What Does Image.new(“L”, (width, height)) Do in Python? (Explained with LSB Recovery)

[Rajeev Bagra]

ߓ Context (Code First)

When recovering a hidden black & white image from the LSB (least significant bit) of another image, we often use this code:

from PIL import Image

def reveal_bw_image(filename):
img = Image.open(filename)
width, height = img.size
pixels = img.load()

secret_img = Image.new("L", (width, height))
secret_pixels = secret_img.load()

for x in range(width):
for y in range(height):
pixel = pixels[x, y]
hidden_bit = extract_end_bits(1, pixel)
secret_pixels[x, y] = hidden_bit * 255

return secret_img

This raises a common question:

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✅ Q&A: What Does Image.new("L", (width, height)) Create?

❓ Q1: On what basis is this new image created?

secret_img = Image.new("L", (width, height))

✅ Answer:
The new image is created based on:

1. Mode "L" → meaning the image is grayscale
2. Size (width, height) → meaning the image has the same dimensions as the carrier image

So it becomes a blank grayscale canvas where we will build the recovered hidden image.

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❓ Q2: What will be its pixel value at each point initially?

✅ Answer:
When you create an image like this (without specifying a color), Pillow fills it with:

0 everywhere

That means every pixel starts as:

• 0 → black

So initially, the secret image is a completely black image.

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❓ Q3: Why does the code create a blank black image first?

✅ Answer:
Because we need an empty image to write the recovered hidden pixels into.

The loop extracts hidden bits from the original image and replaces pixels in the secret image:

secret_pixels[x, y] = hidden_bit * 255

So during the loop, pixels change from the default 0 to:

• 0 (black) if hidden bit = 0
• 255 (white) if hidden bit = 1

That’s how the hidden image gets drawn.

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❓ Q4: What does this line do?

secret_pixels = secret_img.load()

✅ Answer:
This gives you access to the pixels of the new image, so you can change pixel values directly like:

secret_pixels[x, y] = 255

Without .load(), you wouldn’t be able to set pixels easily.

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✅ Final Takeaway

Image.new("L", (width, height)) creates a blank grayscale image of the same size as the original, and its pixels start as 0 (black) until your code fills them with the recovered hidden image data.

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