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我最近在上一篇文章中解决了位图问题。现在我又回来了另一个图像问题。这次是PNG。

我正在使用 LibPng 来读取和写入 PNG 文件。我有一个包含每个像素的 BGRA 信息的结构。我知道像素以 RGBA 格式正确存储。所以我指定交换 B 和 R。效果很好。我想我以某种方式翻转了图像，但我不太确定。

当我尝试将 24 位 PNG 转换为 32 位 PNG 时，我的问题就出现了，反之亦然。目前，我可以做 24 到 24 和 32 到 32 就好了。

你们能看看我的代码并告诉我在尝试从 24 转换为 32 时我做错了什么吗？

下面的代码适用于将相同的 PNG 加载和写回磁盘。我确保将所有内容都包含在这个文件中，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）：在此处输入图像描述 当我将其另存为 24 时，它完美保存。当我尝试将其保存为 32 时，它看起来像：

在此处输入图像描述

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1 回答 1

2

尝试对您的源代码进行以下修改。未测试！
在您的Load函数中更改此：

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

对此：

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

在您的加载和保存功能中更改此：

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

对此：

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

在您的保存功能更改此：

png_write_png(PngPointer, InfoPointer, PNG_TRANSFORM_IDENTITY, NULL);

对此：

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

并在您的SetBitsPerPixel函数中更改此行：

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

对此：

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

实际上，我不确定您为什么要这样做，因为据我所知，24 和 32 bpp 图像的位深度应为8 位。

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，您可能需要进行一些额外的修改。关键是您应该始终将像素保存在 BGRA 缓冲区中（无论您加载没有 alpha 通道的索引、灰度或 RGB 图像）。

于 2012-12-24T01:48:02.217 回答