Bitmap 24 to 32 and back - c++

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.