Saturday, July 26, 2014

Code for reading bmp image files directly in C/C++

We have discussed the process of reading BMP files in earlier post1 and post2 in detail. You can find the code below:

unsigned char* ReadBMP(char* filename, int* array)

{

 FILE* img = fopen(filename, "rb");   //read the file


unsigned char header[54];
fread(header, sizeof(unsigned char), 54, img); // read the 54-byte header


   // extract image height and width from header
 int width = *(int*)&header[18];    
 int height = *(int*)&header[22];    
 int padding=0; while (width*3+padding) % 4!=0 padding++;

 int widthnew=width*3+padding;

 unsigned char* data = new unsigned char[widthnew];

 for (int i=0; i<height; i++ ) {                                    

 fread( data, sizeof(unsigned char), widhthnew, img);

 for (int j=0; j<width*3; j+=3)                                

 { //Convert BGR to RGB and store                      

array[i*width+j+0] = data[j+2];                              

array[i*width+j+1] = data[j+1];                              

array[i* width+j+2] = data[j+0]; }}                        

fclose(img); //close the file
}



Please post incase you face any difficulty in running the codes.

11 comments:

  1. There are a lot of errors. Havent you run it even ones ? I think not. Even 'i' in the first for loop is not defined ..

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  2. This comment has been removed by a blog administrator.

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  3. Is there any chance that we can see the correct code?

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  4. This comment has been removed by the author.

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  5. how to store pixel values into 2D array

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  6. Plz can u give me soln. How to output that stored data to output console in c++

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  7. Plz can u give me soln. How to output that stored data to output console in c++

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  8. Good work and thanks for the code.
    There is a functional bug which took me around 2hours to point out.
    array[i*width+j+0] = data[j+2]; should be array[i*width*3+j+0]. Notice that you have to multiply "i * width * 3".

    Here is the working code:
    static unsigned char *texels;
    static int width, height;

    static void readBmp(char *filename)
    {
    FILE *fd;
    fd = fopen(filename, "rb");
    if (fd == NULL)
    {
    printf("Error: fopen failed\n");
    return;
    }

    unsigned char header[54];

    // Read header
    fread(header, sizeof(unsigned char), 54, fd);

    // Capture dimensions
    width = *(int*)&header[18];
    height = *(int*)&header[22];

    int padding = 0;

    // Calculate padding
    while ((width * 3 + padding) % 4 != 0)
    {
    padding++;
    }

    // Compute new width, which includes padding
    int widthnew = width * 3 + padding;

    // Allocate memory to store image data (non-padded)
    texels = (unsigned char *)malloc(width * height * 3 * sizeof(unsigned char));
    if (texels == NULL)
    {
    printf("Error: Malloc failed\n");
    return;
    }

    // Allocate temporary memory to read widthnew size of data
    unsigned char* data = (unsigned char *)malloc(widthnew * sizeof (unsigned int));

    // Read row by row of data and remove padded data.
    for (int i = 0; i<height; i++)
    {
    // Read widthnew length of data
    fread(data, sizeof(unsigned char), widthnew, fd);

    // Retain width length of data, and swizzle RB component.
    // BMP stores in BGR format, my usecase needs RGB format
    for (int j = 0; j < width * 3; j += 3)
    {
    int index = (i * width * 3) + (j);
    texels[index + 0] = data[j + 2];
    texels[index + 1] = data[j + 1];
    texels[index + 2] = data[j + 0];
    }
    }

    free(data);
    fclose(fd);
    }

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