Lib Png: Png24 To Png32 and Vice-Versa

Question

I recently solved the Bitmap problem in my last post. Now I'm back with another Image problem. This time it's PNG.

I'm using LibPng to read and write PNG files. I have a struct that holds BGRA information of each Pixel. I know that the pixels are stored up right and in RGBA format. So I specified swapping of the B and the R. That works fine. I think I'm somehow flipping the image but I'm not quite sure.

My problem comes in when I try to convert 24 bit PNG to 32 bit PNG and vice-versa. Currently, I can do 24 to 24 and 32 to 32 just fine.

Can you guys look over my code and tell me what I'm doing wrong when attempting to convert from 24 to 32?

The below code works for loading and writing the same PNG back to the disk. I made sure to include everything in this one file so that you guys can compile it and see if necessary.

#include <iostream>
#include <vector>
#include <fstream>
#include <stdexcept>
#include "Libraries/LibPng/Include/png.h"

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


std::vector<RGB> Pixels;
uint32_t BitsPerPixel, width, height;
int bitdepth, colortype, interlacetype, channels;


void ReadFromStream(png_structp PngPointer, png_bytep Data, png_size_t Length) //For reading using ifstream rather than FILE*
{
    std::ifstream *Stream = (std::ifstream*)png_get_io_ptr(PngPointer);
    Stream->read((char*)Data, Length);
}

void WriteToStream(png_structp PngPointer, png_bytep Data, png_size_t Length)  //For writing using ofstream rather than FILE*
{
    std::ofstream *Stream = (std::ofstream*)png_get_io_ptr(PngPointer);
    Stream->write((char*)Data, Length);
}


void Load(const char* FilePath)
{
    std::fstream hFile(FilePath, std::ios::in | std::ios::binary);
    if (!hFile.is_open()){throw std::invalid_argument("File Not Found.");}

    unsigned char Header[8] = {0};
    hFile.read(reinterpret_cast<char*>(&Header), sizeof(Header));

    if (png_sig_cmp(Header, 0, 8))
    {
        hFile.close();
        throw std::invalid_argument("Error: Invalid File Format. Required: Png.");
    }

    png_structp PngPointer = png_create_read_struct(PNG_LIBPNG_VER_STRING, nullptr, nullptr, nullptr);
    if (!PngPointer)
    {
        hFile.close();
        throw std::runtime_error("Error: Cannot Create Read Structure.");
    }

    png_infop InfoPointer = png_create_info_struct(PngPointer);
    if (!InfoPointer)
    {
        hFile.close();
        png_destroy_read_struct(&PngPointer, nullptr, nullptr);
        throw std::runtime_error("Error: Cannot Create InfoPointer Structure.");
    }

    png_infop EndInfo = png_create_info_struct(PngPointer);
    if (!EndInfo)
    {
        hFile.close();
        png_destroy_read_struct(&PngPointer, &InfoPointer, nullptr);
        throw std::runtime_error("Error: Cannot Create EndInfo Structure.");
    }

    if (setjmp(png_jmpbuf(PngPointer)))
    {
        hFile.close();
        png_destroy_read_struct(&PngPointer, &InfoPointer, nullptr);
        throw std::runtime_error("Error: Cannot Set Jump Pointer.");
    }

    png_set_sig_bytes(PngPointer, sizeof(Header));
    png_set_read_fn(PngPointer, reinterpret_cast<void*>(&hFile), ReadFromStream);
    png_read_info(PngPointer, InfoPointer);


    //This is where I start getting the info and storing it..

    channels = png_get_channels(PngPointer, InfoPointer);
    png_get_IHDR(PngPointer, InfoPointer, &width, &height, &bitdepth, &colortype, &interlacetype, nullptr, nullptr);
    png_set_strip_16(PngPointer);
    png_set_packing(PngPointer);

    switch (colortype)
    {
        case PNG_COLOR_TYPE_GRAY:
        {
            png_set_expand(PngPointer);
            break;
        }

        case PNG_COLOR_TYPE_GRAY_ALPHA:
        {
            png_set_gray_to_rgb(PngPointer);
            break;
        }

        case PNG_COLOR_TYPE_RGB:
        {
            png_set_bgr(PngPointer);
            BitsPerPixel = 24;
            break;
        }

        case PNG_COLOR_TYPE_RGBA:
        {
            png_set_bgr(PngPointer);
            BitsPerPixel = 32;
            break;
        }

        default: png_destroy_read_struct(&PngPointer, &InfoPointer, nullptr); throw std::runtime_error("Error: Png Type not supported."); break;
    }

    //Store the new data.

    png_read_update_info(PngPointer, InfoPointer);
    channels = png_get_channels(PngPointer, InfoPointer);
    png_get_IHDR(PngPointer, InfoPointer, &width, &height, &bitdepth, &colortype, &interlacetype, nullptr, nullptr);

    Pixels.resize(width * height);
    std::vector<unsigned char*> RowPointers(height);
    unsigned char* BuffPos = reinterpret_cast<unsigned char*>(Pixels.data());

