was using the Windows API functions GetPixel and SetPixel). I lost a couple of clients. I then spent a couple of weeks browsing through forums trying to find answers. But eventually, I got through this by myself and learned this easy pointer arithmetic method.

Using the Code

We use a pointer to bitmap bits: bitmapzor.GetBits();. Then, we use a pointer offset to get to another line in the bitmap pixel colors array: bitmapzor.GetPitch();.

The complete code with comments is listed here:

// FastImageManipulations.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"


int _tmain(int argc, _TCHAR* argv[])
{
    //Okay Now you need a bitmap to work with. Find any bmp file on your PC and
    //Put it to C:\\1.bmp location. You can enter any name, just alter the file
    //path below:

    CImage bitmapzor;
    bitmapzor.Load(_T("C:\\1.bmp"));

    //I use ATL CImage class because it is easy to use. Low number of code lines
    //You can use HBITMAP, HGDIOBJ of GDIplus and other methods,
    //They work the same way and at the same speed, but take tons of code lines 
    //For instance you can use HBITMAP bitmapzor = (HBITMAP)ImageLoad(...)
    //And then you are to use GDI and API functions
    //Let us see the difference between standard API and pointer way:
    printf ("Now we use ATL (Api) GetPixel and SetPixel functions to " 
            "enforce (for example) all green pixels by 30%\n please " 
            "press any key to start and start counting minutes :)");

    getchar();
    COLORREF PixColor=0; //This is a color data
    int R=0,G=0,B=0; //These are the channel values

    //First we shall for instance allter all pixel colors to green using standard way
    for (int i=0; i<bitmapzor.GetWidth(); i++) //along line
        for (int j=0; j<bitmapzor.GetHeight(); j++) //new line
        {
            //This evil slow function extracts 24bit pixel color as BGR value
            PixColor = bitmapzor.GetPixel(i,j);
            R=GetRValue(PixColor);//This macro extracts red channel value
            G=GetGValue(PixColor);//This macro extracts green channel value
            B=GetBValue(PixColor);//This extracts blue pixel color
            G=(int)((float)G*1.3); //There we enforce green color by 30%.
            if (G>255) G=255; //There we ensure G is within 0-255 range
            //We can assemble the BGR 24 bit number that way
            //PixColor = B*0x10000+G*0x100+R;
            PixColor = RGB(R,G,B);//Or we can use this function
            //Now we save a new "green" pixel value back to bitmap
            bitmapzor.SetPixel(i,j,PixColor);
        }

    //Now we need to save this result to a HD:
    LPCTSTR filename=_T("C:\\GetPixel.bmp");
    //you can change the location of this file

    bitmapzor.Save(filename);
    printf ("Done, file is saved to: %s\n Please press " 
            "any key for another method demo", filename);
    getchar();

    //Okay now another way with the pointer arithmetics:
    printf ("Pointer arithmetics demo (without GetPixel and SetPixel functions): \n");
    bitmapzor.Destroy(); //Unload the changed bitmap from memory
    bitmapzor.Load("C:\\1.bmp");
    //this is a pointer to the exact bitmap pixel color array location
    BYTE* byteptr = (BYTE*)bitmapzor.GetBits();

    //You can use other functions for HBITMAP and HGDIOBJ methods to find out the 
    //container for bitmap color table.
    //For instance when you use GDI and HBITMAPS, you should use
    //BYTE* byteptr = (BYTE*)(HBITMAPINFOHEADER + HBITMAPINFOHEADER->biSize);
    //simple pointer arithmetics

    int pitch = bitmapzor.GetPitch(); //This is a pointer offset to get new line of the bitmap

    for (int i=0; i<bitmapzor.GetWidth();i++)
        for (int j=0; j<bitmapzor.GetHeight();j++)
        {
            //pointer arithmetics to find (i,j) pixel colors:
            R= *(byteptr+pitch*j+3*i);
            G= *(byteptr+pitch*j+3*i+1);
            B= *(byteptr+pitch*j+3*i+2); 

            //allter pixel G color:
            G=(int)((float)G*1.3);
            if (G>255) G=255;
            //write down the new G-Color
            *(byteptr+pitch*j+3*i+1)=G;
        }

    //Save the bitmap:
    LPCTSTR filename2 = _T("C:\\ptrarithm.bmp"); //you can use any other file name
    bitmapzor.Save(filename2);
    printf ("Done, file is saved to: %s\n Please press any key to exit program", filename2);
    getchar();
    bitmapzor.Destroy(); //destroy the bitmap
    return 0;
}

Points of Interest

A normal bitmap is a WxH pixel table with a header. Each value in the pixel table is a 24 bit integer or 3xCHARs. To access these values, we need to know the starting point of the pointer and the offset. Normal bitmaps have a negative offset; e.g., if the pointer to some line of the color table is a byteptr, then the pointer to the next line is byteptr-3*W. But sometimes, bitmaps are upside-down oriented and the pointer to the next line is byteptr+3*W. Anyway, the offset negative or positive is always given by the GetPitch() method.

History

This is version 1.0.2 of the article.