如何使用 stbi_write_png 将图像像素数据写入 png 文件

Question

所以我在一个周末一直在跟踪光线追踪，最初是将所有像素数据写入 ppm 文件，并且只是在尝试写入不同的图像文件格式。在互联网上的一些帮助下，我能够写入 bmp 文件，目前我正在尝试将像素数据写入 png。我一直在尝试使用 stbi_write_png 函数，但创建的结果图像与所需的完全不同

我将在下面详细说明 bmp 部分和 png 以及生成的图像的代码 实现。

这是bmp写入的代码

#include<iostream>
#include<stdint.h>
#include<fstream>
#include<random>
#include "hitableList.h"
#include "sphere.h"
#include "camera.h"
#include "material.h"
#include <float.h> //for float_MAX
#include "main.h" //contains our typedef declarations, #defines and struct declarations.
typedef uint8_t u8;
typedef uint16_t u16;
typedef uint32_t u32;
typedef uint64_t u64;

typedef int8_t s8;
typedef int16_t s16;
typedef int32_t s32;
typedef int64_t s64;
typedef float f32;

#define STBI_MSC_SECURE_CRT
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image_write.h"


#define CHANNEL_NUM 3

internal u32 GetTotalPixelSize(image_32 Image)//take image_32 Image
{
    u32 Result = Image.Width*Image.Height * sizeof(u32);
    return(Result);
}

//Function defintion to allocate image size
internal image_32 AllocateImage(u32 width, u32 height)
{
    image_32 Image = {};//create the image object and initialize it.
    Image.Height = height;
    Image.Width = width;
    u32 OutputPixelSize = GetTotalPixelSize(Image);//old version of the code does this line->sizeof(u32)*image.Width*image.Height;
    Image.Pixels = (u32*)malloc(OutputPixelSize);//main source of the initial nullpointer at main *Out.
    return(Image);
}

internal void WriteImage(image_32 Image, const char* OutputFileName)
{
    u32 OutputPixelSize = GetTotalPixelSize(Image);
    bitmap_header Header = {};

    Header.FileType = 0x4D42;
    Header.FileSize = sizeof(Header) + OutputPixelSize;//Need to set it later
                                                       //Header.Reserved1;//These are reserved and set by the header itself
                                                       //Header.Reserved2;//These are reserved and set by the header itself
    Header.BitmapOffset = sizeof(Header);
    Header.Size = sizeof(Header) - 14;//also need to set the size of the pixels. Since the header is 50 bytes check wikipedia.
    Header.Width = Image.Width;
    Header.Height = Image.Height;
    Header.Planes = 1;
    Header.BitsPerPixel = 32;
    Header.Compression = 0;
    Header.SizeOfBitmap = OutputPixelSize;//writing bottom part first. Very Important.
    Header.HorzResolution = 0;
    Header.VertResolution = 0;
    Header.ColorsUsed = 0;
    Header.ColorsImportant = 0;
FILE *OutFile = fopen(OutputFileName, "wb");

    if (OutFile)
    {
        fwrite(&Header, sizeof(Header), 1, OutFile);//we write it into the header
        fwrite(Image.Pixels, OutputPixelSize, 1, OutFile);
        fclose(OutFile);
    }

    else
    {
        fprintf(stderr, "[Error]Unable to write output file %s. \n", OutputFileName);
    }
}

vec3 color(const ray& r, hitable *world, int depth)
{
    hit_record rec;
    if(world->hit(r, 0.001, FLT_MAX, rec)){
        ray scattered;
        vec3 attenuation;
        if(depth <50 && rec.mat_ptr->scatter(r,rec,attenuation,scattered)){
            return attenuation*color(scattered, world, depth+1);
        }
        else{
            return vec3(0,0,0);
        }
    }
    else{
        vec3 unit_direction = unit_vector(r.direction());
        float t = 0.5*(unit_direction.y() + 1.0);
        return (1.0-t)*vec3(1.0, 1.0, 1.0) + t*vec3(0.5, 0.7, 1.0);
    }
}

int main()
{    
    printf("Raycasting......");
    /*
    int nx=1280;
    int ny =720;
    */
    u32 ns = 10;
    u32 width = 1280;
    u32 height = 720;
hitable *list[5];
    list[0] = new sphere(vec3(0,0,-1), 0.5, new lambertian(vec3(0.8, 0.3, 0.3)));
    list[1] = new sphere(vec3(0,-100.5,-1), 100, new lambertian(vec3(0.8, 0.8, 0.0)));
    list[2] = new sphere(vec3(1,0,-1), 0.5, new metal(vec3(0.8, 0.6, 0.2), 0.3));
    list[3] = new sphere(vec3(-1,0,-1), 0.5, new dielectric(1.5));
    list[4] = new sphere(vec3(-1,0,-1), -0.45, new dielectric(1.5));
    hitable *world = new hitable_list(list,4);
    camera cam;
    //u32 *Out = Image.Pixels;
    u8* pixels = new u8[width * height * CHANNEL_NUM];

    u32 index =0;
    for(u32 y=0 ; y<height; y++)
    {
        for(u32 x=0; x<width; x++)
        {
            vec3 col(0, 0, 0);

            for(u32 s=0; s < ns; s++)
            {
                float u = float(x+drand48())/float(width);
                float v = float(y+drand48())/float(height);
                ray r = cam.get_ray(u, v);
                vec3 p = r.point_at_parameter(2.0);
                col = col + color(r, world, 0); //col returns a vec3
            }
            col/=float(ns);//average sampling per pixel


           vec3 BMPColor = vec3(255*col); //getting bmp color values from raytraced image.
           u32 BMPvalue = BGRPack4x8(BMPColor); //packing the bmp color into an integer to write to the bitmap image.

