A PDF page taller than one raster is drawn at the size it is shown, in bands around the screen, not squeezed into 4096 rows and blown up

The render geometry (pdf_bridge.c) clamped a page's LONGEST side to
max_dimension: 4096 in the SDL policy, 1200 in Kitty's. A page 841.89 x
4818.9 pt, fit to a 1570 px wide pane, came out 715 x 4096 and was drawn
2.2 times too large at 1x, 4.4 times at 2x; Kitty's raster was 210 px
wide. Every request now caps a raster ROW at max_dimension, and caps
the height too only while that keeps the page at least as large as it
is shown (RenderRequest.display_width/height, set from the fit). A
tall page fit to width is rasterized at the pane's own width, in SDL,
and at Kitty's 96 dpi as any other page is there.

Such a page is not one buffer. A raster over 8192 rows (a texture's and
Kitty's 10000-pixel limit) or over 64 MiB is banded (pdf_view.zig): its
slot holds the rows on screen and a screenful either side, in 512-row
chunks, and is rendered again, keeping the chunks it has, once the
screen comes within half a screenful of its edge. Each chunk comes from
its own render reaching 32 rows past both edges, so a row of the page
is the same however it was scrolled to, and no chunk shows its clip
edge: band rows are within 3 levels of a whole-page render (108 pixels
of 45 million on the reported PDF). Highlights paint over the band's
clean rows as over a page's, and the band uses the page's CTM, so
search, selection and pointer geometry are unchanged.

A display-list render now culls to the band (a scissor in page space).
No pixel changes, but a frame that renders new rows of the reported
page at 2x fell from 33-43 ms to 13-16 ms (pdf-scroll-bench, --cell
16x32). design.pdf's scroll-bench pixels are identical.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 62 insertions, 14 deletions

diff --git a/src/pdf_bridge.c b/src/pdf_bridge.c
index 2c723dbb..6fdab46c 100644
--- a/src/pdf_bridge.c
+++ b/src/pdf_bridge.c
@@ -481,6 +481,8 @@ pardes_pdf_render_geometry(
     int minimum_width,
     int minimum_height,
     int max_dimension,
+    int display_width,
+    int display_height,
     fz_matrix *ctm_out,
     fz_irect *bbox_out)
 {
@@ -488,23 +490,38 @@ pardes_pdf_render_geometry(
     fz_rect bounds;
     float page_width;
     float page_height;
-    float longest;
     float scale;
+    float shown = 0.0f;
 
     pardes_pdf_cache_page(document, page_number, 0);
     bounds = document->cached_bounds;
     page_width = bounds.x1 - bounds.x0;
     page_height = bounds.y1 - bounds.y0;
-    longest = fmaxf(page_width, page_height);
-    if (!(longest > 0.0f))
+    if (!(page_width > 0.0f) || !(page_height > 0.0f))
         fz_throw(document->ctx, FZ_ERROR_FORMAT, "PDF page has empty bounds");
     scale = (float)dpi / 72.0f;
     if (minimum_width > 0)
         scale = fmaxf(scale, (float)minimum_width / page_width);
     if (minimum_height > 0)
         scale = fmaxf(scale, (float)minimum_height / page_height);
-    if (longest * scale > (float)max_dimension)
-        scale = (float)max_dimension / longest;
+    /*
+     * A row of the raster is never longer than max_dimension: every band of
+     * the page carries whole rows. The height is held to it too, as the
+     * budget of one whole-page raster, but never below the scale the page is
+     * shown at: a page taller than that (a poster, a long web capture) is
+     * drawn in bands at the display's own resolution (pdf_view.zig) instead
+     * of being squeezed into max_dimension rows and blown up again, blurred.
+     */
+    if (page_width * scale > (float)max_dimension)
+        scale = (float)max_dimension / page_width;
+    if (display_width > 0)
+        shown = fmaxf(shown, (float)display_width / page_width);
+    if (display_height > 0)
+        shown = fmaxf(shown, (float)display_height / page_height);
+    if (page_height * scale > (float)max_dimension)
+        scale = fminf(scale, fmaxf((float)max_dimension / page_height, shown));
+    if (page_height * scale > (float)PARDES_PDF_MAX_RASTER_ROWS)
+        scale = (float)PARDES_PDF_MAX_RASTER_ROWS / page_height;
 
