将 RGB 的值限制为 255

Question

I'm working on CS50 pset4 sepia filter and trying to cap the RGB value at 255 with my cap function, but it seems still doesn't working. Can anyone advice me where to look and how to fix without spoiling my academic honesty commitment?

I've used unsigned char because ide gives error otherwise. Also doesn't accept BYTE as a type. I've tried to cast it to integer but it gives error too. Maybe my casting was not correctly. I'll appreciate if you want to add a brief explaination for me to understand my mistake so I could be more efficient in my future code regarding this issue.

This my ap function:

unsigned char cap(unsigned char a)
{
    if (a > 255)
    {
        a = 255;
    }
    return a;
}

and this the code in sepia function (in the nested loops)

unsigned char x = image[i][j].rgbtRed;
unsigned char y = image[i][j].rgbtGreen;
unsigned char z = image[i][j].rgbtBlue;

image[i][j].rgbtRed = cap(round(x * 0.393 + y * 0.769 + z * 0.189));
image[i][j].rgbtGreen = cap(round(x * 0.349 + y * 0.686 + z * 0.168));
image[i][j].rgbtBlue = cap(round(x * 0.272 + y * 0.534 + z * 0.131));

Answer 1

unsigned char as the type of input parameter excludes values > 255 (if bytes are 8 bits as they commonly are). Just make your function accept an int or float or double . Likewise you should probably clamp negative numbers to zero, though they won't occur in this calculation but might occur in some other.

unsigned char cap(int a)
{
    if (a > 255) {
        return 255;
    }
    if (a < 0) {
        return 0;
    }

    return a;
}

---

As for BYTE , you declare a typedef:

typedef unsigned char BYTE;

and use BYTE thereafter.

Answer 2

Change the cap() function to be more like:

unsigned char cap(unsigned int a)

The problem is that the C compiler truncates "larger than 255" values to make them fit in an unsigned char when trying to pass the values to the cap() function.

Alternatively; use fmin() or fminf() instead (eg image[i][j].rgbtBlue = fmin(round(x * 0.272 + y * 0.534 + z * 0.131), 255.0); ).