    //Set the row pointers to my Pixels vector. This way, the image is stored upright in my vector<BGRA> Pixels.
    //I think this is flipping it for some reason :S

    for (size_t I = 0; I < height; ++I)
    {
        RowPointers[I] = BuffPos + (I * width * ((BitsPerPixel > 24) ? 4 : 3));
    }

    png_read_image(PngPointer, RowPointers.data());                 //Get the pixels as BGRA and store it in my struct vector.
    png_destroy_read_struct(&PngPointer, &InfoPointer, nullptr);
    hFile.close();

    std::cout<<"Loading Parameters..."<<std::endl;
    std::cout<<"Bits:  "<<BitsPerPixel<<std::endl;
    std::cout<<"Depth: "<<bitdepth<<std::endl;
    std::cout<<"CType: "<<colortype<<std::endl;
}

void Save(const char* FilePath)
{
    std::fstream hFile(FilePath, std::ios::out | std::ios::binary);
    if (!hFile.is_open()) {throw std::invalid_argument("Cannot open file for writing.");}

    png_structp PngPointer = png_create_write_struct(PNG_LIBPNG_VER_STRING, nullptr, nullptr, nullptr);
    if (!PngPointer)
    {
        hFile.close();
        throw std::runtime_error("Error: Cannot Create Write Structure.");
    }

    png_infop InfoPointer = png_create_info_struct(PngPointer);
    if (!InfoPointer)
    {
        hFile.close();
        png_destroy_write_struct(&PngPointer, nullptr);
        throw std::runtime_error("Error: Cannot Create InfoPointer Structure.");
    }

    if (setjmp(png_jmpbuf(PngPointer)))
    {
        hFile.close();
        png_destroy_write_struct(&PngPointer, &InfoPointer);
        throw std::runtime_error("Error: Cannot Set Jump Pointer.");
    }

    std::cout<<"\nSaving Parameters..."<<std::endl;
    std::cout<<"Bits:  "<<BitsPerPixel<<std::endl;
    std::cout<<"Depth: "<<bitdepth<<std::endl;
    std::cout<<"CType: "<<colortype<<std::endl;

    //This is where I set all the Information..

    png_set_IHDR (PngPointer, InfoPointer, width, height, bitdepth, BitsPerPixel == 24 ? PNG_COLOR_TYPE_RGB : PNG_COLOR_TYPE_RGBA, PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
    std::vector<unsigned char*> RowPointers(height);
    unsigned char* BuffPos = reinterpret_cast<unsigned char*>(Pixels.data());

    //Set the Row pointers to my vector<BGRA> Pixels. It should have been stored upright already.
    //I think this flips it upside down :S?

    for (size_t I = 0; I < height; ++I)
    {
        RowPointers[I] = BuffPos + (I * width * ((BitsPerPixel > 24) ? 4 : 3));
    }

    png_set_bgr(PngPointer);        //My struct vector holds BGRA and PNG requires RGBA so swap them..
    png_set_write_fn(PngPointer, reinterpret_cast<void*>(&hFile), WriteToStream, nullptr);
    png_set_rows(PngPointer, InfoPointer, RowPointers.data());
    png_write_png(PngPointer, InfoPointer, PNG_TRANSFORM_IDENTITY, NULL);
    png_destroy_write_struct(&PngPointer, &InfoPointer);
    hFile.close();
}

void SetBitsPerPixel(uint32_t BPP)
{
    BitsPerPixel = BPP;
    bitdepth = (BPP > 24 ? 8 : 6);
    channels = (BPP > 24 ? 4 : 3);
    colortype = (BPP > 24 ? PNG_COLOR_TYPE_RGBA : PNG_COLOR_TYPE_RGB);
}


int main()
{
    Load("C:/Images/Png24.png");
    SetBitsPerPixel(32);
    Save("C:/Images/Output/Png32.png");
}

I load (24 bit PNG made with MS-Paint): When I save it back as 24, it saves flawlessly. When I attempt to save it back as 32, it looks like:

Answer 1

Try doing the following modifications to your source code. Not tested!
In your Load function change this:

    case PNG_COLOR_TYPE_RGB:
    {
        png_set_bgr(PngPointer);
        BitsPerPixel = 24;
        break;
    }

to this:

    case PNG_COLOR_TYPE_RGB:
    {
        png_set_filler(PngPointer, 0xFF, PNG_FILLER_AFTER);
        png_set_bgr(PngPointer);
        BitsPerPixel = 32;
        break;
    }

---

In your Load and Save functions change this:

for (size_t I = 0; I < height; ++I)
{
    RowPointers[I] = BuffPos + (I * width * ((BitsPerPixel > 24) ? 4 : 3));
}

to this:

size_t BytesPerLine = width << 2;
unsigned char *ptr = BuffPos;
for (size_t I = 0; I < height; ++I, ptr += BytesPerLine)
    RowPointers[I] = ptr;

---

In your Save function change this:

png_write_png(PngPointer, InfoPointer, PNG_TRANSFORM_IDENTITY, NULL);

to this:

png_write_png(PngPointer, InfoPointer, BitsPerPixel == 24 ? PNG_TRANSFORM_STRIP_FILLER : PNG_TRANSFORM_IDENTITY, NULL);

---

and in your SetBitsPerPixel function change this line:

bitdepth = (BPP > 24 ? 8 : 6);

to this:

bitdepth = (BPP >= 24 ? 8 : 6);

Actually, I'm not sure why you're doing this, since, as far as I know, 24 and 32 bpp images should have a bitdepth of 8 bits.

NOTE: This modifications are intended for 24 and 32 bpp images, so if you want to use indexed or greyscale images you may need to do some extra modifications. The point is that you should always save pixels in a BGRA buffer (no matter if you load indexed, greyscale or RGB images with no alpha channel).