Use glBlendFuncSeparate for alpha masking in tex.c

The draw happens in two steps:

1. Copy the alpha mask to the framebuffer without changing the color
   channels: glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_ONE, GL_ZERO)

   This uses GL_LINEAR filtering and GL_CLAMP_TO_BORDER to create a
   alpha mask with hard edges as opposed to blurred edges.

2. Copy the texture color to the framebuffer using the alpha mask
   stored in the frame buffer as the mask:
   glBlendFuncSeparate(GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA, GL_ZERO, GL_ZERO);

   We can turn on GL_LINEAR filtering and GL_CLAMP here because the
   alpha mask is taken from the framebuffer instead of the filtered
   alpha component of the texture.

This creates a nice looking image without having to do any fancy setup.
The previous version also has issues if the texture is not split in the
middle of the polygon.

Unfortunately glBlendFuncSeparate depends on OpenGL 2.0 and requires
twice as many draws.

diff --git a/examples/tex/tex.c b/examples/tex/tex.c
index bef3a340c309b29124f0606fa2cd1920680446dc..5041226abce59f3dae5b93d463110ca64e66c25f 100644 (file)
--- a/examples/tex/tex.c
+++ b/examples/tex/tex.c
@@ -21,6 +21,28 @@
 #include <GL/gl.h>
 #include <GL/glu.h>
 
 #include <GL/gl.h>
 #include <GL/glu.h>
 

+#define crazy_draw(tex) \
+       for (int _i = 0; ({ \
+               if (_i == 0) { \
+                       /* Clear alpha buffer */ \
+                       glBindTexture(GL_TEXTURE_2D, tex); \
+                       glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST); \
+                       glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST); \
+                       glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_BORDER); \
+                       glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_BORDER); \
+                       glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_ONE, GL_ZERO); \
+               } else if (_i == 1) { \
+                       /* Draw pixels */ \
+                       glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); \
+                       glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); \
+                       glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP); \
+                       glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP); \
+                       glBlendFuncSeparate(GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA, GL_ZERO, GL_ZERO); \
+               } else { \
+                       glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA); \
+               } \
+       }), _i < 2; _i++)
+

 guint tex, texl, texr;
 
 gboolean on_key_press(GtkWidget *widget, GdkEventKey *event, gpointer _)
 guint tex, texl, texr;
 
 gboolean on_key_press(GtkWidget *widget, GdkEventKey *event, gpointer _)


@@ -32,7 +54,7 @@ gboolean on_key_press(GtkWidget *widget, GdkEventKey *event, gpointer _)
 
 gboolean on_expose(GtkWidget *drawing, GdkEventExpose *event, gpointer _)
 {
 
 gboolean on_expose(GtkWidget *drawing, GdkEventExpose *event, gpointer _)
 {

-       gdouble y = 0.875;
+       gdouble y = 0;

 
        /* Setup view */
        glMatrixMode(GL_PROJECTION);
 
        /* Setup view */
        glMatrixMode(GL_PROJECTION);


@@ -55,44 +77,33 @@ gboolean on_expose(GtkWidget *drawing, GdkEventExpose *event, gpointer _)
        glVertex3f( 0.25, -0.75, 0.0);
        glEnd();
 
        glVertex3f( 0.25, -0.75, 0.0);
        glEnd();
 

-       /* Clear background for GL_ONE */
-       glEnable(GL_COLOR_MATERIAL);
-       glDisable(GL_TEXTURE_2D);
-       glColor4f(0.0, 0.0, 0.0, 0.0);
-       glBegin(GL_QUADS);
-       glVertex3f(-0.75,  0.0, 0.0);
-       glVertex3f(-0.75,  0.5, 0.0);
-       glVertex3f( 0.75,  0.5, 0.0);
-       glVertex3f( 0.75,  0.0, 0.0);
-       glEnd();
-

        /* Setup for textures */
        glEnable(GL_TEXTURE_2D);
        glEnable(GL_BLEND);
        glColor4f(1.0, 1.0, 1.0, 1.0);
        glDisable(GL_COLOR_MATERIAL);
        /* Setup for textures */
        glEnable(GL_TEXTURE_2D);
        glEnable(GL_BLEND);
        glColor4f(1.0, 1.0, 1.0, 1.0);
        glDisable(GL_COLOR_MATERIAL);

-       glBlendFunc(GL_ONE, GL_ONE);

 
        /* Left */
 
        /* Left */

-       glBindTexture(GL_TEXTURE_2D, texl);
-       glBegin(GL_QUADS);
-       glTexCoord2f( 0.0, y);     glVertex3f(-0.75,  0.0, 0.0);
-       glTexCoord2f( 0.0, 1.0);   glVertex3f(-0.75,  0.5, 0.0);
-       glTexCoord2f( 2.0, 1.0);   glVertex3f( 0.75,  0.5, 0.0);
-       glTexCoord2f( 2.0, y);     glVertex3f( 0.75,  0.0, 0.0);
-       glEnd();
+       crazy_draw(texl) {
+               glBegin(GL_QUADS);
+               glTexCoord2f( 0.0, y);     glVertex3f(-0.75,  0.0, 0.0);
+               glTexCoord2f( 0.0, 1.0);   glVertex3f(-0.75,  0.5, 0.0);
+               glTexCoord2f( 2.0, 1.0);   glVertex3f( 0.75,  0.5, 0.0);
+               glTexCoord2f( 2.0, y);     glVertex3f( 0.75,  0.0, 0.0);
+               glEnd();
+       }

 
        /* Right */
 
        /* Right */

-       glBindTexture(GL_TEXTURE_2D, texr);
-       glBegin(GL_QUADS);
-       glTexCoord2f(-1.0, y);     glVertex3f(-0.75, 0.0, 0.0);
-       glTexCoord2f(-1.0, 1.0);   glVertex3f(-0.75, 0.5, 0.0);
-       glTexCoord2f( 1.0, 1.0);   glVertex3f( 0.75, 0.5, 0.0);
-       glTexCoord2f( 1.0, y);     glVertex3f( 0.75, 0.0, 0.0);
-       glEnd();
+       crazy_draw(texr) {
+               glBegin(GL_QUADS);
+               glTexCoord2f(-1.0, y);     glVertex3f(-0.75, 0.0, 0.0);
+               glTexCoord2f(-1.0, 1.0);   glVertex3f(-0.75, 0.5, 0.0);
+               glTexCoord2f( 1.0, 1.0);   glVertex3f( 0.75, 0.5, 0.0);
+               glTexCoord2f( 1.0, y);     glVertex3f( 0.75, 0.0, 0.0);
+               glEnd();
+       }

 
        /* Bottom */
 
        /* Bottom */

-       glBlendFunc(GL_ONE, GL_ZERO);

        glBindTexture(GL_TEXTURE_2D, tex);
        glBegin(GL_QUADS);
        glTexCoord2f( 0.0, 0.0);   glVertex3f(-0.75, -0.5, 0.0);
        glBindTexture(GL_TEXTURE_2D, tex);
        glBegin(GL_QUADS);
        glTexCoord2f( 0.0, 0.0);   glVertex3f(-0.75, -0.5, 0.0);


@@ -132,8 +143,8 @@ guint load_tex(gchar *filename)
        glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
        glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
 
        glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
        glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
 

-       glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_BORDER);
-       glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_BORDER);
+       glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
+       glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);

        g_object_unref(pixbuf);
        return tex;
 }
        g_object_unref(pixbuf);
        return tex;
 }