Convert to grayscale and reduce the size

bmpgrayscaleimage processingvb6

I am trying to develop an OCR in VB6 and I have some problems with BMP format. I have been investigating the OCR process and the first step is to convert the image in "black and white" with a threshold. The conversion process is easy to understand and I have done it. However, I'm trying to reduce the size of the resulting image because it uses less colors (each pixel only has 256 possible values in grayscale). In the original image I have 3 colors (red, green and blue) but now I only need one color (the value in grayscale). In this moment I have achieved the conversion but the resulting grayscale images have the same size as the original color image (I assign the same color value in the three channels).

I have tried to modify the header of the BMP file but I haven't achieved anything and now I don't understand how it works. For example, if I convert the image with paint, the offset that is specified in the header changes its value. If the header is constant, why does the offset change its value?.

Best Answer

The thing is that a grey-scale bitmap image is the same size as a color bitmap image because the data that is used to save the grey colors takes just as much space as the color.

The only difference is that grey is just 3 times that same value. (160,160,160) for example with color giving something like (123,200,60). The grey values are just a small subset of the RGB field.

You can trim down the size after converting to grey-scale by converting it from 24 bit to 16 bit or 8-bit for example. Although it depends on what you are using to do the conversion whether that is already supplied to you. Otherwise you'll have to make it yourself.

You can also try using something else than BMP images. PNG files are lossless too, and would even save space with the 24 bit version. Image processing libraries usally give you several options as output formats. Otherwise you can probably find a library that does this for you.

Related Solutions

Css – Convert an image to grayscale in HTML/CSS

Support for CSS filters has landed in Webkit. So we now have a cross-browser solution.

img {
  filter: gray; /* IE6-9 */
  -webkit-filter: grayscale(1); /* Google Chrome, Safari 6+ & Opera 15+ */
  filter: grayscale(1); /* Microsoft Edge and Firefox 35+ */
}

/* Disable grayscale on hover */
img:hover {
  -webkit-filter: grayscale(0);
  filter: none;
}

<img src="https://upload.wikimedia.org/wikipedia/commons/thumb/0/04/Wikimedia_Canadian_Community_Logo.svg/240px-Wikimedia_Canadian_Community_Logo.svg.png">

What about Internet Explorer 10?

You can use a polyfill like gray.

C# – Convert an image to grayscale

"I want a Bitmap d, that is grayscale. I do see a consructor that includes System.Drawing.Imaging.PixelFormat, but I don't understand how to use that."

Here is how to do this

Bitmap grayScaleBP = new 
         System.Drawing.Bitmap(2, 2, System.Drawing.Imaging.PixelFormat.Format16bppGrayScale);

EDIT: To convert to grayscale

             Bitmap c = new Bitmap("fromFile");
             Bitmap d;
             int x, y;

             // Loop through the images pixels to reset color.
             for (x = 0; x < c.Width; x++)
             {
                 for (y = 0; y < c.Height; y++)
                 {
                     Color pixelColor = c.GetPixel(x, y);
                     Color newColor = Color.FromArgb(pixelColor.R, 0, 0);
                     c.SetPixel(x, y, newColor); // Now greyscale
                 }
             }
            d = c;   // d is grayscale version of c

Faster Version from switchonthecode follow link for full analysis:

public static Bitmap MakeGrayscale3(Bitmap original)
{
   //create a blank bitmap the same size as original
   Bitmap newBitmap = new Bitmap(original.Width, original.Height);

   //get a graphics object from the new image
   using(Graphics g = Graphics.FromImage(newBitmap)){

       //create the grayscale ColorMatrix
       ColorMatrix colorMatrix = new ColorMatrix(
          new float[][] 
          {
             new float[] {.3f, .3f, .3f, 0, 0},
             new float[] {.59f, .59f, .59f, 0, 0},
             new float[] {.11f, .11f, .11f, 0, 0},
             new float[] {0, 0, 0, 1, 0},
             new float[] {0, 0, 0, 0, 1}
          });

       //create some image attributes
       using(ImageAttributes attributes = new ImageAttributes()){

           //set the color matrix attribute
           attributes.SetColorMatrix(colorMatrix);

           //draw the original image on the new image
           //using the grayscale color matrix
           g.DrawImage(original, new Rectangle(0, 0, original.Width, original.Height),
                       0, 0, original.Width, original.Height, GraphicsUnit.Pixel, attributes);
       }
   }
   return newBitmap;
}

Best Answer

Related Solutions

Css – Convert an image to grayscale in HTML/CSS

C# – Convert an image to grayscale

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