how can i convert a colored bitmap image to grayscale image?

Use the color matrix and set it to the image attributes.

The color matrix for gray scale is

//Gilles Khouzams colour corrected grayscale shear
        ColorMatrix cm = new ColorMatrix(new float[][]{   new float[]{0.3f,0.3f,0.3f,0,0},
                                  new float[]{0.59f,0.59f,0.59f,0,0},
                                  new float[]{0.11f,0.11f,0.11f,0,0},
                                  new float[]{0,0,0,1,0,0},
                                  new float[]{0,0,0,0,1,0},
                                  new float[]{0,0,0,0,0,1}});

refer

http://www.bobpowell.net/grayscale.htm[^]

About 582,000 results (0.14 seconds) results in google.

First, you must load the image into memory. As you know, there are quite a few different formats around. Depending on which format the original image is, you may need an external library to load it.

Once you have loaded it, you must go through the pixels of the image and convert them to grayscale. Grayscale means, that all components (R, G and B) of the colors have the same value. This way you get 256 colors from black (R = 0, G = 0, B = 0) to white (R = 255, G = 255, B = 255).

For each pixel you must find a grayscale color value with similar brightness and replace the pixel's color value with it. I would simply try to calculate the average of the original R, G and B and use this value for all three components of the grayscale pixel:

R2 = G2 = B2 = (R1 + G1 + B1) / 3

After modifying each pixel, you simply must save the image again. Sorry, but without knowing the image's format and what you use to load it, it's hard to explain more precisely.

All the Answers presented here are correct in certain sense, but I want to add my notes.

How good these methods are depends on the purpose. It this is just to convert the image to gray-scale the way it still can be recognized, all the method would pass in most cases. There is a different point of view of the people who go in for digital black-and-white art photography. This days, only art photography can be black-and-white (and also in some industrial/scientific applications). They are somewhat different people, all of them use computers, usually don't know anything except Photoshop and most of them are amazingly illiterate in computer technology (I used to make fun of whole staff of the Popular Photography Magazine: every one who delivered their "technical expert advice" gave their advices based on their misconception that lostless compression does not exist). Nevertheless, they know something which is missing in the given answers. I know for sure what they say if then see the gray-scale picture converted using the shown methods: what a crap!

A real expert advice from a good photographer (they do it exactly in the same way as we do it at CodeProject) usually goes like that: don't use embedded "convert to gray-scale" method! Depending on the picture, you need to use separate color channel and convert them to gray-scale separately. For example, for a landscape with blue sky, start with red channel only. This is because the most difficult problem is over-exposing of the very bright sky, but the red channel is the least exposed by the sky, so you can reduce unwanted contrast. Then you may want to add some of green channel. Very often you need to add dynamic range to green forest, do the following. Blah-blah…

Anyway, we should always take into account, that there are many different conversions from color to gray-scale; in all cases it is done with loss of information. None of the methods is a priori better then others. Anyway, the considerations presented above should discourage any arguments on which method is really "kosher", and which is really "orthodox". :-)

—SA