我需要学习这些东西才能通过考试，所以
我尝试了这段代码，但是没有用。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);
最佳答案
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中。也就是说，请do not cast
malloc()的返回值和C中的family。