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Digital · Guided lesson

Read RGBA and pixel coordinates

About 14 minutes · 4 reading + 10 practice and checks
Time includes reading, practice, and questions. Extra videos or further study are optional.
See this course’s project requirements

A local original 4×4-to-64×64 pixel image tool with RGBA inspection, grayscale and threshold filters, preserved alpha and checked PNG export.

Keep your working files in your own notebook or software. Save a private description, checklist, and work link in My Proof.

What to include

  • Produce a checked RGBA and coordinate-to-offset inspection table.
  • Demonstrate separate source/result buffers and a working original reset.
  • Calculate and verify equal-channel grayscale with alpha unchanged.
  • Produce a below/equal/above threshold matrix with preserved alpha.
  • Create and inspect the actual 64×64 PNG with intended crisp scaling and alpha behavior.
  • Deliver source, checked channel math, actual rendered/export evidence, and honest unavailable checks.

Review and revise

Use these criteria to check your work and choose what to improve.

  • The implementation or declared simulation follows the supplied contract and retains inspectable original inputs.
  • Normal, boundary and rejected cases are recorded with expected versus observed results.
  • The changed case has a correct explanation and does not rely on hidden state.
  • The project includes source, outputs, instructions and honest tool, privacy and verification limits.
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Before you begin

The idea in everyday life

Files pixels.html, pixel-filters.mjs and test-pixels.mjs supply an original 4×4 pattern: each row repeats red, green, blue, transparent white. Serve locally with python3 -m http.server 8765 --bind 127.0.0.1. Export scales the result to 64×64 PNG.

The idea

Each pixel uses four channels: red, green, blue, alpha. The row-major byte offset is 4×(y×width+x), with coordinates starting at zero.

Follow along

Read RGBA and pixel coordinates — original worked practice

Each pixel uses four channels: red, green, blue, alpha. The row-major byte offset is 4×(y×width+x), with coordinates starting at zero.

Files pixels.html, pixel-filters.mjs and test-pixels.mjs supply an original 4×4 pattern: each row repeats red, green, blue, transparent white. Serve locally with python3 -m http.server 8765 --bind 127.0.0.1. Export scales the result to 64×64 PNG.

  1. 1. Step 1

    Read the first four values 255,0,0,255 in pixel-filters.mjs.

    You should see: The first pixel is opaque red.

    Each pixel uses four channels: red, green, blue, alpha. The row-major byte offset is 4×(y×width+x), with coordinates starting at zero.

  2. 2. Step 2

    Inspect the fourth pixel 255,255,255,0.

    You should see: Its RGB is white but alpha is fully transparent.

    Each pixel uses four channels: red, green, blue, alpha. The row-major byte offset is 4×(y×width+x), with coordinates starting at zero.

  3. 3. Step 3

    Calculate the offset for x=2,y=1,width=4 and inspect that blue pixel.

    You should see: The offset is 24.

    Each pixel uses four channels: red, green, blue, alpha. The row-major byte offset is 4×(y×width+x), with coordinates starting at zero.

  4. 4. Step 4

    Record all four row colors, alpha values, and one coordinate lookup.

    You should see: An RGBA/index inspection table.

    Each pixel uses four channels: red, green, blue, alpha. The row-major byte offset is 4×(y×width+x), with coordinates starting at zero.

Now change one thing

For x=1,y=2,width=4, what byte offset begins the pixel?

Your work

What to finish

Produce a checked RGBA and coordinate-to-offset inspection table.

Common mistake

Watch for this

Confusing pixel index with channel offset or assuming white RGB implies opacity.

How can I tell I understand it?

Demonstrate the rule: Each pixel uses four channels: red, green, blue, alpha. The row-major byte offset is 4×(y×width+x), with coordinates starting at zero. Use the supplied variant to distinguish a changed result from a broken implementation.

Try 3 questions

Quick knowledge check

Answer all 3 questions. Get at least 2 right and the lesson completes automatically.

Not yet passed

You’ll get an explanation for each answer. Signed in? Your results save to your account.

1What does alpha 0 mean in the supplied fourth pixel?
2How many channel entries represent the 4×4 pattern?
3What offset begins x=2,y=1 in a width-4 row-major image?
0 of 3 answeredApply the lesson to a short question, then review the explanation.
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Finish this lesson

Pass the check. We’ll save the progress.

Answer all 3 questions above and get at least 2 right. You can review the explanations and try again.

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Sources and further reading

Explore the references behind this lesson.

developer.mozilla.org — Pixel_manipulation_with_canvas
Source & access notes · checked 2026-10-01

Relevant documented module/API behavior inspected during expansion authoring; actual execution evidence is recorded separately. No grant eligibility is established.

Access: Official page retrieved with the web reader on 2026-10-01. Documentation reading does not certify execution in every native tool or browser.

Reuse: Original local practice files, fictional data, worked examples and questions. Concepts paraphrased and linked; source exercises are not copied.

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