figure out optimal DPI for pdf page conversion

dlibrary/dlibrary.py

@@ -742,6 +742,7 @@ def get_displayed_image_xref(page):
 def display_sixel_pixmap(pixmap_bytes):
     s = BytesIO()
     image = Image.open(BytesIO(pixmap_bytes))
+    image.thumbnail(size=(800, 800))
     width, height = image.size
 
     try:
@@ -778,6 +779,25 @@ def display_sixel_pixmap(pixmap_bytes):
     finally:
         sixel_output_unref(output)
 
+def naive_convert_page(page):
+    return { 'ext': 'png', 'image': page.get_pixmap(dpi=PDF_CONVERSION_DPI).tobytes('png') }
+
+def convert_page(pdf, page):
+    xref = get_displayed_image_xref(page)
+    if xref is None:
+        debug('Page has multiple images, converting with naive DPI approach')
+        return naive_convert_page(page)
+
+    image_rect = page.get_image_rects(xref)[0]
+    image = extract_image(pdf, xref)
+    scale_factor = image['width'] / image_rect.width
+    if scale_factor < PDF_CONVERSION_DPI / 72:
+        debug(f'Image scale factor is lower than {PDF_CONVERSION_DPI}dpi, using higher dpi instead')
+        return naive_convert_page(page)
+
+    debug(f'Page has single image, converting with scale factor {scale_factor}')
+    return { 'ext': 'png', 'image': page.get_pixmap(matrix=fitz.Matrix(scale_factor, scale_factor)).tobytes('png') }
+
 def pdf_image_extractors(pdf, strategy):
     print(f'0/{pdf.page_count} pages analyzed...', end='')
     image_extractors = []
@@ -795,7 +815,7 @@ def pdf_image_extractors(pdf, strategy):
                 if choice.lower().startswith('c'):
                     if choice == strategy:
                         print(f'Converting page {idx+1}')
-                    image_extractors.append(lambda p=page: { 'ext': 'png', 'image': p.get_pixmap(dpi=PDF_CONVERSION_DPI).tobytes('png') })
+                    image_extractors.append(lambda p=pdf, pp=page: convert_page(p, pp))
                     break
                 if xref is not None and (choice.lower().startswith('x') or choice.lower() == 'extract'):
                     if choice == strategy:
@@ -811,7 +831,6 @@ def pdf_image_extractors(pdf, strategy):
                     display_sixel_pixmap(page.get_pixmap(dpi=PDF_PREVIEW_DPI).tobytes('png'))
                     if xref is not None:
                         pixmap = fitz.Pixmap(pdf, xref)
-                        pixmap.shrink(2)
                         display_sixel_pixmap(pixmap.tobytes('png'))
 
                 choice = input(f'[N]ope out / [c]onvert page{"" if xref is None else " / e[x]tract image"} / [d]rop page / [s]how page? [n/c{"" if xref is None else "/x"}/d/s] ')