Agfdhyk

I know this question is very noob. I am trying to understand how the pointer thing works. I studied basics of C but still did not understand this.

Given this piece of function:

+ (void)nv21ToRgbWithWidth:(unsigned int)width height:(unsigned int)height yuyv:(unsigned char *)yuyv rgb:(unsigned char *)rgb

{

    const int nv_start = width * height ;

    UInt32 i, j, index = 0, rgb_index = 0;

    UInt8 y, u, v;

    int r, g, b, nv_index = 0;



    for(i = 0; i <  height ; i++)

    {

        for(j = 0; j < width; j ++){

            //nv_index = (rgb_index / 2 - width / 2 * ((i + 1) / 2)) * 2;

            nv_index = i / 2  * width + j - j % 2;



            y = yuyv[rgb_index];

            u = yuyv[nv_start + nv_index ];

            v = yuyv[nv_start + nv_index + 1];





            r = y + (140 * (v-128))/100;  //r

            g = y - (34 * (u-128))/100 - (71 * (v-128))/100; //g

            b = y + (177 * (u-128))/100; //b



            if(r > 255)   r = 255;

            if(g > 255)   g = 255;

            if(b > 255)   b = 255;

            if(r < 0)     r = 0;

            if(g < 0)     g = 0;

            if(b < 0)     b = 0;



            index = rgb_index % width + (height - i - 1) * width;

            rgb[index * 3+0] = b;

            rgb[index * 3+1] = g;

            rgb[index * 3+2] = r;

            rgb_index++;

        }

    }

}

How am I suppose to know how the unsigned char * for rgb should be initialized before passing in to the function?

I tried calling the function like this:

unsigned char *rgb = NULL;



[MyClass nv21ToRgbWithWidth:imageWidth height:imageHeight yuyv:yuyvValues rgb:rgb];

But the the program crashes on this line:

rgb[index * 3+0] = b;

I see rgb was initialized with NULL, so you can't assign values. So, I thought of initializing an array and pass it to pointer rgb like this:

unsigned char rgbArr[10000];

unsigned char *rgb = rgbArr;

but the function still crashes. I really don't know how should I pass the rgb parameter in this function. Please help me understand this.

The expected size in bytes seems to be at least height*width*3; it might be that allocating such an array as a local variable (as you do with unsigned char rgbArr[10000]) exceeds a stack limit; The program likely crashes in such a case. I'd try to use the heap instead:

unsigned char* rgb = malloc(imageHeight*imageWidth*3);

[MyClass nv21ToRgbWithWidth:imageWidth height:imageHeight yuyv:yuyvValues rgb:rgb];

...

free(rgb);

That is what the malloc(), calloc(), realloc() and free() functions are for. Don't forget to use the free() function to prevent memory leaks... I hope that helps.