将 RGB 值转换为二进制到十进制

Question

I have the following java code, which takes three double values (between 0 and 1) of the colors RGB and converts them to decimal format. I understood how the first 8 bit save color x, the second 8 bit color y... and also how to get the decimal value from the resulting binary. what i dont understand 100% is why we multiply with 255 (i know 128+64+32+16+8+4+2+1). What exactly do we get from multiplying the double value with 255. is it a value which can be stored in 8 bit? And why dont we use 256 (possible amount of one color)?

public final double getR() {
    return (1 - cyan);
}

public final double getG() {
    return (1 - magenta);
}

public final double getB() {
    return (1 - yellow);
}

/**
 * Gets the rgb color in one integer.
 * 
 * @return an integer containing the red component in bits 16-23, the green component in bits 8-15
 *            and the blue component in bits 0-7. Bits 24-32 are zero.
 */
public int getRGB() {
    int r = (int) Math.round(getB() * 255);
    r |= (int) Math.round(getR() * 255) << 16;
    r |= (int) Math.round(getG() * 255) << 8;
    return r;
}

Thanks

Answer 1

You need the conversion from double because you cannot store the double value into 8 bit.

Your double values are between 0.0 and 1.0 . You can see them as the proportion of color used (eg 0.33333333 in yellow means that one-third of the possible yellow is used). As you can see such a double can have many decimal places, which means we need a lot of memory (64-bit) to store such a color.

Your function now tries to store the double value into only 8 bit (256 values). As we said the double value can be seen as a portion (between 0 and 1) and the function calculates the same for 8 bit (between 0 and 255). This can simply be done by multiplying the double value with 255. For the example with yellow (0.33333333 of yellow is used) it is: 0.33333333 * 255 = 84,99999915 . The meaning is still the same 84,99999915 yellow parts of 255 yellow parts are used, which is still a third. In order to get a compressed version of this number, it is rounded to the next integer value. In our example, this is 85 , which is really close to the actual portion, but we save a lot of memory.

It makes also sense for the lowest double value 0.0 , which is converted to the lowest int value 0 . The highest double value 1.0 is converted to 255 (highest 8-bit number).

In conclusion, we convert a double (64-bit) into an only 8-bit number, which has the same proportion of the color, but it is not as accurate.

Edit: As there is also a confusion with the 255: 8 bit can store 256 values (0 to 255). If you can choose 256 as a color value somewhere they use the range 1-256 without 0. Essentially it is the same one shifted by 1.