Getting RGB values for each pixel from a 24bpp Bitmap for conversion to GBA format in C

Question

I want to read the RGB values for each pixel from a .bmp file, so I can convert the bmp into a format suitable for GBA (GameBoy Advance).

I

Answer 1

I did a bit of testing and extended Patrice's program a bit. I'm not a good C programmer, so all credit goes to him and my parts aren't as elegant as his - sorry for that.

Warning: huge source code ahead.

#include 
#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; jred, 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 out the pixel information stored in the file. It takes the padding into account but only works with bmps with a 24 bits per pixel color depth (If you need other depths, you'll have to customize the Rgb struct). Hope this helps you, but as I said, it's just an extension of Patrice's code.

Here's a sample output from my testfile:

$ ./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 is displaying a red cross. Note that the image is stored upside-down so row 1 of the file is actually row 5 of the image. D'oh forgot to write something to file the code opens, but this is left as an excercise up to you ;).