           *Out++ = BMPvalue;


           if((y%64) ==0)
        {
            printf("\rRaycasting row %d%%....",100*y / height);
            fflush(stdout);
        }
}

         WriteImage(Image, "..\\data\\Hollow_Glass_Sphere.bmp");//getting the raytraced image plane on test.bmp.

         printf("\nDone.....\n");

return 0;
    }

#include<iostream>
#include<stdint.h>
#include<fstream>
#include<random>
#include "hitableList.h"
#include "sphere.h"
#include "camera.h"
#include "material.h"
#include <float.h> //for float_MAX
#include "main.h" //contains our typedef declarations, #defines and struct declarations.
typedef uint8_t u8;
typedef uint16_t u16;
typedef uint32_t u32;
typedef uint64_t u64;

typedef int8_t s8;
typedef int16_t s16;
typedef int32_t s32;
typedef int64_t s64;
typedef float f32;

#define STBI_MSC_SECURE_CRT
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image_write.h"


#define CHANNEL_NUM 3

vec3 color(const ray& r, hitable *world, int depth)
{
    hit_record rec;
    if(world->hit(r, 0.001, FLT_MAX, rec)){
        ray scattered;
        vec3 attenuation;
        if(depth <50 && rec.mat_ptr->scatter(r,rec,attenuation,scattered)){
            return attenuation*color(scattered, world, depth+1);
        }
        else{
            return vec3(0,0,0);
        }
    }
    else{
        vec3 unit_direction = unit_vector(r.direction());
        float t = 0.5*(unit_direction.y() + 1.0);
        return (1.0-t)*vec3(1.0, 1.0, 1.0) + t*vec3(0.5, 0.7, 1.0);
    }
}

int main()
{    
    printf("Raycasting......");
    /*
    int nx=1280;
    int ny =720;
    */
    u32 ns = 10;
    u32 width = 1280;
    u32 height = 720;

    hitable *list[5];
    list[0] = new sphere(vec3(0,0,-1), 0.5, new lambertian(vec3(0.8, 0.3, 0.3)));
    list[1] = new sphere(vec3(0,-100.5,-1), 100, new lambertian(vec3(0.8, 0.8, 0.0)));
    list[2] = new sphere(vec3(1,0,-1), 0.5, new metal(vec3(0.8, 0.6, 0.2), 0.3));
    list[3] = new sphere(vec3(-1,0,-1), 0.5, new dielectric(1.5));
    list[4] = new sphere(vec3(-1,0,-1), -0.45, new dielectric(1.5));
    hitable *world = new hitable_list(list,4);
    camera cam;
    //u32 *Out = Image.Pixels;
    u8* pixels = new u8[width * height * CHANNEL_NUM];

    u32 index =0;
    for(u32 y=0 ; y<height; y++)
    {
        for(u32 x=0; x<width; x++)
        {
            vec3 col(0, 0, 0);

            for(u32 s=0; s < ns; s++)
            {
                float u = float(x+drand48())/float(width);
                float v = float(y+drand48())/float(height);
                ray r = cam.get_ray(u, v);
                vec3 p = r.point_at_parameter(2.0);
                col = col + color(r, world, 0); //col returns a vec3
            }
            col/=float(ns);//average sampling per pixel

            col = vec3( sqrt(col[0]), sqrt(col[1]), sqrt(col[2]));
            int ir = int(255.99*col[0]);
            int ig = int(255.99*col[1]);
            int ib = int(255.99*col[2]);

            pixels[++index] = ir;
            pixels[++index] = ig;
            pixels[++index] = ib;


            if((y%64) ==0)
        {
            printf("\rRaycasting row %d%%....",100*y / height);
            fflush(stdout);
        }
    }

    stbi_write_png("testpng_4.png", width, height, CHANNEL_NUM, pixels, width*CHANNEL_NUM);

    printf("\nDone.....\n");

    return 0;
}

这是对应于 bmp 写入代码的 BMP 图像，基于我在周末书籍中阅读和实现的光线追踪，这是正确的所需图像

这是与 png 编写代码对应的 PNG 图像，看起来图像在颜色和方向上被翻转了。我已尝试调试此图像中可能存在的问题，但失败了。想到的唯一想法是它是否是字节顺序问题。如果有人能帮我解决 png 写入的这个问题，我会非常高兴，如果这是一个字节顺序问题，我该如何解决它。

Answer 1

这是从 RGB 转换为 BRG 时的输出：

1. 图像应在 y 轴上翻转。而不是循环为：

for(u32 y=0; y < height; y++)

应该是这样的：

for (u32 y = height - 1; y >= 0; y--)

1. ir、ig 和 ib 值的类型为 int；但是，像素缓冲区和stbi_write_png 函数的输入在您的情况下需要unsigned char, u8。将它们乘以 255.9 可能会产生远高于 255 的值，您可以考虑将它们限制在 [0, 255] 之间，然后再将它们转换为 unsigned char. 您可以在此处查阅类似的讨论：stb image write issue

2. 虽然将像素顺序从 RGB 更改为 BRG 可以解决问题，但您可能还需要考虑为什么图像整体看起来更亮。这很可能是因为 sqrt 对 col 值的操作，这将返回比原始 col 值更高的值，因为 cols 是小数。在多重采样中，将总和除以样本数就足够了。如果这些不起作用，我会检查 stbi_write_png 是否仍在编写 Alpha 通道，或者您的观众将其中一个通道视为 Alpha 通道，这是最不可能的。

这是RGB通道改为BRG时的输出，图像垂直翻转，每一列都是平方的，看起来是正确的：