From 365bc1ce1197914ef21af9673c7a5d06e40fc2a1 Mon Sep 17 00:00:00 2001
From: Kornel <kornel@geekhood.net>
Date: Sat, 23 Dec 2017 01:29:16 +0000
Subject: [PATCH] Make IDCT method customizable

---
 jddctmgr.c | 14 ++++++++++++++
 jdmaster.h |  5 +++++
 jpeglib.h  |  8 ++++++++
 3 files changed, 27 insertions(+)

diff --git a/jddctmgr.c b/jddctmgr.c
index 3a5ba7e8..a9f876f2 100644
--- a/jddctmgr.c
+++ b/jddctmgr.c
@@ -27,6 +27,7 @@
 #include "jdct.h"               /* Private declarations for DCT subsystem */
 #include "jsimddct.h"
 #include "jpegcomp.h"
+#include "jdmaster.h"
 
 
 /*
@@ -86,6 +87,11 @@ typedef union {
 #endif
 #endif
 
+EXTERN(void) jpeg_set_idct_method_selector (j_decompress_ptr cinfo, jpeg_idct_method_selector selector){
+  my_master_ptr master = (my_master_ptr) cinfo->master;
+  master->custom_idct_selector = selector;
+}
+
 
 /*
  * Prepare for an output pass.
@@ -225,6 +231,14 @@ start_pass (j_decompress_ptr cinfo)
       ERREXIT1(cinfo, JERR_BAD_DCTSIZE, compptr->_DCT_scaled_size);
       break;
     }
+
+    // Allow custom idct function to be set dynamically
+    my_master_ptr master = (my_master_ptr) cinfo->master;
+
+    if (master->custom_idct_selector != NULL) {
+      master->custom_idct_selector(cinfo, compptr, &method_ptr, &method);
+    }
+
     idct->pub.inverse_DCT[ci] = method_ptr;
     /* Create multiplier table from quant table.
      * However, we can skip this if the component is uninteresting
diff --git a/jdmaster.h b/jdmaster.h
index 76897e28..d08d1372 100644
--- a/jdmaster.h
+++ b/jdmaster.h
@@ -23,6 +23,11 @@ typedef struct {
    */
   struct jpeg_color_quantizer *quantizer_1pass;
   struct jpeg_color_quantizer *quantizer_2pass;
+
+  /*
+   * Permit users to replace the IDCT method
+  */
+  jpeg_idct_method_selector custom_idct_selector;
 } my_decomp_master;
 
 typedef my_decomp_master *my_master_ptr;
diff --git a/jpeglib.h b/jpeglib.h
index 73a24e87..fac60fb4 100644
--- a/jpeglib.h
+++ b/jpeglib.h
@@ -514,6 +514,8 @@ struct jpeg_compress_struct {
   int script_space_size;
 };
 
+typedef void (*jpeg_idct_method) (j_decompress_ptr cinfo, jpeg_component_info *compptr, JCOEFPTR coef_block, JSAMPARRAY output_buf, JDIMENSION output_col);
+typedef void (*jpeg_idct_method_selector) (j_decompress_ptr cinfo, jpeg_component_info *compptr, jpeg_idct_method * set_idct_method, int * set_idct_category);
 
 /* Master record for a decompression instance */
 
@@ -1128,6 +1130,12 @@ EXTERN(void) jpeg_c_set_int_param (j_compress_ptr cinfo, J_INT_PARAM param,
                                    int value);
 EXTERN(int) jpeg_c_get_int_param (const j_compress_ptr cinfo, J_INT_PARAM param);
 
+/*
+ * Permit users to replace the IDCT method dynamically.
+ * The selector callback is called after the default idct implementation was choosen,
+ * and is able to override it.
+ */
+EXTERN(void) jpeg_set_idct_method_selector (j_decompress_ptr cinfo, jpeg_idct_method_selector selector);
 
 /* These marker codes are exported since applications and data source modules
  * are likely to want to use them.