Add heuristic for improving text rendering in CPDF_CharPosList::Load

This CL uses the following algorithm for improving substitute font spacing (we
are assuming the text layout is horizontal):
* Calculate PDFWidth, the width that the PDF says the glyph should have.
* Calculate FTWidth, the width calculated by freetype for the glyph, using the
substitute font that we'll use to render it. Note that some embedded fonts have
PDFWidth == FTWidth + 1, so we consider that to be matching widths.
* If PDFWidth > FTWidth + 1 , move the x coordinate by the difference / 2 so
that the glyph is rendered in a more centered spot and the text looks better.
* If PDFWidth < FTWidth, transform the glyph horizontally by PDFWidth / FTWidth
so that the glyph gets compressed and does not overlap with surrounding glyphs.

Bug: chromium:431507
Change-Id: Ia378344253fabe44d93af4daab98bb3b7bca22de
Reviewed-on: https://pdfium-review.googlesource.com/6630
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Nicolás Peña <npm@chromium.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/core/fpdfapi/font/cpdf_font.cpp b/core/fpdfapi/font/cpdf_font.cpp
index 1faa4ec4be..bdd3464ed2 100644
--- a/core/fpdfapi/font/cpdf_font.cpp
+++ b/core/fpdfapi/font/cpdf_font.cpp
@@ -159,6 +159,10 @@ uint32_t CPDF_Font::CharCodeFromUnicode(wchar_t unicode) const {
   return m_pToUnicodeMap ? m_pToUnicodeMap->ReverseLookup(unicode) : 0;
 }
 
+bool CPDF_Font::HasFontWidths() const {
+  return true;
+}
+
 void CPDF_Font::LoadFontDescriptor(CPDF_Dictionary* pFontDesc) {
   m_Flags = pFontDesc->GetIntegerFor("Flags", FXFONT_NONSYMBOLIC);
   int ItalicAngle = 0;