SDL code should greyscale but just loads the image normally - image-processing

Question

The code below should load an image, then grey scale the image in a window. Instead it just loads the image. I've used printf("hello") in the loop starting with "for (int y = 0; y < image->h; y++)" however the console doesn't show "hello", unless I removed SDL_Delay(20000) which makes the console print it, but the image flashes for a second and i cant tell if that's in greyscale of the same image.
#include <SDL2/SDL.h>
#include <SDL2/SDL_image.h>
#include <stdio.h>
#include "SDL2/SDL_ttf.h"
SDL_Window *window = NULL;
SDL_Surface *windowSurface = NULL;
SDL_Surface *image = NULL;
SDL_Event *event = NULL;
SDL_Texture *texture = NULL;
int main(int argc, char *argv[])
{
if(SDL_Init(SDL_INIT_VIDEO) < 0)
{
perror("Cannot initialise SDL");
SDL_Quit();
return 1;
}
else
{
window = SDL_CreateWindow("Loading_image", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 640, 480, SDL_WINDOW_SHOWN | SDL_WINDOW_RESIZABLE);
if(window == NULL)
perror("Cannot load image");
else
{
windowSurface = SDL_GetWindowSurface(window);
image = IMG_Load("image.bmp");
if(image == NULL)
perror("Cannot load image");
else
{
SDL_BlitSurface(image, NULL, windowSurface, NULL);
}
SDL_UpdateWindowSurface(window);
SDL_Delay(20000);
}
}
SDL_UpdateTexture(texture, NULL, image->pixels, image->w * sizeof(Uint32));
image = SDL_ConvertSurfaceFormat(image,SDL_PIXELFORMAT_ARGB8888,0);
Uint32 * pixels = (Uint32 *)image->pixels;
int x = 0;
int y = 0;
for (int y = 0; y < image->h; y++)
{
for (int x = 0; x < image->w; x++)
{
Uint32 pixel = pixels[y * image->w + x];
Uint8 r=0,g=0,b=0;
SDL_GetRGB(pixel, image->format, &r,&g,&b);
Uint8 v = 0.212671f * r + 0.715160f * g + 0.072169f * b;
SDL_MapRGB(image->format,v,v,v);
}
}
int quit = 0;
while (!quit) //This loop will loop until the conditions are met e.g. You quit the renderer//
{
SDL_WaitEvent(event);// waits for the event (quitting the renderer)//
switch (event->type)
{
case SDL_QUIT:
quit = 1;
break;
}
}
SDL_FreeSurface(image);
image = NULL;
window = NULL;
windowSurface = NULL;
SDL_DestroyWindow(window);
IMG_Quit();
SDL_Quit();
return 0;
}

Answer 1

There are several issues with your code. Mostly SDL specifics, but also some issues with the grayscale conversion.
I removed any unnecessary stuff I could spot and annotated some changes by comments.
#include <SDL.h>
#include <SDL_image.h>
#include <stdio.h>
SDL_Window *window = NULL;
SDL_Surface *windowSurface = NULL;
SDL_Surface *image = NULL;
SDL_Event event; // You may want to use an object instead of a pointer
SDL_Texture *texture = NULL;
int main(int argc, char *argv[])
{
if (SDL_Init(SDL_INIT_VIDEO) < 0)
{
perror("Cannot initialise SDL");
SDL_Quit();
return 1;
}
else
{
window = SDL_CreateWindow("Loading_image", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 640, 480, SDL_WINDOW_SHOWN | SDL_WINDOW_RESIZABLE);
if (window == NULL)
perror("Cannot load image"); // You may want to change this error message
else
{
windowSurface = SDL_GetWindowSurface(window);
image = IMG_Load("image.bmp");
if (image == NULL)
perror("Cannot load image");
// I removed the blitting code here, you basically don't need it here
// Rather do it in the render loop below
}
}
image = SDL_ConvertSurfaceFormat(image, SDL_PIXELFORMAT_ARGB8888, 0);
Uint32 * pixels = (Uint32 *)image->pixels;
int x = 0;
int y = 0;
for (int y = 0; y < image->h; y++)
{
for (int x = 0; x < image->w; x++)
{
Uint32 pixel = pixels[y * image->w + x];
Uint8 r = 0, g = 0, b = 0;
SDL_GetRGB(pixel, image->format, &r, &g, &b);
Uint8 v = 0.212671f * r + 0.715160f * g + 0.072169f * b;
pixel = SDL_MapRGB(image->format, v, v, v); // Get the return value which is the pixel value
pixels[y * image->w + x] = pixel; // ...and assign it back to the pixels
}
}
int quit = 0;
while (!quit)
{
while (SDL_PollEvent(&event)) // Continous checking for events
{
switch (event.type)
{
case SDL_QUIT:
quit = 1;
break;
}
}
// "Render loop"
SDL_BlitSurface(image, NULL, windowSurface, NULL);
SDL_UpdateWindowSurface(window);
}
SDL_FreeSurface(image);
image = NULL;
window = NULL;
windowSurface = NULL;
SDL_DestroyWindow(window);
IMG_Quit();
SDL_Quit();
return 0;
}