bug#74476: [PATCH] Explore JPEG loading without quantization

[Manuel Giraud via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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Tags: patch

Hi,

While trying to speed up "emacs as an image viewer", I found that emacs
is using libjpeg with color quantization and it seems that removing this
quantization could speed up JPEG loading a bit.

My simple limited benchmark:

          - M-: (clear-image-cache)
          - Open an image in folder with some large enough pictures in
            it (4000x3000 here)
          - M-: (benchmark-run 10 (image-next-file 1))

Here are the results I get:

without this path: (5.415405491 1 0.09232176400000025)
with: (3.079911418 1 0.0751190459999993)

I don't think that this patch could be applied as is (it is rather
ugly).  And I also think that I probably have missed some (many?) use
case (where color quantization is mandatory).  But I'm submitting this
patch anyway as a conversation starter on the subject.

Thanks,

In GNU Emacs 31.0.50 (build 9, x86_64-unknown-openbsd7.6, X toolkit) of
 2024-11-22 built on computer
Repository revision: c66c0942ea9ac10e6d6324e472150de403a03b69
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101014
System Description: OpenBSD computer 7.6 GENERIC.MP#437 amd64

Configured using:
 'configure CC=egcc CPPFLAGS=-I/usr/local/include
 LDFLAGS=-L/usr/local/lib MAKEINFO=gmakeinfo --prefix=/home/manuel/emacs
 --bindir=/home/manuel/bin --with-x-toolkit=lucid
 --with-toolkit-scroll-bars=no --without-cairo
 --without-compress-install'


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From e360545b65427d6f0eec05cdeb26b8a05d8a0d61 Mon Sep 17 00:00:00 2001
From: Manuel Giraud <manuel@ledu-giraud.fr>
Date: Thu, 21 Nov 2024 17:19:59 +0100
Subject: [PATCH] Explore JPEG loading without quantization

---
 src/image.c | 63 +++++++++++++----------------------------------------
 1 file changed, 15 insertions(+), 48 deletions(-)

diff --git a/src/image.c b/src/image.c
index db7f6acd171..374d2f9c454 100644
--- a/src/image.c
+++ b/src/image.c
@@ -8949,9 +8949,9 @@ jpeg_load_body (struct frame *f, struct image *img,
   FILE *fp = NULL;
   JSAMPARRAY buffer;
   int row_stride, x, y;
-  int width, height;
-  int i, ir, ig, ib;
-  unsigned long *colors;
+  int width, height, ncomp;
+  int i, off;
+  unsigned long color;
   Emacs_Pix_Container volatile ximg_volatile = NULL;
 
   /* Open the JPEG file.  */
@@ -9049,12 +9049,11 @@ jpeg_load_body (struct frame *f, struct image *img,
 
   jpeg_read_header (&mgr->cinfo, 1);
 
-  /* Customize decompression so that color quantization will be used.
-	 Start decompression.  */
-  mgr->cinfo.quantize_colors = 1;
+  /* Start decompression.  */
   jpeg_start_decompress (&mgr->cinfo);
   width = img->width = mgr->cinfo.output_width;
   height = img->height = mgr->cinfo.output_height;
+  ncomp = mgr->cinfo.output_components;
 
   if (!check_image_size (f, width, height))
     {
@@ -9073,53 +9072,22 @@ jpeg_load_body (struct frame *f, struct image *img,
       sys_longjmp (mgr->setjmp_buffer, 1);
     }
 
-  /* Allocate colors.  When color quantization is used,
-     mgr->cinfo.actual_number_of_colors has been set with the number of
-     colors generated, and mgr->cinfo.colormap is a two-dimensional array
-     of color indices in the range 0..mgr->cinfo.actual_number_of_colors.
-     No more than 255 colors will be generated.  */
-  USE_SAFE_ALLOCA;
-  {
-    if (mgr->cinfo.out_color_components > 2)
-      ir = 0, ig = 1, ib = 2;
-    else if (mgr->cinfo.out_color_components > 1)
-      ir = 0, ig = 1, ib = 0;
-    else
-      ir = 0, ig = 0, ib = 0;
-
-    /* Use the color table mechanism because it handles colors that
-       cannot be allocated nicely.  Such colors will be replaced with
-       a default color, and we don't have to care about which colors
-       can be freed safely, and which can't.  */
-    init_color_table ();
-    SAFE_NALLOCA (colors, 1, mgr->cinfo.actual_number_of_colors);
-
-    for (i = 0; i < mgr->cinfo.actual_number_of_colors; ++i)
-      {
-	/* Multiply RGB values with 255 because X expects RGB values
-	   in the range 0..0xffff.  */
-	int r = mgr->cinfo.colormap[ir][i] << 8;
-	int g = mgr->cinfo.colormap[ig][i] << 8;
-	int b = mgr->cinfo.colormap[ib][i] << 8;
-	colors[i] = lookup_rgb_color (f, r, g, b);
-      }
-
-#ifdef COLOR_TABLE_SUPPORT
-    /* Remember those colors actually allocated.  */
-    img->colors = colors_in_color_table (&img->ncolors);
-    free_color_table ();
-#endif /* COLOR_TABLE_SUPPORT */
-  }
-
   /* Read pixels.  */
-  row_stride = width * mgr->cinfo.output_components;
+  row_stride = width * ncomp;
   buffer = mgr->cinfo.mem->alloc_sarray ((j_common_ptr) &mgr->cinfo,
 					 JPOOL_IMAGE, row_stride, 1);
   for (y = 0; y < height; ++y)
     {
       jpeg_read_scanlines (&mgr->cinfo, buffer, 1);
-      for (x = 0; x < mgr->cinfo.output_width; ++x)
-	PUT_PIXEL (ximg, x, y, colors[buffer[0][x]]);
+      for (x = 0; x < width; ++x)
+	{
+	  color = 0;
+	  off = x * ncomp;
+	  /* XXX I suck at bit twiddling.  */
+	  for (i = 0; i < ncomp; ++i)
+	    color += (buffer[0][off + i] << ((ncomp - 1 - i) * 8));
+	  PUT_PIXEL (ximg, x, y, color);
+	}
     }
 
   /* Clean up.  */
@@ -9135,7 +9103,6 @@ jpeg_load_body (struct frame *f, struct image *img,
 
   /* Put ximg into the image.  */
   image_put_x_image (f, img, ximg, 0);
-  SAFE_FREE ();
   return 1;
 }
 
-- 
2.47.0


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-- 
Manuel Giraud