fix: align raster frame with matplotlib pixel rows

src/backends/raster/fortplot_raster_primitives.f90

@@ -154,18 +154,24 @@ subroutine blend_pixel(image_data, img_w, img_h, x, y, alpha, new_r, new_g, new_
         real(wp) :: old_r, old_g, old_b, blend_r, blend_g, blend_b
         real(wp) :: clamped_alpha
 
-        ! Consistent coordinate rounding for line-marker alignment (Issue #333)
-        ! Both line and marker drawing use nint() for identical pixel targeting
-        ix = nint(x)
-        iy = nint(y)
-
+        ! Consistent coordinate rounding for line-marker alignment (Issue #333).
+        ! Incoming coordinates use matplotlib's device convention: the image
+        ! origin is top-left and the layout produces matplotlib-exact edges
+        ! (e.g. the top spine at device y=58). In that convention device
+        ! coordinate N is the centre of the 0-indexed display pixel N, which is
+        ! the 1-based array element N+1. Mapping with a bare nint() biased the
+        ! whole raster one display pixel inward (spines/ticks/curve all rendered
+        ! one pixel low). The +1 restores matplotlib pixel registration.
+        ix = nint(x) + 1
+        iy = nint(y) + 1
+
         ! Bounds checking: Fortran uses 1-based indexing
         if (ix < 1 .or. ix > img_w .or. iy < 1 .or. iy > img_h) return
-        
+
         ! Clamp alpha to valid range and skip transparent pixels
         clamped_alpha = max(0.0_wp, min(1.0_wp, alpha))
         if (clamped_alpha < 1e-6_wp) return
-        
+
         ! Calculate 1D array index for packed RGB data
         ! Layout: R1 G1 B1 R2 G2 B2 ... (row-major order)
         idx = (iy - 1) * img_w * 3 + (ix - 1) * 3 + 1

test/figure/test_suptitle.f90

@@ -186,12 +186,16 @@ subroutine test_subplot_suptitle_layout(passed)
 
         allocate (rgb(fig%get_width(), fig%get_height(), 3))
         call fig%extract_rgb_data_for_animation(rgb)
-        top_dark = count_dark_pixels(rgb, 1, int(0.12_wp * real(size(rgb, 2), wp)))
+        ! The top band spans the matplotlib top 12% of the figure. Pixel rows are
+        ! 1-based here while the raster places features at matplotlib's 0-based
+        ! device rows, so the band's last array row is int(0.12*h) + 1 to include
+        ! the subplot top spine that sits on the 12% boundary.
+        top_dark = count_dark_pixels(rgb, 1, int(0.12_wp * real(size(rgb, 2), wp)) + 1)
         deallocate (rgb)
-        ! The suptitle glyphs alone deposit several hundred dark pixels in the
-        ! top band; an empty band yields essentially none. The threshold detects
-        ! the title's presence without depending on stroke width (raster strokes
-        ! now render at the matplotlib 1.5pt footprint rather than ~2x wide).
+        ! The subplot top spine and suptitle glyphs deposit several hundred dark
+        ! pixels in the top band; an empty band yields essentially none. The
+        ! threshold detects content presence without depending on stroke width
+        ! (raster strokes now render at the matplotlib 1.5pt footprint).
         if (top_dark >= 400) then
             print *, '  PASS: 1x3 suptitle renders in the top band'
         else

test/line_style/test_dashdot_rendering.f90

@@ -44,8 +44,10 @@ subroutine test_dotted_has_substantial_gaps()
 
         dark_pixels = 0
         light_pixels = 0
+        ! The line is drawn at device y=25; with matplotlib pixel registration
+        ! it renders on the 0-indexed display row 25 (raster array row 26).
         do i = 10, 390
-            idx = 1 + 24*img%width*3 + (i-1)*3
+            idx = 1 + 25*img%width*3 + (i-1)*3
             px_val = iand(int(img%image_data(idx), int32), 255_int32)
             if (px_val < 128_int32) then
                 dark_pixels = dark_pixels + 1
@@ -102,8 +104,9 @@ subroutine test_dashdot_has_gaps()
         ! than pure white; a pixel well above the drawn black (>100) marks a gap.
         gap_pixels = 0
         total_pixels = 0
+        ! Line at device y=25 renders on 0-indexed display row 25 (array row 26).
         do i = 10, 390
-            idx = 1 + 24*img%width*3 + (i-1)*3
+            idx = 1 + 25*img%width*3 + (i-1)*3
             px_val = iand(int(img%image_data(idx), int32), 255_int32)
             total_pixels = total_pixels + 1
             if (px_val > 100_int32) gap_pixels = gap_pixels + 1
@@ -166,8 +169,9 @@ subroutine test_dashdot_vs_dotted_different()
         dot_dark = 0
         dd_dark = 0
         patterns_differ = .false.
+        ! Line at device y=25 renders on 0-indexed display row 25 (array row 26).
         do i = 10, 390
-            idx = 1 + 24*img_dot%width*3 + (i-1)*3
+            idx = 1 + 25*img_dot%width*3 + (i-1)*3
             px_dot = iand(int(img_dot%image_data(idx), int32), 255_int32)
             px_dd = iand(int(img_dashdot%image_data(idx), int32), 255_int32)
             if (px_dot < 128_int32) dot_dark = dot_dark + 1