/sketch

Upscale & Post-Process Reference

Purpose: Ship print-ready or retina-grade outputs from Gemini renders via upscaling, masked inpainting, outpainting, and final format selection. Invoke when the base generation is approved but the raster is too small, has a localized artifact, or needs canvas extension before handoff.

Scope Boundary

• sketch upscale: upscaler choice, mask authoring, inpainting prompts, artifact detection heuristics, export format tradeoffs (WebP / AVIF / PNG / JPEG).
• sketch generate / batch (siblings): produce the base raster. Do not upscale at generation time — render at native ceiling first (1K / 2K / 4K), then post-process if needed.
• Artisan (elsewhere): picks the final format per surface (web / app / print) and wires it into components.
• Muse (elsewhere): if the post-processed asset is a token (icon / hero), Muse registers it.

If the issue is "image is too small" → upscale. If "wrong subject" → re-run generate with a new prompt, not inpainting.

Workflow

INSPECT    →  read base image: dimensions, artifact map, safe margins
           →  decide: upscale only / inpaint region / outpaint canvas / multi
           →  confirm color space (sRGB for web, Display P3 only if warranted)

CHOOSE     →  pick upscaler (see "Upscaler Choice")
           →  set target: 2x / 4x, target DPI for print (300 dpi)
           →  author mask if inpainting (see "Mask Authoring")

PROCESS    →  run upscaler; for inpainting, re-prompt Gemini with mask + source
           →  verify no new artifacts introduced (see "Artifact Detection")

EXPORT     →  pick format (see "Export Formats")
           →  strip EXIF unless provenance matters; preserve SynthID note
           →  write sidecar metadata.json with tool versions + params

Upscaler Choice

Default: regenerate at Nano Banana 2 / Pro 4K if the base prompt is still valid — avoids upscaler hallucinations. Fall back to Real-ESRGAN when the base is fixed or regeneration is budget-blocked.

# Real-ESRGAN CLI invocation (illustrative)
# pip install realesrgan
# realesrgan-ncnn-vulkan -i base.png -o out_4x.png -n realesrgan-x4plus -s 4

Mask Authoring

Inpainting needs a mask that marks "change here, keep everything else". Three authoring paths:

1. Alpha channel: open in GIMP / Photoshop, paint transparent over the region, export PNG-RGBA. Transparent = editable.
2. Separate grayscale PNG: white = editable, black = keep. Same dimensions as source.
3. SVG → raster: author mask as SVG <rect> / <path>, rasterize at source resolution. Reproducible and versionable.

# Example: SVG mask → PNG at source resolution
import cairosvg, pathlib

cairosvg.svg2png(
    url="mask.svg",
    write_to="mask.png",
    output_width=1024,
    output_height=1024,
)

Rule: feather the mask edge by 4–8 px. Hard edges produce visible seams after inpainting.

Inpainting Prompt Strategy

Gemini-native inpainting via reference: send source + mask + targeted edit prompt. Keep the edit prompt narrow — describe only the region.

EDIT: within the masked region only, replace the coffee cup with a glass of water.
Preserve lighting direction, shadow falloff, and surrounding table texture.
Do not alter anything outside the mask.

Good prompts:

• name only the replacement subject
• repeat lighting / palette constraints
• explicitly forbid changes outside the mask

Bad prompts:

• full scene re-description (model re-renders everything)
• style pivots ("make it anime now") — that is a generate call, not upscale

Outpainting (Canvas Extension)

When the canvas is too tight (hero needs 21:9 but base is 16:9):

1. Pad the source with transparent pixels to target ratio.
2. Use the transparent region as the inpaint mask.
3. Prompt: "extend the scene naturally into the transparent region, matching composition, palette, and lighting of the visible area."

Limit single-pass extension to 20–30% of the original canvas. Larger extensions produce repetition or seam artifacts — do it in 2–3 passes.

Artifact Detection Heuristics

Quick pre-export checks:

Quick gate: diff against base at 50% opacity in a viewer. Anomalies outside the intended mask region are artifacts.

Export Formats

Rules:

• strip EXIF (pillow: im.save(..., exif=b"")) unless provenance is explicitly needed
• keep color profile: embed sRGB for web, Display P3 only for verified P3 surfaces
• preserve SynthID notice in the delivered sidecar — post-processing does not remove the watermark

Anti-Patterns

• Upscale a 512 px base to 4K — upscaler invents detail; regenerate at native 4K instead.
• Inpaint a face and expect identity preservation — Gemini does not have face-lock; warn the user or route to a face-preserving pipeline.
• Use a hard-edged mask — seams show up at the exact rectangle. Feather 4–8 px minimum.
• Outpaint 2x the canvas in one pass — produces repetition. Stage it.
• Save the final as PNG-24 for a 4K photo hero on a marketing page — 8 MB asset; use WebP or AVIF.
• Strip the SynthID disclosure from the sidecar because "the watermark is invisible anyway" — the disclosure is the contract with the user, not the watermark itself.
• Run the upscaler before confirming the base is approved — wastes GPU minutes if the base is rejected.

Handoff

• To Artisan: final assets + export table (which format for which surface) + dimensions / DPI note.
• To Muse: if the asset becomes a registered token, hand over the 1x and 2x exports plus color profile.
• To Growth: print-spec variant (300 DPI CMYK TIFF) only if marketing needs offset print; otherwise WebP hero.
• To Vitrine: post-processed catalog entry with upscaler, mask, and inpaint_prompt recorded in sidecar metadata.