A scene by design — Parallax Sketchbook

SKETCHBOOK / 04 The composition desk · 2-week chapter

Page 1 of 6: Compose a world from values

One scene, no format detour

Compose a world from values

A scene is a collection of values describing a camera, geometry, colors, and light. In this course you construct that description directly in C++23. A text configuration format is not required to learn rendering, and adding one would introduce a second problem before the image problem is understood.

The native demo_scene() returns an owned Scene. Its string and vector members retain their own data; no result points into a temporary input buffer. Make a local copy, change a sphere or camera value, and pass that candidate to validation. The reference composition is a starting point, not a portfolio result you have already authored.

Keep representation separate from action. Constructing a Scene should not open an output file or create a graphics context. That gives you a small, repeatable object to inspect in a debugger and reuse in tests.

TRY IT IN YOUR STUDIO

Create your own scene function with one deliberate composition change. Draw which object owns the sphere vector and the returned image bytes.

C.20Compose ordinary owning values using the rule of zero.

LOOK CLOSER

#include "prism/renderer.hpp"

void scene_study() {
    auto candidate = prism::demo_scene();
    candidate.width = 320;
    candidate.height = 200;
    candidate.background = {0.05, 0.08, 0.14};
    const auto checked = prism::validate_scene(candidate);
    if (!checked) {
        return; // Present checked.error() at the application boundary.
    }
    auto image = prism::render(candidate);
    if (!image) return;
    // image now owns the rendered RGB bytes.
}

Compilable C++23 function using the native project API. Call it from an application or test; it intentionally performs no file output.

From the page to your own work

Your studio assignment.

Build a typed scene in C++ and make unsupported states fail at a visible boundary.

A finish line you can see

Your own typed scene variation, one accepted boundary test, one rejected boundary test, and a written validation contract.

Change the demo scene through typed values; no configuration parser is required.
Test an accepted dimension or radius boundary and its rejected neighbor.
Show that render validates caller-constructed scenes too.
Explain one arithmetic or non-finite-value failure that the member types alone cannot prevent.

Keep the evidence in your weekly notebook ↗

Explain it like an engineer

Try answering aloud before opening the review notes. Use your own implementation as evidence.

Why does a Scene with correctly typed fields still need validation?

Types do not establish positive dimensions, finite vectors, or cross-field budgets.
Check factors before multiplying dimensions or work counts.
Return a complete result or error without publishing a partial change.

These prompts support practice; they are not an automatic assessment of your work.

A tip for this exercise

Keep a valid scene beside each experimental candidate. That gives you a visible recovery point and makes rejecting a bad edit part of the design rather than an emergency repair.

Hints, when you need them.

Try a first step, then reveal as much help as you need. Each hint gets a little more specific.

1Hint 1 · Find a starting point

Pick one field whose type permits unsupported values. An unsigned image dimension, for example, can still be zero or above the supported limit.

2Hint 2 · Choose an approach

Read the validator for that field and choose an accepted boundary and its rejected neighbor. Hold other scene values fixed so the failure has one explanation.

3Hint 3 · Check your reasoning

Pass the rejected candidate to render and check its error before accessing a value. Then render the saved valid scene and confirm your failed edit did not replace it.

A small creative variation ↗

Create a second scene function with a different composition while retaining the same validated domain.

Optional. Save it for later if this chapter already feels full.

Coming back after a break? ↩

Call demo_scene(), change one sphere color, and render once. Then run just one validation test.

Books & principles behind this study +

Original Parallax explanations and assignments, with these works for further reading.

YOUR MARGIN NOTES

Leave yourself a good question.

What changed your mind? What would you try next?

Notes for A scene by design

Notes are private to this browser and saved as you write.

A CHECKPOINT, NOT A SCORE

Can you explain the idea and try the exercise yourself?

Completion records your own assessment. It does not compile or test your C++.

Companion source 9 files