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image-treatment

Design & creation

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Updated 10-15 17:18

About this skill

Image Treatment

Apply stunning visual effects to images using CSS filters, blend modes, masks, and SVG filters.

Prerequisites

  • Read .agents/design-context.md (see design-context) for brand colors (duotone and overlay colors should come from the Color System), style archetype, and imagery direction. Colors below use the shared --brand-* / --gray-* scale.
  • Identify the mood and purpose: editorial, product, marketing, artistic.
  • If source images don't exist yet, create them with the image-generation skill (python <image-generation>/scripts/gemini-generate.py), then treat them here. For treatments that must be baked into the file (social images, email, OG images), apply them with Pillow instead of CSS.

Step 1: CSS Filter Combinations

Individual Filters

Filter Range Effect
brightness() 0-2+ Darken (< 1) or lighten (> 1)
contrast() 0-2+ Reduce (< 1) or boost (> 1)
saturate() 0-3+ Desaturate (0) or vivid (> 1)
grayscale() 0-1 Remove color
sepia() 0-1 Warm vintage tone
hue-rotate() 0-360deg Shift all hues
blur() 0-20px+ Gaussian blur

Preset Filter Recipes

.filter-vintage  { filter: sepia(0.3) contrast(1.1) brightness(1.05) saturate(0.9); }
.filter-cool     { filter: saturate(0.85) contrast(1.1) brightness(1.02) hue-rotate(-10deg); }
.filter-bw       { filter: grayscale(1) contrast(1.4) brightness(1.05); }
.filter-dreamy   { filter: brightness(1.1) contrast(0.9) saturate(1.2) blur(0.5px); }
.filter-faded    { filter: contrast(0.85) brightness(1.1) saturate(0.7); }
.filter-moody    { filter: brightness(0.85) contrast(1.2) saturate(0.8); }
.filter-vivid    { filter: contrast(1.15) saturate(1.4) brightness(1.02); }

Hover Effect with Filters

.image-card img { filter: grayscale(0.8) brightness(0.9); transition: filter 0.4s ease; }
.image-card:hover img { filter: grayscale(0) brightness(1); }

Step 2: Blend Mode Techniques

Common Blend Modes

Mode Effect Use case
multiply Darkens (white disappears) Color tints, knocking out white backgrounds
screen Lightens (black disappears) Glow effects
overlay Boosts contrast Color overlays
soft-light Subtle overlay Gentle tinting
color Applies hue/sat, preserves luminosity Tinting photos
luminosity Takes brightness from the layer, hue/saturation from what's below Recoloring textures

Color Overlay on Image

.image-container { position: relative; }
.image-container::after {
  content: ''; position: absolute; inset: 0;
  background: var(--brand-500); mix-blend-mode: multiply; opacity: 0.6;
}

Step 3: Duotone Image Effect

Maps the image's lights to one color and darks to another.

CSS-Only Duotone

Grayscale image multiplied onto the highlight color (white → highlight, black → black), then the shadow color lightens the darks (black → shadow). The highlight must be lighter than the shadow in every RGB channel.

.duotone { position: relative; isolation: isolate; background: #e2b94c; /* highlight */ }
.duotone img { display: block; width: 100%; filter: grayscale(1) contrast(1.2); mix-blend-mode: multiply; }
.duotone::after {
  content: ''; position: absolute; inset: 0; pointer-events: none;
  background: #1a1a2e; /* shadow */ mix-blend-mode: lighten;
}

SVG Filter Duotone (exact mapping, works on any element)

tableValues are each channel of shadow → highlight in 0-1 (hex ÷ 255): #1a1a2e → #e2b94c.

<svg width="0" height="0" aria-hidden="true" style="position:absolute">
  <filter id="duotone-gold" color-interpolation-filters="sRGB">
    <feColorMatrix type="matrix" values="0.2126 0.7152 0.0722 0 0  0.2126 0.7152 0.0722 0 0  0.2126 0.7152 0.0722 0 0  0 0 0 1 0"/>
    <feComponentTransfer>
      <feFuncR type="table" tableValues="0.102 0.886"/>
      <feFuncG type="table" tableValues="0.102 0.725"/>
      <feFuncB type="table" tableValues="0.180 0.298"/>
    </feComponentTransfer>
  </filter>
</svg>
<img src="photo.jpg" alt="…" style="filter: url(#duotone-gold)">

Popular Duotone Pairs

Shadow color Highlight color Feel
#1a1a2e #e2b94c Luxury, gold+navy
#0d1b2a #3498db Tech, cool blue
#2d1b4e #ff6b6b Creative, bold
#1b3a2a #a7f3d0 Natural, green

