Get RGB values ​​for each pixel from 24bpp bitmap for conversion to GBA format in C

I tested and extended Patrice a bit. I'm not a very good C programmer, so all his merit and my details are not as elegant as his - sorry about that.

Warning: There is a huge source code ahead.

#include <stdio.h>
#pragma pack(2)


typedef struct
{
    char signature[2];
    unsigned int fileSize;
    unsigned int reserved;
    unsigned int offset;
} BmpHeader;

typedef struct
{
    unsigned int headerSize;
    unsigned int width;
    unsigned int height;
    unsigned short planeCount;
    unsigned short bitDepth;
    unsigned int compression;
    unsigned int compressedImageSize;
    unsigned int horizontalResolution;
    unsigned int verticalResolution;
    unsigned int numColors;
    unsigned int importantColors;

} BmpImageInfo;

typedef struct
{
    unsigned char blue;
    unsigned char green;
    unsigned char red;
    //unsigned char reserved; Removed for convenience in fread; info.bitDepth/8 doesn't seem to work for some reason
} Rgb;


int main( int argc, char **argv ) {

    FILE *inFile;
    BmpHeader header;
    BmpImageInfo info;
    Rgb *palette;
    int i = 0;

    printf( "Opening file %s for reading.\n", argv[1] );

    inFile = fopen( argv[1], "rb" );
    if( !inFile ) {
        printf( "Error opening file %s.\n", argv[1] );
        return -1;
    }

    if( fread(&header, 1, sizeof(BmpHeader), inFile) != sizeof(BmpHeader) ) {
        printf( "Error reading bmp header.\n" );
        return -1;
    }

    if( fread(&info, 1, sizeof(BmpImageInfo), inFile) != sizeof(BmpImageInfo) ) {
        printf( "Error reading image info.\n" );
        return -1;
    }

    if( info.numColors > 0 ) {
        printf( "Reading palette.\n" );
        palette = (Rgb*)malloc(sizeof(Rgb) * info.numColors);
        if( fread(palette, sizeof(Rgb), info.numColors, inFile) != (info.numColors * sizeof(Rgb)) ) {
            printf( "Error reading palette.\n" );
            return -1; // error
        }
    }

    printf( "Opening file %s for writing.\n", argv[2] );
    FILE *outFile = fopen( argv[2], "wb" );
    if( !outFile ) {
        printf( "Error opening outputfile.\n" );
        return -1;
    }
    Rgb *pixel = (Rgb*) malloc( sizeof(Rgb) );
    int read, j;
    for( j=0; j<info.height; j++ ) {
        printf( "------ Row %d\n", j+1 );
        read = 0;
        for( i=0; i<info.width; i++ ) {
            if( fread(pixel, 1, sizeof(Rgb), inFile) != sizeof(Rgb) ) {
                printf( "Error reading pixel!\n" );
                return -1;
            }
            read += sizeof(Rgb);
            printf( "Pixel %d: %3d %3d %3d\n", i+1, pixel->red, pixel->green, pixel->blue );
        }
        if( read % 4 != 0 ) {
            read = 4 - (read%4);
            printf( "Padding: %d bytes\n", read );
            fread( pixel, read, 1, inFile );
        }
    }

    printf( "Done.\n" );
    fclose(inFile);
    fclose(outFile);

    printf( "\nBMP-Info:\n" );
    printf( "Width x Height: %i x %i\n", info.width, info.height );
    printf( "Depth: %i\n", (int)info.bitDepth );

    return 0;

}

      

        
        
        
      

    

This program reads the pixel information stored in the file. It takes shims into account, but only works with bmps with 24 bits per pixel color depth (if you need other depths, you'll have to tweak the Rgb structure). Hope this helps you, but as I said, this is just an extension of Patrice's code.

Here's an example of the output from my test file:

$ ./a.out test.bmp out.txt
Opening file test.bmp for reading.
Opening file out.txt for writing.
------ Row 1
Pixel 1:   0   0   0
Pixel 2:   0   0   0
Pixel 3:   0   0   0
Pixel 4:   0   0   0
Pixel 5:   0   0   0
Padding: 1 bytes
------ Row 2
Pixel 1:   0   0   0
Pixel 2: 232  33  33
Pixel 3:   0   0   0
Pixel 4: 232  33  33
Pixel 5:   0   0   0
Padding: 1 bytes
------ Row 3
Pixel 1:   0   0   0
Pixel 2:   0   0   0
Pixel 3: 232  33  33
Pixel 4:   0   0   0
Pixel 5:   0   0   0
Padding: 1 bytes
------ Row 4
Pixel 1:   0   0   0
Pixel 2: 232  33  33
Pixel 3:   0   0   0
Pixel 4: 232  33  33
Pixel 5:   0   0   0
Padding: 1 bytes
------ Row 5
Pixel 1:   0   0   0
Pixel 2:   0   0   0
Pixel 3:   0   0   0
Pixel 4:   0   0   0
Pixel 5:   0   0   0
Padding: 1 bytes
Done.

BMP-Info:
Width x Height: 5 x 5
Depth: 24

      

        
        
        
      

    

Edit: Yes, my image shows a red cross. Note that the image is stored in reverse order, so line 1 of the file is actually line 5 of the image. D'oh forgot to write something to open the code, but this is left as an exercise for you;).