改变颜色没有用C

Question

用“graphics.h中”库我需要学习这些东西，为了通过考试，以便 我想这个代码，但没有奏效。我怎样才能使它工作？

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

（ 'img_header.h' 包含了一些功能）

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); 
} 

---

我就与指针直接处理，但不成功！该代码仍然生成9像素位图图像，颜色（红色= 205，蓝色= 205，绿色= 205），这似乎是一个奇怪的结果，因为我在编译代码时，会打印出以下代码：

blue = 0 
green = 72 
red = 45 

blue = 255 
green = 0 
red = 0 

Press any key to continue . . . 

而且代码：

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，你是要修改局部变量red，​​，blue。 img内没有反映这些chages。

相反，摆脱这些局部变量和直接处理指针，就像

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，如果你做

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

这将反映在img。

这就是说，请do not cast返回值malloc()和家人在C。