Step 4: Image Masking

clip-path Shapes

.clip-circle   { clip-path: circle(50%); }
.clip-rounded  { clip-path: inset(0 round 1rem); }
.clip-diagonal { clip-path: polygon(0 0, 100% 0, 100% 85%, 0 100%); }
.clip-hexagon  { clip-path: polygon(25% 0%, 75% 0%, 100% 50%, 75% 100%, 25% 100%, 0% 50%); }

mask-image for Gradient Fades

.image-fade-bottom {
  mask-image: linear-gradient(to bottom, black 60%, transparent 100%);
  -webkit-mask-image: linear-gradient(to bottom, black 60%, transparent 100%);
}
.image-vignette {
  mask-image: radial-gradient(ellipse 70% 70% at center, black 50%, transparent 100%);
  -webkit-mask-image: radial-gradient(ellipse 70% 70% at center, black 50%, transparent 100%);
}

Text-Shaped Mask

.image-text-mask {
  background-image: url('photo.jpg'); background-size: cover;
  -webkit-background-clip: text; -webkit-text-fill-color: transparent;
  background-clip: text; font-size: 8rem; font-weight: 900;
  color: transparent; /* fallback if background-clip: text is unsupported */
}

Only for display-size decorative text; the photo under the letters must still give 3:1 contrast with the page background, and the element keeps real text for screen readers.

Step 5: Background Image Techniques

Full Cover Background

.hero-bg {
  background: url('hero.jpg') center/cover no-repeat;
  min-height: 80vh;
}

Fixed / Parallax Background

.parallax-section {
  background: url('bg.jpg') center/cover no-repeat;
  background-attachment: fixed;
  min-height: 60vh;
}
@media (hover: none), (prefers-reduced-motion: reduce) {
  .parallax-section { background-attachment: scroll; } /* iOS ignores fixed; parallax can trigger vestibular issues */
}

Multiple Background Layers

.layered-bg {
  background:
    linear-gradient(to bottom, rgba(0,0,0,0.4), rgba(0,0,0,0.7)),
    url('pattern.svg') repeat center / 64px 64px,
    url('photo.jpg') no-repeat center / cover;
}

Step 6: Image Overlay Patterns

Gradient Overlay (for text readability)

.image-overlay-gradient { position: relative; }
.image-overlay-gradient::after {
  content: ''; position: absolute; inset: 0;
  background: linear-gradient(to top, rgba(0,0,0,0.8) 0%, rgba(0,0,0,0.3) 40%, transparent 100%);
}
.image-overlay-gradient .content { position: relative; z-index: 1; color: white; }

Text over images must still meet WCAG contrast (4.5:1, or 3:1 for ≥24px / ≥18.66px bold) against the lightest pixels behind it. Check the region under the text in the rendered screenshot, not the average of the image; strengthen the overlay until it passes.

Hover Reveal Overlay

.card-image { position: relative; overflow: hidden; }
.card-image .overlay {
  position: absolute; inset: 0;
  background: oklch(from var(--brand-500) l c h / 0.85);
  display: grid; place-items: center; opacity: 0; transition: opacity 0.3s ease;
}
.card-image:hover .overlay { opacity: 1; }

Step 7: Image Grid and Collage Layouts

Masonry-Style Grid

.image-grid-masonry { columns: 3; column-gap: 1rem; }
.image-grid-masonry img { display: block; width: 100%; margin-bottom: 1rem; border-radius: 0.5rem; break-inside: avoid; }
/* Note: columns flow top-to-bottom, so DOM/reading order runs down each column, not across rows */
@media (max-width: 768px) { .image-grid-masonry { columns: 2; } }
@media (max-width: 480px) { .image-grid-masonry { columns: 1; } }

CSS Grid Collage

.image-collage {
  display: grid; grid-template-columns: repeat(4, 1fr);
  grid-template-rows: repeat(3, 200px); gap: 0.5rem;
}
.image-collage img:nth-child(1) { grid-column: 1 / 3; grid-row: 1 / 3; }
.image-collage img { width: 100%; height: 100%; object-fit: cover; border-radius: 0.5rem; }

Step 8: Responsive Image Art Direction

Resolution Switching (same crop, many sizes)

w descriptors list each file's real pixel width; sizes tells the browser how wide the image will be displayed so it can pick before layout. Without sizes, the browser assumes 100vw.

<img src="card-800.jpg"
     srcset="card-400.jpg 400w, card-800.jpg 800w, card-1200.jpg 1200w, card-1600.jpg 1600w"
     sizes="(max-width: 640px) 100vw, (max-width: 1024px) 50vw, 400px"
     alt="Team reviewing designs at a whiteboard" width="800" height="600"
     loading="lazy" decoding="async">

Art Direction + Modern Formats (<picture>)

Browsers take the first matching <source>, so order by media query, then by format (AVIF before WebP). Each source gets its own srcset/sizes; width/height on <source> handle a different aspect ratio per breakpoint.

