How to read bmp files

This is necessary in case the user decides to save the processed image. In the 24 bits per pixel case, if the width is divisible by 4, we write the data just like for the 32 bits per pixel case. If the width is not divisible by 4, we increase the row stride, by adding 1 repeatedly, until it is divisible by 4.

We fill a padding vector with zeros that will be used at the end of each line. We modify the bitmap headers to take into account the new file size and write the headers like in the previous cases.

The data vector is written one row at a time: we write a row, we write the padding data, we write the next row and so on …. Next, we can write the code for creating a BMP image in memory.

For this, like before, we consider only images with 24 and 32 bits per pixel. The constructor needs to set the width and height for the image, the header sizes, the file size, the offset data the position at which the pixel data is written in the file , the bits per pixel count, the compression type and resize the data vector to accommodate the image size:.

The only part that remains to be implemented is the modify pixel data part. A quick and dirty approach, without error checking, is to fill a rectangular region from the image with a particular color. For example, we could write:. You can find the complete source code on the GitHub repository for this article. The next step is to try to interpret a real. I first created a small file by selecting a small part of my desktop that contains an icon for QuickC, an old Microsoft MS-DOS C programming environment that I have used for many years.

Here's the image: Although this sort of puts the cart before the programming horse, here are the results from running my program, written in QuickC. The third line shows the 14 bytes of the file header. These 14 bytes contain 5 values: The uppercase characters BM, ASCII codes 66 and 77 expressed as a base integer 2 bytes File size, bytes 4 bytes Two "reserved values" that are not needed 2 bytes each Offset to beginning of image data 4 bytes.

In principle, it is possible to read these integer values directly by choosing appropriately declared data types that will automatically select the appropriate number of bytes. However, this may give unpredictable results with different C compilers, so I chose a more simple-minded, although probably more tedious solution: read the header one character at a time characters occupy a single byte and then use explicit typecasting to force C to interpret each character as an integer.

For this to work, the C declaration must be unsigned char rather than char. Here's the code:. The two important values in the header are the file size, in bytes, and the offset from the beginning of the file to the start of the image itself, in bytes. It not only lets you view BMP files but also offers many tools to edit them. For example, it can rotate, flip, scale, crop images, add text to the image , and more.

And it offers selection tools and pen tools. Read this post now! How to convert BMP files? If not, you can try the following online BMP file converters. Online-Convert is a powerful online conversion tool that allows you to convert images, videos, audio files, documents, etc. Free Download.