Key Concepts from the PDF Specification:


1. Highlight Annotations:
o In ISO 32000, highlight annotations are a type of text markup annotation.
o Highlight annotations are used to apply a transparent overlay of color over a selection of text. The key behavior is that they do not obscure the text; instead, the text is rendered at full opacity with the highlight acting as a translucent color layer above it.
o This feature is specified under the Text Markup Annotations section (7.6.2.4 in ISO 32000-1).

2. Subtypes of Text Markup Annotations:
o The standard defines several types of text markup annotations, including:
 Highlight: Applies a transparent color overlay to the text.
 Underline, Strikeout, and Squiggly: Other visual effects applied to text.
o These annotations are represented in the PDF by the /Subtype entry in the annotation dictionary. The highlight annotation is marked with /Subtype /Highlight.

3. Appearance Stream (Transparency Settings):
o Highlight annotations in PDF files use appearance streams, which are descriptions of how the annotation should be rendered, including its color and transparency.
o The transparency of the highlight is controlled by setting a blending mode in the appearance stream that specifies how the color overlay interacts with the underlying content.
o This blending mode ensures that the underlying text remains visible at 100% opacity, similar to what is observed in Bluebeam Revu's "Highlight" mode.

4. CA (Constant Alpha):
o The CA (constant alpha) value in the PDF standard controls the opacity of the annotation's stroke and fill. For highlight annotations, this allows for the creation of a semi-transparent overlay that doesn’t obscure the content beneath.
o The CA value for highlight annotations ensures that the color overlays but does not blend with the underlying text.

How This Maps to Bluebeam Revu:
• In Bluebeam Revu's "Highlight" mode, the highlight is essentially a text markup annotation with specific transparency and blending settings.
• The ISO 32000 standard describes this behavior through the use of highlight annotations (subtype /Highlight), which apply a transparent color overlay that preserves the visibility of the text at 100% opacity.
• Bluebeam leverages this PDF functionality to create a tool where the fill color acts like a highlight but doesn't blend into the text, which is consistent with the PDF specification for highlight annotations.

Conclusion:
The PDF ISO 32000 standard defines the behavior of highlight annotations in a way that aligns closely with Bluebeam Revu's "Highlight" mode. This is achieved using the highlight annotation subtype and appearance stream settings that allow for a transparent color overlay while maintaining the full opacity of underlying content, ensuring the text is not affected by the color overlay.