如何在python中获取图像像素的RGBA颜色值

Question

I'm trying to convert this java code to python:

BufferedImage image;
FileInputStream fstream1 = new FileInputStream("image.png")
image = ImageIO.read(fstream1);
int max = -40000000; //Java rgb returns negative values

for (int i = 0; i < 128; i++)
{
    for (int j = 0; j < 255; j++)
    {
        color = image.getRGB(j, i); //returns integer
.....

I tried this in python:

from PIL import Image

image = Image.open("image.png").convert("RGBA")
pixels = image.load()

for i in range(128):
    for j in range(255):
        color = pixels[j, i] #returns (R, G, B, A);

The problem however is that i'm getting different values in python.

Why does java returns negative integer values and how do i get the same result in python?

Answer 1

This function should convert a colour from the Python format to the Java format:

def convertPixel(c):
    x = c[0] << 16 | c[1] << 8 | c[2] | c[3] << 24
    if x >= 1<<31:
        x -= 1<<32
    return x

Note that Python's format is perfectly sane - it gives you the exact R, G, B and A values directly. It's Java that has the weird format.

You get negative values in Java because of integer overflow - for example 0xFFFFFFFF (or 4294967295), which is pure white, wraps around to -1.

Answer 2

The Java getRGB() value is a signed 32-bit integer with the alpha, R, G and B values in each of the 8 bits from most to least significant bytes.

You can 'reproduce' the same value with:

def packRGBA(r, g, b, a)
    val = a << 24 | r << 16 | g << 8 | b
    if a & 0x80:
        val -= 0x100000000
    return val

color = packRGBA(*pixels[j, i])