Why Does the Pixel Loop Start with x and Not y? (Image Coordinates Explained)

[Rajeev Bagra]

ߓ Context (What this code is doing)

In LSB steganography, a hidden black-and-white image is stored inside the least significant bit (LSB) of each pixel of another “carrier” image. To recover it, we loop through every pixel and rebuild a new secret image like this:

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

This often leads to a beginner question:

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✅ Q&A: Why Does the Loop Start with x and Not y?

❓ Q: Why did we start with x first instead of y?

✅ Answer:
Starting with x first is mainly a coding style choice, not a mathematical requirement.

In images, pixels are accessed using:

pixels[x, y]

So it feels natural to loop in the same order:

• x → horizontal direction (width)
• y → vertical direction (height)

This means we scan the image column-by-column.

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❓ Q: Will the output change if I start with y first?

✅ Answer:
No. The output will be the same as long as you visit every pixel exactly once.

This alternative version also works perfectly:

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

Both loops cover all pixel coordinates, just in a different order.

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❓ Q: So which loop order is better?

✅ Answer:
Both are correct, but here are the typical reasons people choose one order:

✅ Starting with x first

• Matches pixels[x, y] readability
• Easy to understand for beginners

✅ Starting with y first

• Often considered more “row-by-row” scanning
• In some systems it can be slightly faster (but not important here)

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✅ Final Takeaway

You can start with x or y — it doesn’t affect correctness.
The only requirement is: every pixel (x, y) must be processed exactly once.

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