tests/gentle_compare.py: added pixmaps_rms().

Does simple RMS comparison of two bitmaps.

tests/gentle_compare.py

@@ -1,3 +1,5 @@
+import math
+
 import pymupdf
 
 
@@ -24,3 +26,29 @@ def gentle_compare(w0, w1):
             print(f"word {i}: rectangle mismatch {delta}")
             return False
     return True
+
+
+def pixmaps_rms(a, b, out_prefix=''):
+    '''
+    Returns RMS diff of raw bytes of two pixmaps. We assert that the pixmaps
+    are the same size.
+    <a> and <b> can each be a pymupdf.Pixmap or path of a bitmap file.
+    '''
+    if isinstance(a, str):
+        print(f'{out_prefix}pixmaps_rms(): reading pixmap from {a=}.')
+        a = pymupdf.Pixmap(a)
+    if isinstance(b, str):
+        print(f'{out_prefix}pixmaps_rms(): reading pixmap from {b=}.')
+        b = pymupdf.Pixmap(b)
+    assert a.irect == b.irect, f'Differing rects: {a.irect=} {b.irect=}.'
+    a_mv = a.samples_mv
+    b_mv = b.samples_mv
+    assert len(a_mv) == len(b_mv)
+    e = 0
+    for i, (a_byte, b_byte) in enumerate(zip(a_mv, b_mv)):
+        if i % 100000 == 0:
+            print(f'{out_prefix}compare_pixmaps(): {i=} {e=} {a_byte=} {b_byte=}.')
+        e += (a_byte - b_byte) ** 2
+    rms = math.sqrt(e / len(a_mv))
+    print(f'{out_prefix}compare_pixmaps(): {e=} {rms=}.')
+    return rms