# CSS Sprites - Image Request Optimization

## Overview

The CV website uses CSS sprites to dramatically reduce HTTP requests for company, project, and course logos. Instead of loading 44+ individual image files, we load only 3 sprite sheets (6 files total including retina versions).

## Performance Impact

| Metric | Before | After | Improvement |
|--------|--------|-------|-------------|
| Image Requests | 44+ | 3-6 | ~93% reduction |
| Cache Invalidation | Per image | Per sprite | Simplified |
| HTTP Overhead | 44 round-trips | 3-6 round-trips | Dramatic reduction |

## Architecture

### File Structure

```
static/
├── images/
│   ├── companies/          # Source images (any size)
│   ├── projects/           # Source images (any size)
│   ├── courses/            # Source images (any size)
│   └── sprites/            # Generated sprites
│       ├── sprite-companies.png
│       ├── sprite-companies@2x.png
│       ├── sprite-projects.png
│       ├── sprite-projects@2x.png
│       ├── sprite-courses.png
│       ├── sprite-courses@2x.png
│       └── sprite-map.json
├── sprite-showcase.html    # Visual QA page
└── css/
    └── 04-interactive/
        └── _sprites.css    # Sprite CSS classes
```

### Go Sprite Generator Tool

Located at `cmd/sprites/main.go`, this tool:

1. **Scans source directories** for PNG images
2. **Normalizes images** to standard sizes (60x60px for 1x, 120x120px for 2x)
3. **Maintains aspect ratio** and centers on transparent background
4. **Combines into horizontal strips** for each category
5. **Generates sprite-map.json** for documentation
6. **Creates sprite-showcase.html** for visual QA

### Image Size Standards

- **Base size**: 60x60px (optimal for 80px display box with 10px padding)
- **Retina size**: 120x120px (@2x for high-DPI displays)
- **Section display**: 80x80px box (60px icon + 10px padding each side)

## Usage

### Makefile Targets

```bash
# Generate sprites from source images
make sprites

# Clean generated sprite files
make sprites-clean
```

### JSON Data Structure

Add `logoIndex` to entries in cv-en.json and cv-es.json:

```json
{
  "company": "Olympic Broadcasting Services",
  "companyLogo": "olympic-broadcasting.png",
  "logoIndex": 15
}
```

**Important**: Only add `logoIndex` when there's an actual PNG file. Entries without a logo file should not have `logoIndex`.

### Template Integration

Templates automatically use sprites when `logoIndex` is present:

```html
{{if .LogoIndex}}
<span class="icon-sprite icon-section icon-company"
      style="--icon-index: {{.LogoIndex}};"
      role="img"
      aria-label="{{.Company}} logo"></span>
{{else if .CompanyLogo}}
<img src="/static/images/companies/{{.CompanyLogo}}" alt="{{.Company}} logo">
{{else}}
<iconify-icon icon="mdi:office-building" width="80" height="80"></iconify-icon>
{{end}}
```

### CSS Classes

```css
/* Base sprite class */
.icon-sprite {
  display: inline-block;
  width: 50px;
  height: 50px;
  background-repeat: no-repeat;
  background-size: auto 50px;
}

/* Category-specific classes */
.icon-company { background-image: url('/static/images/sprites/sprite-companies.png'); }
.icon-project { background-image: url('/static/images/sprites/sprite-projects.png'); }
.icon-course { background-image: url('/static/images/sprites/sprite-courses.png'); }

/* Size variants */
.icon-sprite.icon-section {
  width: 80px;
  height: 80px;
  padding: 10px;
  background-size: auto 60px;
  background-origin: content-box;
  background-clip: content-box;
}
.icon-sprite.icon-small { width: 32px; height: 32px; }
.icon-sprite.icon-large { width: 64px; height: 64px; }
```

## Adding New Icons

1. **Drop source image** into appropriate directory:
   - `static/images/companies/` for company logos
   - `static/images/projects/` for project logos
   - `static/images/courses/` for course logos

2. **Run sprite generation**:
   ```bash
   make sprites
   ```

3. **Update JSON files** with new `logoIndex` based on sprite-map.json

4. **Verify** in showcase page at `/static/sprite-showcase.html`

## Retina Display Support

The CSS automatically loads @2x sprites on retina displays:

```css
@media (-webkit-min-device-pixel-ratio: 2), (min-resolution: 192dpi) {
  .icon-company {
    background-image: url('/static/images/sprites/sprite-companies@2x.png');
    background-size: auto 60px; /* Display at 1x size */
  }
}
```

## Sprite Map JSON

The `sprite-map.json` file documents icon positions:

```json
{
  "companies": [
    {"index": 0, "name": "accenture.png"},
    {"index": 1, "name": "aena-long.png"},
    ...
  ],
  "projects": [...],
  "courses": [...]
}
```

This file is for documentation/debugging only - CSS calculates offset from index using `calc(var(--icon-index) * -60px)`.

## Verification

### Showcase Page

Visit `/static/sprite-showcase.html` to:
- View full sprite sheets
- See all individual icons with index labels
- Test zoom levels (100%, 200%, 300%)
- Verify retina rendering

### Network Verification

In browser DevTools (Network tab, filter Images):
- **Should see**: sprite-companies.png, sprite-projects.png, sprite-courses.png
- **Should NOT see**: individual logo files (unless fallback triggers)

## Troubleshooting

### Invalid PNG Warning

If you see "png: invalid format: not a PNG file", the source file is not a valid PNG. Check the file with `file <filename>` to verify format.

### Icon Not Displaying

1. Verify `logoIndex` is present in JSON
2. Check sprite-map.json for correct index
3. Verify CSS is loaded
4. Check browser console for errors

### Wrong Icon Displayed

Verify the `logoIndex` value matches the icon's position in sprite-map.json (0-indexed).