Merge pull request #275 from mozilla/idct-selector

Make IDCT method customizable

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

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;

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 */
 
@@ -1137,6 +1139,12 @@ EXTERN(boolean) jpeg_read_icc_profile(j_decompress_ptr cinfo,
                                       JOCTET **icc_data_ptr,
                                       unsigned int *icc_data_len);
 
+/*
+ * 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.