detect yellow color in image using PIL

Question

I have taken a JPG image using a camera and wish to detect a yellow matchbox in the picture. I was trying to use the Python Imaging Library. I tried to find the RGB combination for yellow, and it turns out that R and G are always 255 for any shade of yellow. In the image however, there are no values of 255. What am I doing wrong? How can I solve this problem? This is my attempted code:

for x in range(1,2592):
    for y in range(1,1456):
        p = im.getpixel((x,y))
        if p[0]>150 and p[1]>150:
            print p

Unfortunately, no coordinates turn up.

Answer 1

Two issues:

• Depending on the image format, the color may be in a 0.0 to 1.0 space.

• (150, 150, 255) , while matching your test would not be considered yellow. In fact it's blue. You should test that red and green are large enough, but also that blue is small enough.

---

Putting those together:

for x in range(1,2592):
    for y in range(1,1456):
        p=im.getpixel((x,y))
        if p[0] > 150 and p[1] > 150 and p[2] < 150: # added p[2] < 150 for blue
            print p, "is yellow"
        else:
            print p, "is not yellow"                 # see what range you have