Question

This may be a long post but I really need to know how to Convert between 24 and 32 bit bitmaps. For the sake of the length of this post, I removed the PNG part of my question.

Here goes:

I have a struct like the one below that holds all pixel information:

typedef union RGB
{
    uint32_t Color;
    struct
    {
        unsigned char B, G, R, A;
    } RGBA;
} *PRGB;

std::vector<RGB> Pixels;   //Holds all pixels.

All of the bitmap writing works except when going from 24 to 32 or vice-versa. I don’t know what I’m doing wrong or why 24-32 conversions don’t work. My bitmap reading and writing code is as follows:

Bitmap(const void* Pointer, int Width, int Height, uint32_t BitsPerPixel) //Constructor initialization here...
{
    Pixels.clear();
    if (Pointer == nullptr) {throw std::logic_error("Null Pointer Exception. Pointer is NULL.");}
    if (Width < 1 || Height < 1) {throw std::invalid_argument("Invalid Arguments. Width and Height cannot equal 0.");}
    std::memset(&Info, 0, sizeof(BITMAPINFO));
    size = ((width * BitsPerPixel + 31) / 32) * 4 * height;

    Info.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
    Info.bmiHeader.biWidth = width;
    Info.bmiHeader.biHeight = height;
    Info.bmiHeader.biPlanes = 1;
    Info.bmiHeader.biBitCount = BitsPerPixel;
    Info.bmiHeader.biCompression = BI_RGB;
    Info.bmiHeader.biSizeImage = size;
    bFileHeader.bfType = 0x4D42;
    bFileHeader.bfOffBits = sizeof(BITMAPFILEHEADER) + sizeof(Info.bmiHeader);
    bFileHeader.bfSize = bFileHeader.bfOffBits + size;

    const unsigned char* BuffPos = static_cast<const unsigned char*>(Pointer);
    height = (height < 0 ? -height : height);
    Pixels.resize(width * height);

    for (int I = 0; I < height; I++)
    {
        for (int J = 0; J < width; J++)
        {
            Pixels[(height - 1 - I) * width + J].RGBA.B = *(BuffPos++);
            Pixels[(height - 1 - I) * width + J].RGBA.G = *(BuffPos++);
            Pixels[(height - 1 - I) * width + J].RGBA.R = *(BuffPos++);
            Pixels[(height - 1 - I) * width + J].RGBA.A = (Info.bmiHeader.biBitCount > 24 ? *(BuffPos++) : 0);
        }
        if(Info.bmiHeader.biBitCount == 24)
            BuffPos += width % 4;
    }
}

bool SaveBitmap(const char* FilePath)
{
    std::vector<unsigned char> ImageData(size);
    unsigned char* BuffPos = ImageData.data();

    for (int I = 0; I < height; ++I)
    {
        for (int J = 0; J < width; ++J)
        {
            *(BuffPos++) = Pixels[(height - 1 - I) * width + J].RGBA.B;
            *(BuffPos++) = Pixels[(height - 1 - I) * width + J].RGBA.G;
            *(BuffPos++) = Pixels[(height - 1 - I) * width + J].RGBA.R;

            if (Info.bmiHeader.biBitCount > 24)
                *(BuffPos++) = Pixels[(height - 1 - I) * width + J].RGBA.A;
        }
        if(Info.bmiHeader.biBitCount == 24)
            BuffPos += width % 4;
    }

    std::fstream hFile(FilePath, std::fstream::out | std::ofstream::binary);
    if (!hFile.is_open()) return false;

    hFile.write(reinterpret_cast<char*>(&bFileHeader), sizeof(BITMAPFILEHEADER));
    hFile.write(reinterpret_cast<char*>(&Info.bmiHeader), sizeof (BITMAPINFOHEADER));
    hFile.write(reinterpret_cast<char*>(&ImageData[0]), Size());
    hFile.close();

    return true;
}

Any idea what the two problems could be? I want it so that if I called Bitmap(24BmpBuff, W, H, 32); It’ll save as 32. If I do Bitmap(32BmpBuff, W, H, 24) it’ll save as 24 bit. I just can’t see it so I’m hoping one of you will.

