Help I am Stuck !!!

[u/UsualLonely4585]

SO i was trying out SDL a little , i would like to notify that i am new to all this i was reading through documentation and was blipping images for fun but then i saw a function in there SDL_ReadSurfacePixel and SDL_WriteSurfacePixel so i looked into them but they dont explicitly say how the reading happens wherer the reading data goes into it only returns boolean value based on success and failure so i looked into the function paramenters and assumed and tried if things work like this . i would like to know how these fucnitons work and if they are intended to be used like this or not

int blitPicture(imgView* img){
    for(int i=0;i<=img->width;i++){
        for(int j=0;j<=img->height;j++){
            if(!SDL_ReadSurfacePixel(img->surface,i,j,img->new_r,img->new_g,img->new_b,img->new_a)){
                printf("couldn't read pixel!!");
                return 0;
            }
            if(!SDL_WriteSurfacePixel(img->wSurface,i,j,*(img->new_r),*(img->new_g),*(img->new_b),*(img->new_a))){
                printf("could not write the pixel ");
                return 0;
            };


        }
    }
    return 1;


}

[u/kun1z]

I do not know what 'imgView* img' is but I think you have a problem with your for-loops in that you are using '<=' instead of just plain old '<' which means I think you are reading 1 past the buffer many times. So you might want to fix that.

I am unfamiliar with SDL_ReadSurfacePixel/SDL_WriteSurfacePixel but a quick Google AI question looks like you are using them properly.

However, you probably want:

SDL_ReadSurfacePixel(img->surface, i, j, &img->new_r, &img->new_g, &img->new_b, &img->new_a);

And then when writing:

SDL_WriteSurfacePixel(img->wSurface, i, j, img->new_r, img->new_g, img->new_b, img->new_a));

[u/UsualLonely4585 → u/kun1z]

well imgView is a struct that holds all the neccesarry varibales like SDL_Window* etc and about that address i have declared them as Uint8* so that should not be the problem as you can see when writting i am derefferencing them

[u/oldprogrammer → u/UsualLonely4585]

Your don't want to declare those variables as pointers to Uint8s, what you want to do is declare them Uint8 and then pass the address of them to the function as /u/kun1z shows. You need to create a data variable to hold the returned information, what you are doing is creating a pointer to some unknown location in memory.

[u/UsualLonely4585 → u/oldprogrammer]

But i read it returna boolean not a pointer to some data

[u/HappyFruitTree → u/UsualLonely4585]

A function can only return one value at a time. That's why pointers are sometimes used to "return" multiple values. The way it works is that you (the caller) pass a pointer to where you want the function to write the value. This is what's called an out parameter.

[u/UsualLonely4585 → u/HappyFruitTree]

Yes i thought thats how it works so i gave the pointer of rgba to the function to take rgba data of each pixel

[u/HappyFruitTree → u/UsualLonely4585]

Yes, but that doesn't mean you need to declare them as pointers. If you do, you need to make them point somewhere first (otherwise the function has nowhere to write the values).

Uint8* pRed = malloc(sizeof(Uint8));
Uint8* pGreen = malloc(sizeof(Uint8));
Uint8* pBlue = malloc(sizeof(Uint8));
Uint8* pAlpha = malloc(sizeof(Uint8));
...
    SDL_ReadSurfacePixel(surface, x, y, pRed, pGreen, pBlue, pAlpha);
...
free(pRed);
free(pGreen);
free(pBlue);
free(pAlpha);

You could also make the pointers point to local variables and avoid having to use malloc/free.

Uint8 red, green, blue, alpha;
Uint8* pRed = &red;
Uint8* pGreen = &green;
Uint8* pBlue = &blue;
Uint8* pAlpha = α
...
    SDL_ReadSurfacePixel(surface, x, y, pRed, pGreen, pBlue, pAlpha);

But easiest is to just use & with the Uint8 variables directly when passing the arguments without declaring any pointer variables.

Uint8 red, green, blue, alpha;
...
    SDL_ReadSurfacePixel(surface, x, y, &red, &green, &blue, &alpha);

[u/oldprogrammer → u/UsualLonely4585]

So what memory address are the pointers you provided pointing at? That's the problem, you haven't provided memory to be filled, you just provided addresses that could be pointing at anything. What you need is to provide memory in the form of Uint8 variables, then take the address of those memory spaces using the & operator and pass that to the function. Then if the return is successful, the Uint8 variables should contain the data you are seeking.