glamor: tidy up some gradient color formulas

- Avoid some needlessly complex equations for calculating the color at a
  point in a gradient field.
- Avoid calculating certain values multiple times.
- Use similar variable names across the two versions of the get_color
  function where practical.

Signed-off-by: Jeff Smith <whydoubt@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit ff410c56bfd6dcdf5d252ef0ba3f4c6fde91774b)

1 files changed, 30 insertions, 36 deletions

diff --git a/glamor/glamor_gradient.c b/glamor/glamor_gradient.c
index 5deaf5ff4..f0b7d129c 100644
--- a/glamor/glamor_gradient.c
+++ b/glamor/glamor_gradient.c
@@ -54,8 +54,9 @@ _glamor_create_getcolor_fs_source(ScreenPtr screen, int stops_count,
 	    "vec4 get_color(float stop_len)\n"\
 	    "{\n"\
 	    "    int i = 0;\n"\
-	    "    float new_alpha; \n"\
+	    "    vec4 stop_color_before;\n"\
 	    "    vec4 gradient_color;\n"\
+	    "    float stop_delta;\n"\
 	    "    float percentage; \n"\
 	    "    \n"\
 	    "    if(stop_len < stops[0])\n"\
@@ -67,19 +68,19 @@ _glamor_create_getcolor_fs_source(ScreenPtr screen, int stops_count,
 	    "    if(i == n_stop)\n"\
 	    "        return vec4(0.0, 0.0, 0.0, 0.0); \n"\
 	    "    \n"\
-	    "    if(stops[i] - stops[i-1] > 2.0)\n"\
+	    "    stop_color_before = stop_colors[i-1];\n"\
+	    "    stop_delta = stops[i] - stops[i-1];\n"\
+	    "    if(stop_delta > 2.0)\n"\
 	    "        percentage = 0.0;\n" /*For comply with pixman, walker->stepper overflow.*/\
-	    "    else if(stops[i] - stops[i-1] < 0.000001)\n"\
+	    "    else if(stop_delta < 0.000001)\n"\
 	    "        percentage = 0.0;\n"\
 	    "    else \n"\
-	    "        percentage = (stop_len - stops[i-1])/(stops[i] - stops[i-1]);\n"\
-	    "    new_alpha = percentage * stop_colors[i].a + \n"\
-	    "                       (1.0-percentage) * stop_colors[i-1].a; \n"\
-	    "    gradient_color = vec4((percentage * stop_colors[i].rgb \n"\
-	    "                          + (1.0-percentage) * stop_colors[i-1].rgb)*new_alpha, \n"\
-	    "                          new_alpha);\n"\
+	    "        percentage = (stop_len - stops[i-1])/stop_delta;\n"\
 	    "    \n"\
-	    "    return gradient_color;\n"\
+	    "    gradient_color = stop_color_before;\n"\
+	    "    if(percentage != 0.0)\n"\
+	    "        gradient_color += (stop_colors[i] - gradient_color)*percentage;\n"\
+	    "    return vec4(gradient_color.rgb * gradient_color.a, gradient_color.a);\n"\
 	    "}\n"
 
