CODEWITHSUNDEEP
image-processingvb6bmpgrayscale
jesusgonzalezrivera
jesusgonzalezrivera

Reputation: 385

Convert to grayscale and reduce the size

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?.

Upvotes: 0

Views: 7888

Answers (2)

Rethunk
Rethunk

Reputation: 4113

You can write your own conversion in a "lockbits" method. It takes a while to understand how to lock/unlock bits correctly, but the effort is worth it, and once you have the code working you'll see how it can be applied to other scenarios. For example, using an lock/unlock bits technique you can access the pixel values from a bitmap, copy those pixel values into an array, manipulate the array, and then copy the modified array back into a bitmap. That's much faster than calling GetPixel() and SetPixel(). That's still not the fastest image manipulation code one can write, but it's relatively easy to implement and maintain the code.

It's been a while since I've written VB6 code, but Bob Powell's often has good examples, and he has a page about lock bits:

https://web.archive.org/web/20121203144033/http://www.bobpowell.net/lockingbits.htm

In a pinch you could create a new Bitmap of the appropriate format and call SetPixel() for every pixel:

  1. Every pixel (x,y) in your 24-bit color image will have a color value (r,g,b)
  2. After conversion to a 24-bit gray image, each pixel (x,y) will have a three equal values for each color channel; that can be expressed as (n,n,n) as Willem wrote in his reply. If all three colors R,G,B have the same value, then you can say that color value is the "grayscale" value of that pixel. This is the same shade of gray that you will see in your final 8-bit bitmap.
  3. Call SetPixel for each pixel (x,y) in a newly created 8-bit bitmap that is the same width and height as the original color image.

Upvotes: 0

Chrono
Chrono

Reputation: 1473

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.

Upvotes: 2

Related Questions