You can detect if a PDF page has transparency using ElementReader (as shown in ElementReader rample - http://www.pdftron.com/pdfnet/samplecode.html#ElementReader).
To detect if there is transparency on a page you would need to check if there is any
a) Transparency groups
b) Blend modes
c) Opacity values smaller than 1
For this you can extend ProcessElements() along the following lines:
// Pseudo-code to check PDF page for transparency…
static bool CheckIfPageHasTransparency(ElementReader reader)
{
Element element;
while ((element = reader.Next()) != null) // Read page contents
{
if (ProcessGSChanges(element)) return true;
if (element.GetType() == Element.Type.e_form)
{
reader.FormBegin();
Obj grp = element.GetXObject().FindObj(“Group”);
if (grp != null) {
Obj grp_subtype = grp.FindObj(“S”);
if(grp_subtype!=null && grp_subtype.IsName() && grp_subtype.GetName() == “Transparency”))
return true;
}
ProcessElements(reader);
reader.End();
break;
}
}
}
static bool ProcessGSChanges(Element element)
{
GState gs=element.GetGState();
for(GSChangesIterator itr=m_reader.GetChangesIterator(); itr.HasNext(); itr.Next())
{
switch(itr.Current()) {
case GState.GStateAttribute.e_opacity_fill :
if (gs.GetFillOpacity()<1) return true;
break;
case GState.GStateAttribute.e_opacity_stroke :
if (gs.GetStrokeOpacity()<1) return true;
break;
case GState.GStateAttribute.e_soft_mask :
if(gs.GetSoftMask() != null) return true;
break;
case GState.GStateAttribute.e_blend_mode:
if(gs.GetBlendMode() != GState::e_bl_normal)
return true;
break;
default:
break;
}
}
return false;
}