Question

我最近在上一篇文章中解决了Bitmap问题。现在我回来了另一个Image问题。这次是PNG。

我正在使用LibPng来读写PNG文件。我有一个结构，可以保存每个像素的BGRA信息。我知道像素是以RGBA格式存储的。所以我指定了交换B和R.这很好。我想我是在某种程度上翻转图像，但我不太确定。

当我尝试将24位PNG转换为32位PNG时出现问题，反之亦然。目前，我可以做24到24和32到32就好了。

你们可以查看我的代码并告诉我在尝试从24转换为32时我做错了吗？

以下代码适用于将相同的PNG加载和写入磁盘。我确保在这个文件中包含所有内容，以便你们可以编译它并查看是否有必要。

#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");
}

我加载（使用MS-Paint制作的24位PNG）： enter image description here 当我将它保存为24时，它可以完美无缺地保存。当我尝试将其保存为32时，它看起来像：

enter image description here

Answer 1

尝试对源代码进行以下修改。 未经测试！
在加载功能中更改此项：

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

到此：

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

<小时/> 在加载和保存功能中更改此设置：

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

到此：

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

<小时/> 在保存功能中，将其更改为：

png_write_png(PngPointer, InfoPointer, PNG_TRANSFORM_IDENTITY, NULL);

到此：

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

<小时/> 并在 SetBitsPerPixel 函数中更改此行：

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

到此：

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

实际上，我不确定你为什么这样做，因为据我所知，24和32 bpp图像应该有8位的 bitdepth 。

注意：此修改适用于24和32 bpp图像，因此如果您想使用索引或灰度图像，则可能需要做一些额外的修改。关键是你应该总是在BGRA缓冲区中保存像素（无论你是否加载没有alpha通道的索引图像，灰度图像或RGB图像）。

Lib Png：Png24到Png32和副Versa

1 个答案: