M_alpha now no longer squares the alpha component it writes to the framebuffer.

To re-enable the old behaviour, set old-alpha-blend in Config.prc.

panda/src/display/config_display.cxx

@@ -298,6 +298,15 @@ ConfigVariableBool allow_incomplete_render
           "geometry is always paged in immediately when needed, holding up "
           "the frame render if necessary."));
 
+ConfigVariableBool old_alpha_blend
+("old-alpha-blend", false,
+ PRC_DESC("Set this to true to enable the old alpha blending behavior from "
+          "Panda 1.9 in which the alpha value written out to the framebuffer "
+          "is squared.  The new behavior is more intuitive when compositing "
+          "an semitransparent image produced using render-to-texture.  You "
+          "should generally leave this false unless you have an effect that "
+          "relies on the old behavior, or you suspect an implementation bug."));
+
 ConfigVariableInt win_size
 ("win-size", "800 600",
  PRC_DESC("This is the default size at which to open a new window.  This "

panda/src/display/config_display.h

@@ -68,6 +68,7 @@ extern EXPCL_PANDA_DISPLAY ConfigVariableBool default_stereo_camera;
 extern EXPCL_PANDA_DISPLAY ConfigVariableBool color_scale_via_lighting;
 extern EXPCL_PANDA_DISPLAY ConfigVariableBool alpha_scale_via_texture;
 extern EXPCL_PANDA_DISPLAY ConfigVariableBool allow_incomplete_render;
+extern EXPCL_PANDA_DISPLAY ConfigVariableBool old_alpha_blend;
 
 extern EXPCL_PANDA_DISPLAY ConfigVariableInt win_size;
 extern EXPCL_PANDA_DISPLAY ConfigVariableInt win_origin;

panda/src/dxgsg9/dxGraphicsStateGuardian9.cxx

@@ -3798,7 +3798,14 @@ do_issue_blending() {
   case TransparencyAttrib::M_multisample_mask:
   case TransparencyAttrib::M_dual:
     set_render_state(D3DRS_ALPHABLENDENABLE, TRUE);
-    set_render_state(D3DRS_SEPARATEALPHABLENDENABLE, FALSE);
+    if (old_alpha_blend) {
+      set_render_state(D3DRS_SEPARATEALPHABLENDENABLE, FALSE);
+    } else {
+      set_render_state(D3DRS_SEPARATEALPHABLENDENABLE, TRUE);
+      set_render_state(D3DRS_BLENDOPALPHA, D3DBLENDOP_ADD);
+      set_render_state(D3DRS_SRCBLENDALPHA, D3DBLEND_ONE);
+      set_render_state(D3DRS_DESTBLENDALPHA, D3DBLEND_INVSRCALPHA);
+    }
     set_render_state(D3DRS_BLENDOP, D3DBLENDOP_ADD);
     set_render_state(D3DRS_SRCBLEND, D3DBLEND_SRCALPHA);
     set_render_state(D3DRS_DESTBLEND, D3DBLEND_INVSRCALPHA);

panda/src/glstuff/glGraphicsStateGuardian_src.cxx

@@ -7120,11 +7120,20 @@ do_issue_blending() {
     enable_multisample_alpha_mask(false);
     enable_blend(true);
     _glBlendEquation(GL_FUNC_ADD);
-    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
+
+    if (old_alpha_blend) {
+      glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
+    } else {
+      _glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
+    }
 
     if (GLCAT.is_spam()) {
       GLCAT.spam() << "glBlendEquation(GL_FUNC_ADD)\n";
-      GLCAT.spam() << "glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)\n";
+      if (_supports_blend_equation_separate && !old_alpha_blend) {
+        GLCAT.spam() << "glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE_MINUS_SRC_ALPHA)\n";
+      } else {
+        GLCAT.spam() << "glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)\n";
+      }
     }
     return;
 

panda/src/tinydisplay/zbuffer.h

@@ -107,23 +107,26 @@ typedef unsigned int PIXEL;
 #define PCOMPONENT_BLEND(c1, c2, a2) \
   ((((unsigned int)(c1) * ((unsigned int)0xffff - (unsigned int)(a2)) + (unsigned int)(c2) * (unsigned int)(a2))) >> 16)
 
-#define _BLEND_RGB(r1, g1, b1, r2, g2, b2, a2) \
+#define PALPHA_BLEND(a1, a2) \
+  ((((unsigned int)(a1) * ((unsigned int)0xffff - (unsigned int)(a2))) >> 16) + (unsigned int)(a2))
+
+#define _BLEND_RGB(r1, g1, b1, a1, r2, g2, b2, a2) \
   RGBA_TO_PIXEL(PCOMPONENT_BLEND(r1, r2, a2),   \
                 PCOMPONENT_BLEND(g1, g2, a2),   \
                 PCOMPONENT_BLEND(b1, b2, a2),   \
-                a2)
+                PALPHA_BLEND(a1, a2))
 
-#define _BLEND_SRGB(r1, g1, b1, r2, g2, b2, a2) \
+#define _BLEND_SRGB(r1, g1, b1, a1, r2, g2, b2, a2) \
   SRGBA_TO_PIXEL(PCOMPONENT_BLEND(r1, r2, a2),   \
                  PCOMPONENT_BLEND(g1, g2, a2),   \
                  PCOMPONENT_BLEND(b1, b2, a2),   \
-                 a2)
+                 PALPHA_BLEND(a1, a2))
 
 #define PIXEL_BLEND_RGB(rgb, r, g, b, a) \
-  _BLEND_RGB(PIXEL_R(rgb), PIXEL_G(rgb), PIXEL_B(rgb), r, g, b, a)
+  _BLEND_RGB(PIXEL_R(rgb), PIXEL_G(rgb), PIXEL_B(rgb), PIXEL_A(rgb), r, g, b, a)
 
 #define PIXEL_BLEND_SRGB(rgb, r, g, b, a) \
-  _BLEND_SRGB(PIXEL_SR(rgb), PIXEL_SG(rgb), PIXEL_SB(rgb), r, g, b, a)
+  _BLEND_SRGB(PIXEL_SR(rgb), PIXEL_SG(rgb), PIXEL_SB(rgb), PIXEL_A(rgb), r, g, b, a)
 
 
 typedef struct {