LibPng背景

Question

好的，在photoshop里，我创建了一个8位的彩色图像，有一个透明的背景和一些文本。然后，我创建了一个16位颜色的图像，背景透明，还有一些文字。

当我右键单击两个图像并转到属性时，它显示了两个图像的32位深度:l，因此我决定使用以下设置使用LibPng读取图像：

typedef union RGB     //Holds all the pixels..
{
    uint32_t Color;
    struct
    {
        unsigned char B, G, R, A;
    } RGBA;
} *PRGB;


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

    switch (colortype)
    {
        case PNG_COLOR_TYPE_GRAY:
        {
            png_set_expand_gray_1_2_4_to_8(PngPointer);
            png_set_expand(PngPointer);
            png_set_bgr(PngPointer);
            break;
        }

        case PNG_COLOR_TYPE_PALETTE:
        {
            png_set_palette_to_rgb(PngPointer);
            if (png_get_valid(PngPointer, InfoPointer, PNG_INFO_tRNS))
                png_set_tRNS_to_alpha(PngPointer);
            png_set_filler(PngPointer, 0xFF, PNG_FILLER_AFTER);
            png_set_bgr(PngPointer);
        BitsPerPixel = 24;
            break;
        }

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

        case PNG_COLOR_TYPE_RGB:
        {
            png_set_bgr(PngPointer);
            png_set_filler(PngPointer, 0xFF, PNG_FILLER_AFTER);
            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;
    }

    //SET BACKGROUND
    /*png_color_16 my_background, *image_background;
    my_background.red = 255;
    my_background.green = 255;
    my_background.blue = 255;
    image_background = &my_background;

    if (png_get_bKGD(PngPointer, InfoPointer, &image_background))
    {
        png_set_filler(PngPointer, 0xFF, PNG_FILLER_AFTER);
        png_set_background(PngPointer, image_background, PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
    }
    else
    {
        png_set_filler(PngPointer, 0xFF, PNG_FILLER_AFTER);
        png_set_background(PngPointer, &my_background, PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
    }*/
    //END SET BACKGROUND

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

我的保存设置是：

png_set_write_fn(PngPointer, reinterpret_cast<void*>(&hFile), WriteToStream, nullptr);
    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);
    png_write_info(PngPointer, InfoPointer);

    png_set_bgr(PngPointer);
    png_set_packing(PngPointer);
    png_set_interlace_handling(PngPointer);
    if (colortype == PNG_COLOR_TYPE_RGB) png_set_filler(PngPointer, 0xFF, PNG_FILLER_AFTER);

巴布亚新几内亚的情况如下：

​

当我通过C++将其保存为位图时，它看起来像是(8位一位这样做)：

​

把它保存回PNG，它看起来和原来的一模一样。8岁和16岁的人都能省回罚款。

问题是，如果我取消注释"SetBackground“节，它可以完美地保存为位图，但当我将其保存回PNG时，由于代码将其设置为白色(255、255、255)，背景不再透明。

我怎么才能解决这个问题？

Answer 1

我对你的代码做了一些修改。未测试！

channels = png_get_channels(PngPointer, InfoPointer);
png_get_IHDR(PngPointer, InfoPointer, &width, &height, &bitdepth, &colortype, &interlacetype, nullptr, nullptr);
png_set_interlace_handling(PngPointer);
png_set_strip_16(PngPointer);
png_set_packing(PngPointer);
bool BGneeded = (colortype > PNG_COLOR_TYPE_PALETTE); 
switch (colortype)
{
    case PNG_COLOR_TYPE_GRAY:
    {
        png_set_expand_gray_1_2_4_to_8(PngPointer);
        //png_set_expand(PngPointer);
        //png_set_bgr(PngPointer);  //R=G=B so there's no need to swap RGB values
        png_set_gray_to_rgb(PngPointer);
        break;
    }

    case PNG_COLOR_TYPE_PALETTE:
    {
        png_set_palette_to_rgb(PngPointer);
        if (png_get_valid(PngPointer, InfoPointer, PNG_INFO_tRNS))
        {
            png_set_tRNS_to_alpha(PngPointer);
            BGneeded = true;
        }
        else
            png_set_filler(PngPointer, 0xFF, PNG_FILLER_AFTER);
        png_set_bgr(PngPointer);
        BitsPerPixel = 32;
        break;
    }

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

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

//SET BACKGROUND
png_color_16 my_background;
png_color_16p image_background;

my_background.red = 255;
my_background.green = 255;
my_background.blue = 255;

if (png_get_bKGD(PngPointer, InfoPointer, &image_background))
    png_set_background(PngPointer, image_background, PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
else if (BGneeded)
    png_set_background(PngPointer, &my_background, PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
//END SET BACKGROUND

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