<picture>
  <source media="(max-width: 640px)" type="image/avif"
          srcset="hero-mobile-640.avif 640w, hero-mobile-1280.avif 1280w" sizes="100vw" width="1280" height="1600">
  <source media="(max-width: 640px)" type="image/webp"
          srcset="hero-mobile-640.webp 640w, hero-mobile-1280.webp 1280w" sizes="100vw" width="1280" height="1600">
  <source type="image/avif" srcset="hero-1600.avif 1600w, hero-2400.avif 2400w" sizes="100vw">
  <source type="image/webp" srcset="hero-1600.webp 1600w, hero-2400.webp 2400w" sizes="100vw">
  <img src="hero-1600.jpg" alt="Describe what the hero shows" width="1600" height="900"
       fetchpriority="high" decoding="async">
</picture>

Key Rules

  1. Always include width and height attributes (the intrinsic aspect ratio) to prevent layout shift; CSS can still make it fluid with width: 100%; height: auto.
  2. Use loading="lazy" for below-fold images. Never lazy-load the LCP/hero image -- leave it eager (the default) and add fetchpriority="high".
  3. Use decoding="async" so decode doesn't block rendering.
  4. Formats: AVIF (smallest, supported in all current major browsers) → WebP → JPEG fallback in <img src>. Use PNG/SVG only for graphics needing transparency or sharp edges; quality ~50-65 for AVIF and ~75-80 for WebP is a good start.
  5. alt describes the image's purpose; decorative images use alt="".

Generate the Variants

from PIL import Image  # pip install Pillow  (AVIF built in since Pillow 11.2; check features.check("avif"))
src = Image.open("hero.png").convert("RGB")
for w in (640, 1280, 1600, 2400):
    im = src.resize((w, round(src.height * w / src.width)), Image.LANCZOS)
    im.save(f"hero-{w}.webp", quality=78)
    im.save(f"hero-{w}.avif", quality=55)
    im.save(f"hero-{w}.jpg", quality=82, optimize=True, progressive=True)

Step 9: SVG Filters for Advanced Effects

Frosted Glass

.frosted-glass { backdrop-filter: blur(12px) saturate(180%); background: rgba(255,255,255,0.7); }
@supports not (backdrop-filter: blur(1px)) { .frosted-glass { background: rgba(255,255,255,0.92); } } /* keep text readable */

Noise/Grain Texture

<svg width="0" height="0" aria-hidden="true" style="position:absolute">
  <filter id="noise">
    <feTurbulence type="fractalNoise" baseFrequency="0.65" numOctaves="3" stitchTiles="stitch"/>
    <feColorMatrix type="saturate" values="0"/>
    <feBlend in="SourceGraphic" mode="multiply"/>
  </filter>
</svg>
<!-- apply with: .grain { filter: url(#noise); } -->

Drop Shadow (Following Shape)

.shaped-shadow { filter: drop-shadow(0 4px 8px rgba(0,0,0,0.2)); }

Step 10: Object-Fit and Object-Position

Value Behavior
cover Fills container, crops excess (most common)
contain Fits entirely, may have empty space
fill Stretches to fill (distorts)
scale-down Like contain but never scales up 
.card-image { width: 100%; height: 200px; object-fit: cover; object-position: center; }
.portrait   { object-fit: cover; object-position: center top; }
.thumbnail  { aspect-ratio: 1; object-fit: cover; border-radius: 0.75rem; }

Quick Reference

  1. Use CSS filter for quick photo effects (combine brightness, contrast, saturate).
  2. Use mix-blend-mode: multiply for color overlays on images.
  3. Duotone = grayscale image + two-color blend layers.
  4. clip-path for geometric masks; mask-image for gradient fades.
  5. Always disable background-attachment: fixed on mobile (@media (hover: none)).
  6. Layer overlays: gradient bottom-to-top for text readability on images.
  7. Use <picture> for art direction (different crops per breakpoint).
  8. Always include width, height, alt, and srcset + sizes on content images; lazy-load only below the fold.
  9. object-fit: cover + aspect-ratio for consistent image containers.
  10. SVG filters (noise, goo, displacement) for effects CSS cannot achieve.

Verify the Treatment

Render the page and screenshot it (npx playwright screenshot --full-page "file://$PWD/index.html" out.png, plus --viewport-size=375,800 for mobile and --color-scheme=dark), then view the images with Read. Check: faces and products aren't cropped by object-fit/clip-path at any width, text over images passes contrast in its worst spot, duotone/filters still look on-brand (not muddy), and nothing looks broken where a blend mode or filter is unsupported. In the browser, confirm the right srcset candidate loads (img.currentSrc) and that the hero is not lazy-loaded.