Tweak art_pdf_composite_pixel_alpha_8.

art_pdf_composite_pixel_alpha_8 is used to blend src and
destination pixels together with a given blend mode.

In the general case, we have n colour compoents, of which the
first n-s are process ones, and the next s are spots (followed
by alpha).

In some cases, we need to blend the first n-s with the given blend
mode, and the remaining s in Normal mode.

This is currently achieved in the code by making 2 different calls
down to art_pdf_composite_pixel_8 for each section of the pixel
(with different 'n' values).

Unfortunately, due to alpha being assumed to be held as the nth
component, this means we need to do a dance where we stash away
a components value, copy the alpha in, call the routine, then
restore the alpha afterwards.

To avoid this, we modify art_pdf_composite_pixel_alpha_8 to take
another int parameter, specifying the position of the first
component that needs to be blended with the Normal mode.

This enables us to simplify the outer routines that call this so
that we have a single call, rather than 3. It also means we don't
recaulate 'scale' etc twice.

I am not expecting any significant speedups from this alone, but
it simplifies the code so that hopefully we might get some in
later commits.

1 files changed, 25 insertions, 37 deletions

diff --git a/base/gxblend.c b/base/gxblend.c
index 94a13491a..ddcb4964a 100644
--- a/base/gxblend.c
+++ b/base/gxblend.c
@@ -883,7 +883,7 @@ art_pdf_knockout_composite_pixel_alpha_8(byte *backdrop, byte tos_shape, byte *d
 
 void
 art_pdf_composite_pixel_alpha_8(byte *dst, const byte *src, int n_chan,
-        gs_blend_mode_t blend_mode,
+        gs_blend_mode_t blend_mode, int first_spot,
         const pdf14_nonseparable_blending_procs_t * pblend_procs, pdf14_device *p14dev)
 {
     byte a_b, a_s;
@@ -918,20 +918,12 @@ art_pdf_composite_pixel_alpha_8(byte *dst, const byte *src, int n_chan,
     /* Compute a_s / a_r in 16.16 format */
     src_scale = ((a_s << 16) + (a_r >> 1)) / a_r;
 
-    if (blend_mode == BLEND_MODE_Normal) {
-        /* Do simple compositing of source over backdrop */
-        for (i = 0; i < n_chan; i++) {
-            c_s = src[i];
-            c_b = dst[i];
-            tmp = (c_b << 16) + src_scale * (c_s - c_b) + 0x8000;
-            dst[i] = tmp >> 16;
-        }
-    } else {
+    if (first_spot != 0) {
         /* Do compositing with blending */
         byte blend[ART_MAX_CHAN];
 
-        art_blend_pixel_8(blend, dst, src, n_chan, blend_mode, pblend_procs, p14dev);
-        for (i = 0; i < n_chan; i++) {
+        art_blend_pixel_8(blend, dst, src, first_spot, blend_mode, pblend_procs, p14dev);
+        for (i = 0; i < first_spot; i++) {
             int c_bl;		/* Result of blend function */
             int c_mix;		/* Blend result mixed with source color */
 
@@ -945,6 +937,20 @@ art_pdf_composite_pixel_alpha_8(byte *dst, const byte *src, int n_chan,
         }
     }
     dst[n_chan] = a_r;
+
+    dst += first_spot;
+    src += first_spot;
+    n_chan -= first_spot;
+    if (n_chan == 0)
+        return;
+
+    /* Do simple compositing of source over backdrop */
+    for (i = 0; i < n_chan; i++) {
+        c_s = src[i];
+        c_b = dst[i];
+        tmp = (c_b << 16) + src_scale * (c_s - c_b) + 0x8000;
+        dst[i] = tmp >> 16;
+    }
 }
 
 void
@@ -1086,6 +1092,9 @@ art_pdf_recomposite_group_8(byte *dst, byte *dst_alpha_g,
     } else {
         /* "interesting" blend mode */
         byte ca[ART_MAX_CHAN + 1];	/* $C, \alpha$ */
+        int first_blend_spot = n_chan;
+        if (num_spots > 0 && !blend_valid_for_spot(blend_mode))
+            first_blend_spot = n_chan - num_spots;
 
         dst_alpha = dst[n_chan];
         if (src_alpha_g == 255 || dst_alpha == 0) {
@@ -1119,29 +1128,8 @@ art_pdf_recomposite_group_8(byte *dst, byte *dst_alpha_g,
             tmp = (255 - *dst_alpha_g) * (255 - tmp) + 0x80;
             *dst_alpha_g = 255 - ((tmp + (tmp >> 8)) >> 8);
         }
-        if (num_spots > 0 && !blend_valid_for_spot(blend_mode)) {
-            /* Split and do the spots with normal blend mode.
-               Blending functions assume alpha is last component so do some
-               movements here */
-            byte temp_spot_src = ca[n_chan - num_spots];
-            byte temp_spot_dst = dst[n_chan - num_spots];
-            ca[n_chan - num_spots] = ca[n_chan];
-            dst[n_chan - num_spots] = dst[n_chan];
-
-            /* Blend process */
-            art_pdf_composite_pixel_alpha_8(dst, ca, n_chan - num_spots,
-                blend_mode, pblend_procs, p14dev);
-
-            /* Restore colorants that were blown away by alpha and blend spots */
-            dst[n_chan - num_spots] = temp_spot_dst;
-            ca[n_chan - num_spots] = temp_spot_src;
-            art_pdf_composite_pixel_alpha_8(&(dst[n_chan - num_spots]),
-                &(ca[n_chan - num_spots]), num_spots,
-                BLEND_MODE_Normal, pblend_procs, p14dev);
-        } else {
-            art_pdf_composite_pixel_alpha_8(dst, ca, n_chan, blend_mode,
+        art_pdf_composite_pixel_alpha_8(dst, ca, n_chan, blend_mode, first_blend_spot,
                 pblend_procs, p14dev);
-        }
     }
     /* todo: optimize BLEND_MODE_Normal buf alpha != 255 case */
 }
@@ -1193,7 +1181,7 @@ art_pdf_composite_knockout_group_8(byte *backdrop, byte tos_shape, byte *dst,
 
 void
 art_pdf_composite_group_8(byte *dst, byte *dst_alpha_g,
-        const byte *src, int n_chan, byte alpha, gs_blend_mode_t blend_mode,
+        const byte *src, int n_chan, byte alpha, gs_blend_mode_t blend_mode, int first_spot,
         const pdf14_nonseparable_blending_procs_t * pblend_procs,
         pdf14_device *p14dev)
 {
@@ -1212,7 +1200,7 @@ art_pdf_composite_group_8(byte *dst, byte *dst_alpha_g,
     }
 
     art_pdf_composite_pixel_alpha_8(dst, src, n_chan,
-            blend_mode, pblend_procs, p14dev);
+            blend_mode, first_spot, pblend_procs, p14dev);
     if (dst_alpha_g != NULL) {
         tmp = (255 - *dst_alpha_g) * (255 - src[n_chan]) + 0x80;
         *dst_alpha_g = 255 - ((tmp + (tmp >> 8)) >> 8);
@@ -1270,7 +1258,7 @@ art_pdf_knockoutisolated_group_aa_8(byte *dst, const byte *src, byte src_alpha,
         temp_src[i] = src[i];
     temp_src[n_chan] = aa_alpha;
     art_pdf_composite_pixel_alpha_8(dst, temp_src, n_chan, BLEND_MODE_Normal,
-        NULL, p14dev);
+        n_chan, NULL, p14dev);
     dst[n_chan] = src_alpha;
 }