     /*
      * Keep this identical to fz_new_pixmap_from_page/display_list: transform
@@ -526,6 +543,8 @@ pardes_pdf_measure_render(
     int minimum_width,
     int minimum_height,
     int max_dimension,
+    int display_width,
+    int display_height,
     pardes_pdf_raster_layout *out)
 {
     fz_context *ctx;
@@ -541,14 +560,16 @@ pardes_pdf_measure_render(
     memset(out, 0, sizeof(*out));
     if (document == NULL || page_number < 0 ||
         page_number >= document->page_count || dpi < 1 ||
-        minimum_width < 0 || minimum_height < 0 || max_dimension < 1)
+        minimum_width < 0 || minimum_height < 0 || max_dimension < 1 ||
+        display_width < 0 || display_height < 0)
         return PARDES_PDF_ERROR;
 
     ctx = document->ctx;
     fz_try(ctx)
     {
         pardes_pdf_render_geometry(document, page_number, dpi,
-            minimum_width, minimum_height, max_dimension, &ctm, &bbox);
+            minimum_width, minimum_height, max_dimension,
+            display_width, display_height, &ctm, &bbox);
         width = fz_irect_width(bbox);
         height = fz_irect_height(bbox);
         if (width > INT_MAX / 4)
@@ -629,6 +650,8 @@ pardes_pdf_render_into(
     int minimum_width,
     int minimum_height,
     int max_dimension,
+    int display_width,
+    int display_height,
     const pardes_pdf_highlight *highlights,
     size_t highlight_count,
     unsigned char *samples,
@@ -649,6 +672,7 @@ pardes_pdf_render_into(
     if (document == NULL || page_number < 0 ||
         page_number >= document->page_count || dpi < 1 ||
         minimum_width < 0 || minimum_height < 0 || max_dimension < 1 ||
+        display_width < 0 || display_height < 0 ||
         (highlight_count != 0 && highlights == NULL) ||
         highlight_count > PARDES_PDF_MAX_RESULT_QUADS || samples == NULL ||
         width < 1 || height < 1 || stride < 1 || width > INT_MAX / 4 ||
@@ -664,7 +688,8 @@ pardes_pdf_render_into(
     fz_try(ctx)
     {
         pardes_pdf_render_geometry(document, page_number, dpi,
-            minimum_width, minimum_height, max_dimension, &ctm, &bbox);
+            minimum_width, minimum_height, max_dimension,
+            display_width, display_height, &ctm, &bbox);
         if (fz_irect_width(bbox) != width ||
             fz_irect_height(bbox) != height)
             fz_throw(ctx, FZ_ERROR_ARGUMENT,
@@ -676,9 +701,13 @@ pardes_pdf_render_into(
          * discards everything outside these rows. The rows are NOT always
          * bit-identical to the full page's: rows at the band's edges, and a
          * resampled image anywhere in it, can differ (measured on
-         * docs/registry.pdf and docs/design.pdf). Good enough for a band that
-         * is only shown while flinging; never use a band to patch rows into a
-         * whole-page raster -- `pardes_pdf_paint_highlights` exists for that.
+         * docs/registry.pdf and docs/design.pdf), and an edge crossing the
+         * clip is stepped from a different start along its whole length,
+         * a level or two off. Good enough for a band shown while flinging,
+         * and for the chunks of a page too tall to render whole (pdf_view.zig
+         * renders those past their edges and keeps the middle); never use a
+         * band to patch rows into a whole-page raster --
+         * `pardes_pdf_paint_highlights` exists for that.
          */
         bbox.y0 += band_y;
         bbox.y1 = bbox.y0 + band_height;
@@ -708,10 +737,21 @@ pardes_pdf_render_into(
 