I also tried making helper functions:

Convert From 24 bit to 32 bit.

void T24To32(std::vector<RGB> &Input, std::vector<RGB> &Output, int Width, int Height)
{
    Output.resize(Input.size());
    for (int I = 0; I < Height; ++I)
    {
        for (int J = 0; J < Width; ++J)
        {
            Output[J].RGBA.B = Input[J].RGBA.B;
            Output[J].RGBA.G = Input[J].RGBA.G;
            Output[J].RGBA.R = Input[J].RGBA.R;
            Output[J].RGBA.A = 0;
        }
    }
}

Take the unsigned char* of pixels and store them upside down within the struct.

void Pack(int width, int height, int BPP, unsigned char* Input, std::vector<RGB> &Pixels)
{
    unsigned char* BuffPos = Input;
    height = (height < 0 ? -height : height);
    Pixels.resize(width * height);

    for (int I = 0; I < height; I++)
    {
        for (int J = 0; J < width; J++)
        {
            Pixels[(height - 1 - I) * width + J].RGBA.B = *(BuffPos++);
            Pixels[(height - 1 - I) * width + J].RGBA.G = *(BuffPos++);
            Pixels[(height - 1 - I) * width + J].RGBA.R = *(BuffPos++);
            Pixels[(height - 1 - I) * width + J].RGBA.A = (BPP > 24 ? *(BuffPos++) : 0);
        }
        if(BPP == 24)
            BuffPos += width % 4;
    }
}

Take the struct of pixels and store them upright in the unsigned char*.

void Unpack(int width, int height, int BPP, std::vector<RGB> Pixels, unsigned char* &Output)
{
    unsigned char* BuffPos = Output;

    for (int I = 0; I < height; ++I)
    {
        for (int J = 0; J < width; ++J)
        {
            *(BuffPos++) = Pixels[(height - 1 - I) * width + J].RGBA.B;
            *(BuffPos++) = Pixels[(height - 1 - I) * width + J].RGBA.G;
            *(BuffPos++) = Pixels[(height - 1 - I) * width + J].RGBA.R;

            if (BPP > 24)
                *(BuffPos++) = Pixels[(height - 1 - I) * width + J].RGBA.A;
        }
        if(BPP == 24)
            BuffPos += width % 4;
    }
}

I use all of the above like so.. Input image(32 bit):

Code:

void Bitmap32ToBitmap24(int Width, int Height)
{
    Bitmap Image("C:/Images/Bitmap32.bmp");
    std::vector<unsigned char> Pixels(((Width * 32 + 31) / 32) * 4 * Height);   //Array large enough to hold 32 bit bmp.
    unsigned char* BuffPos = Pixels.data();

    Unpack(Width, Height, 32, Image.Get(), BuffPos);                            //Fill the array of unsigned char with image pixels being upright

    Bitmap BMP(Pixels.data(), Width, Height, 24);                               //Convert image to 24 bit bmp and save it.
    BMP.Save("C:/Images/Output/Bitmap32ToBitmap24.png");
}

Output image (24 bit):
24 to 32 results in:

Answer 1

In all your code snippets

if(Info.bmiHeader.biBitCount == 24)
        BuffPos += width % 4;

or

if(BPP == 24)
        BuffPos += width % 4;

occur. I assume this should add the padding value to each line. But it isn’t the padding, it is the number of pixels per line %4.

The correct adding value is (4 - ((width * 3) % 4)) % 4. The width*3 is the number of bytes in that line. The %4 calculates the number of bytes which are to many for a 4 byte padding, but to fill up to the next higher limes we need 4-this value. This again is 4 if no padding offset is needed -> %4 to avoid that.

A faster way to compute the same value is (-width * 3) & 3. See wiki.