在不使用C语言的'graphics.h'库的情况下更改像素的颜色

Question

为了通过考试，我需要学习这些内容，所以我尝试了此代码，但没有用。 我如何使它工作？

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <math.h>
#include "img_header.h"   

（“ img_header.h”包含一些功能）

void simple_rgb_image_init(Simple_RGB_Image* sink, int32_t  width, int32_t  height);


typedef struct {
int32_t width;
int32_t height;
uint8_t* data;
} Simple_RGB_Image;


int main()
{

Simple_RGB_Image img;
int32_t width = 3;
int32_t height = 3;
FILE* out_file;

int32_t w;
int32_t x,y ;

uint8_t red,green,blue;

uint8_t* p_red;
uint8_t* p_green;
uint8_t* p_blue;

p_red   = &red;
p_green = &green;
p_blue  = &blue;

simple_rgb_image_init(&img,width,height);  

x = 1 ;
y = 1 ;
w = calculate_stride(width);   //calculate the stride

blue  = img.data[3 *(w*y + x) + 0];
green = img.data[3 *(w*y + x) + 1];
red   = img.data[3 *(w*y + x) + 3];

printf("blue = %i \n" , blue);  //205
printf("green = %i \n" , green);//205
printf("red = %i \n" , red);    //205

printf("\n\n");

*p_red   = 0;
*p_green = 0;
*p_blue  = 255;

printf("blue = %i \n" , blue);  //255
printf("green = %i \n" , green);//0
printf("red = %i \n" , red);    //0


out_file = fopen("My_picture.bmp","wb");
simple_rgb_image_to_bitmap_stream(&img,out_file); //save the picture as a Bitmap file
fclose(out_file);
simple_rgb_image_clear(&img); //Free memory



return 0;
}


void simple_rgb_image_init(Simple_RGB_Image* sink, int32_t  width, int32_t  height)
{
sink->width = width;
sink->height = height;
sink->data = (uint8_t*)malloc(3 * width * height);
}

---

我确实直接处理过指针，但是徒劳！ 该代码仍在生成一个9像素的位图图像，其颜色为（红色= 205，蓝色= 205，绿色= 205），当我编译代码时，这似乎是一个奇怪的结果，它会打印出以下内容：

blue = 0
green = 72
red = 45 

blue = 255
green = 0 
red = 0 

Press any key to continue . . . 

和代码是：

p_blue  = &(img.data[3 *(w*y + x) + 0]);
p_green = &(img.data[3 *(w*y + x) + 1]);
p_red   = &(img.data[3 *(w*y + x) + 2]);

printf("blue = %i \n" , *p_blue);  
printf("green = %i \n" , *p_green);
printf("red = %i \n" , *p_red);    

printf("\n\n");

*p_red   = 0;
*p_green = 0;
*p_blue  = 255;

printf("blue = %i \n" , *p_blue);  
printf("green = %i \n" , *p_green);
printf("red = %i \n" , *p_red);    

Answer 1

这里的问题是，您正在更改局部变量red ， green ， blue 。 这些变化没有反映在img内部。

相反，摆脱这些局部变量并直接处理指针，例如

p_blue  = &(img.data[3 *(w*y + x) + 0]);
p_green = &(img.data[3 *(w*y + x) + 1]);
p_red   = &(img.data[3 *(w*y + x) + 3]);  //are you sure, this is 3. not 2?

然后，如果您这样做

*p_red   = 0;
*p_green = 0;
*p_blue  = 255;

它将反映在img 。

也就是说，请不要在C中malloc()和family的返回值。