         if (document->cached_display_list != NULL ||
             document->display_list_candidate_page_number == page_number) {
+            /*
+             * Only the nodes that reach the band: a band of a tall page
+             * skips the rest of the page instead of handing every node to
+             * the draw device to clip away. The scissor is in page space
+             * (the list runs untransformed under the device's CTM), a pixel
+             * wider than the band all round. Culling never changes a pixel:
+             * a node outside it draws nothing here.
+             */
+            fz_rect scissor = fz_rect_from_irect(bbox);
+            scissor.x0 -= 1; scissor.y0 -= 1; scissor.x1 += 1; scissor.y1 += 1;
+            scissor = fz_transform_rect(scissor, fz_invert_matrix(ctm));
             pardes_pdf_cache_display_list(document);
             device = fz_new_draw_device(ctx, ctm, pixmap);
             fz_run_display_list(ctx, document->cached_display_list, device,
-                fz_identity, fz_infinite_rect, NULL);
+                fz_identity, scissor, NULL);
         } else {
             device = fz_new_draw_device(ctx, ctm, pixmap);
             fz_run_page(ctx, document->cached_page, device, fz_identity, NULL);
@@ -767,6 +807,8 @@ pardes_pdf_paint_highlights(
     int minimum_width,
     int minimum_height,
     int max_dimension,
+    int display_width,
+    int display_height,
     const pardes_pdf_highlight *highlights,
     size_t highlight_count,
     unsigned char *samples,
@@ -786,6 +828,7 @@ pardes_pdf_paint_highlights(
     if (document == NULL || page_number < 0 ||
         page_number >= document->page_count || dpi < 1 ||
         minimum_width < 0 || minimum_height < 0 || max_dimension < 1 ||
+        display_width < 0 || display_height < 0 ||
         (highlight_count != 0 && highlights == NULL) ||
         highlight_count > PARDES_PDF_MAX_RESULT_QUADS || samples == NULL ||
         width < 1 || height < 1 || width > INT_MAX / 4 || stride != width * 4 ||
@@ -802,7 +845,8 @@ pardes_pdf_paint_highlights(
     fz_try(ctx)
     {
         pardes_pdf_render_geometry(document, page_number, dpi,
-            minimum_width, minimum_height, max_dimension, &ctm, &bbox);
+            minimum_width, minimum_height, max_dimension,
+            display_width, display_height, &ctm, &bbox);
         if (fz_irect_width(bbox) != width || fz_irect_height(bbox) != height)
             fz_throw(ctx, FZ_ERROR_ARGUMENT,
                 "PDF raster layout changed between measure and paint");
@@ -835,6 +879,8 @@ pardes_pdf_highlight_rows(
     int minimum_width,
     int minimum_height,
     int max_dimension,
+    int display_width,
+    int display_height,
     const pardes_pdf_highlight *highlights,
     size_t highlight_count,
     int *row_start,
@@ -853,6 +899,7 @@ pardes_pdf_highlight_rows(
     if (document == NULL || page_number < 0 ||
         page_number >= document->page_count || dpi < 1 ||
         minimum_width < 0 || minimum_height < 0 || max_dimension < 1 ||
+        display_width < 0 || display_height < 0 ||
         (highlight_count != 0 && highlights == NULL))
         return PARDES_PDF_ERROR;
 
@@ -860,7 +907,8 @@ pardes_pdf_highlight_rows(
     fz_try(ctx)
     {
         pardes_pdf_render_geometry(document, page_number, dpi,
-            minimum_width, minimum_height, max_dimension, &ctm, &bbox);
+            minimum_width, minimum_height, max_dimension,
+            display_width, display_height, &ctm, &bbox);
         for (i = 0; i < highlight_count; ++i) {
             fz_rect r;
             if (highlights[i].rgba[3] == 0 ||