transparency support

[spandan-k]

Add Comprehensive Transparency Support to python-pptx

🎯 Overview

This PR adds complete transparency/opacity support to python-pptx, enabling developers to control the transparency of shape fills and colors. The implementation follows the DrawingML specification and provides a clean, intuitive API across all layers of the library.

✨ Features Added

🎨 ColorFormat Transparency

• Property: ColorFormat.transparency (read/write, 0.0-1.0 range)
• Behavior:
  • 0.0 = completely opaque (no alpha element in XML)
  • 1.0 = completely transparent (alpha=0 in XML)
  • 0.5 = 50% transparent (alpha=50000 in XML)
• Validation: Raises ValueError for values outside 0.0-1.0 range
• NoneColor Handling: Returns 0.0 for get, raises ValueError for set

🎨 FillFormat Transparency

• Property: FillFormat.transparency (read/write, 0.0-1.0 range)
• Delegation: Automatically delegates to underlying solid fill
• Error Handling: Raises TypeError for non-solid fill types (gradient, pattern, etc.)
• Integration: Works seamlessly with existing fill API

🎨 _SolidFill Transparency

• Property: _SolidFill.transparency (read/write, 0.0-1.0 range)
• Delegation: Delegates to fore_color.transparency
• Consistency: Maintains transparency across color changes

🏗️ Implementation Details

XML Layer (OXML)

• New Elements: Added support for <a:alpha>, <a:alphaOff>, <a:alphaMod>
• Element Class: CT_PositiveFixedPercentage for alpha value handling
• Registration: Proper element registration in __init__.py
• Methods: add_alpha(), clear_alpha() for XML manipulation

Domain Model Layer (DML)

• ColorFormat: Core transparency property with validation
• FillFormat: High-level transparency access for fills
• _SolidFill: Solid fill transparency delegation
• _Color: Low-level transparency implementation

🎯 Usage Examples

from pptx import Presentation
from pptx.util import Inches

# Create presentation and add shape
prs = Presentation()
slide = prs.slides.add_slide(prs.slide_layouts[0])
shape = slide.shapes.add_shape(MSO_SHAPE.RECTANGLE, 
                               Inches(1), Inches(1), 
                               Inches(2), Inches(2))

# Set fill color and transparency
shape.fill.solid()
shape.fill.fore_color.rgb = RGBColor(255, 0, 0)  # Red
shape.fill.transparency = 0.5  # 50% transparent

# Alternative: Set transparency via color
shape.fill.fore_color.transparency = 0.3  # 30% transparent

# Check transparency
print(f"Fill transparency: {shape.fill.transparency}")
print(f"Color transparency: {shape.fill.fore_color.transparency}")

XML Structure

<a:solidFill>
  <a:srgbClr val="FF0000">
    <a:alpha val="50000"/>  <!-- 50% opaque = 50% transparent -->
  </a:srgbClr>
</a:solidFill>

📝 Notes

• Transparency is only supported for solid fills (not gradients or patterns)
• Setting transparency to 0.0 removes the alpha element for optimal XML
• NoneColor objects cannot have transparency set (must set RGB/theme color first)
• All transparency values are validated to be in the 0.0-1.0 range

Closes #62