     /* Because the array access for shader is very slow, the performance is very low
@@ -106,73 +107,66 @@ _glamor_create_getcolor_fs_source(ScreenPtr screen, int stops_count,
         "\n"
         "vec4 get_color(float stop_len)\n"
         "{\n"
-        "    float stop_after;\n"
-        "    float stop_before;\n"
         "    vec4 stop_color_before;\n"
         "    vec4 stop_color_after;\n"
-        "    float new_alpha; \n"
         "    vec4 gradient_color;\n"
+        "    float stop_before;\n"
+        "    float stop_delta;\n"
         "    float percentage; \n"
         "    \n"
         "    if((stop_len < stop0) && (n_stop >= 1)) {\n"
         "        stop_color_before = vec4(0.0, 0.0, 0.0, 0.0);\n"
-        "        stop_color_after = vec4(0.0, 0.0, 0.0, 0.0);\n"
-        "        stop_after = 0.0;\n"
-        "        stop_before = 0.0;\n"
+        "        stop_delta = 0.0;\n"
         "    } else if((stop_len < stop1) && (n_stop >= 2)) {\n"
         "        stop_color_before = stop_color0;\n"
         "        stop_color_after = stop_color1;\n"
-        "        stop_after = stop1;\n"
         "        stop_before = stop0;\n"
+        "        stop_delta = stop1 - stop0;\n"
         "    } else if((stop_len < stop2) && (n_stop >= 3)) {\n"
         "        stop_color_before = stop_color1;\n"
         "        stop_color_after = stop_color2;\n"
-        "        stop_after = stop2;\n"
         "        stop_before = stop1;\n"
+        "        stop_delta = stop2 - stop1;\n"
         "    } else if((stop_len < stop3) && (n_stop >= 4)){\n"
         "        stop_color_before = stop_color2;\n"
         "        stop_color_after = stop_color3;\n"
-        "        stop_after = stop3;\n"
         "        stop_before = stop2;\n"
+        "        stop_delta = stop3 - stop2;\n"
         "    } else if((stop_len < stop4) && (n_stop >= 5)){\n"
         "        stop_color_before = stop_color3;\n"
         "        stop_color_after = stop_color4;\n"
-        "        stop_after = stop4;\n"
         "        stop_before = stop3;\n"
+        "        stop_delta = stop4 - stop3;\n"
         "    } else if((stop_len < stop5) && (n_stop >= 6)){\n"
         "        stop_color_before = stop_color4;\n"
         "        stop_color_after = stop_color5;\n"
-        "        stop_after = stop5;\n"
         "        stop_before = stop4;\n"
+        "        stop_delta = stop5 - stop4;\n"
         "    } else if((stop_len < stop6) && (n_stop >= 7)){\n"
         "        stop_color_before = stop_color5;\n"
         "        stop_color_after = stop_color6;\n"
-        "        stop_after = stop6;\n"
         "        stop_before = stop5;\n"
+        "        stop_delta = stop6 - stop5;\n"
         "    } else if((stop_len < stop7) && (n_stop >= 8)){\n"
         "        stop_color_before = stop_color6;\n"
         "        stop_color_after = stop_color7;\n"
-        "        stop_after = stop7;\n"
         "        stop_before = stop6;\n"
+        "        stop_delta = stop7 - stop6;\n"
         "    } else {\n"
         "        stop_color_before = vec4(0.0, 0.0, 0.0, 0.0);\n"
-        "        stop_color_after = vec4(0.0, 0.0, 0.0, 0.0);\n"
-        "        stop_after = 0.0;\n"
-        "        stop_before = 0.0;\n"
+        "        stop_delta = 0.0;\n"
         "    }\n"
-        "    if(stop_after - stop_before > 2.0)\n"
+        "    if(stop_delta > 2.0)\n"
         "        percentage = 0.0;\n" //For comply with pixman, walker->stepper overflow.
-        "    else if(stop_after - stop_before < 0.000001)\n"
+        "    else if(stop_delta < 0.000001)\n"
         "        percentage = 0.0;\n"
-        "    else \n"
-        "        percentage = (stop_len - stop_before)/(stop_after - stop_before);\n"
-        "    new_alpha = percentage * stop_color_after.a + \n"
-        "                       (1.0-percentage) * stop_color_before.a; \n"
-        "    gradient_color = vec4((percentage * stop_color_after.rgb \n"
-        "                          + (1.0-percentage) * stop_color_before.rgb)*new_alpha, \n"
-        "                          new_alpha);\n"
+        "    else\n"
+        "        percentage = (stop_len - stop_before)/stop_delta;\n"
         "    \n"
-        "    return gradient_color;\n"
+        "    gradient_color = stop_color_before;\n"
+        "    if(percentage != 0.0)\n"
+        "        gradient_color += (stop_color_after - gradient_color)*percentage;\n"
+        "    return vec4(gradient_color.rgb * gradient_color.a, gradient_color.a);\n"
         "}\n";
 
     if (use_array) {