Use multi-texturing for alpha masking in tex.c

Multi-texturing uses two textures for each fragment. The example uses
GL_TEXTURE0 as the regular texture color and a second texture as the
alpha mask. Since there are two textures they can have separate
filtering an wrapping parameters.

The color texture uses GL_MODULATE which combines it with the existing
color. The alpha mask uses GL_REPLACE which causes it to replace the
linearly filtered alpha mask from the color texture with a hard-edged
alpha mask that is clamped to transparent at the border.

This is similar to how drawing is done in NASA World Wind

diff --git a/examples/tex/tex.c b/examples/tex/tex.c
index 5041226abce59f3dae5b93d463110ca64e66c25f..cdd03228591875fb122a661a3be478c5e16f152b 100644 (file)
--- a/examples/tex/tex.c
+++ b/examples/tex/tex.c
@@ -21,29 +21,7 @@
 #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;
+guint tex, texl, texr, mask;
 
 gboolean on_key_press(GtkWidget *widget, GdkEventKey *event, gpointer _)
 {
@@ -80,28 +58,40 @@ gboolean on_expose(GtkWidget *drawing, GdkEventExpose *event, gpointer _)
        /* Setup for textures */
        glEnable(GL_TEXTURE_2D);
        glEnable(GL_BLEND);
-       glColor4f(1.0, 1.0, 1.0, 1.0);
+       glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
        glDisable(GL_COLOR_MATERIAL);
 
+       /* Setup mask */
+       glActiveTexture(GL_TEXTURE1);
+       glEnable(GL_TEXTURE_2D);
+       glBindTexture(GL_TEXTURE_2D, mask);
+       glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
+
+       glActiveTexture(GL_TEXTURE0);
+       glEnable(GL_TEXTURE_2D);
+       glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
+
        /* Left */
-       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();
-       }
+       glBindTexture(GL_TEXTURE_2D, texl);
+       glBegin(GL_QUADS);
+       glMultiTexCoord2f(GL_TEXTURE0,  0.0, y);     glMultiTexCoord2f(GL_TEXTURE1,  0.0, y);     glVertex3f(-0.75,  0.0, 0.0);
+       glMultiTexCoord2f(GL_TEXTURE0,  0.0, 1.0);   glMultiTexCoord2f(GL_TEXTURE1,  0.0, 1.0);   glVertex3f(-0.75,  0.5, 0.0);
+       glMultiTexCoord2f(GL_TEXTURE0,  2.0, 1.0);   glMultiTexCoord2f(GL_TEXTURE1,  2.0, 1.0);   glVertex3f( 0.75,  0.5, 0.0);
+       glMultiTexCoord2f(GL_TEXTURE0,  2.0, y);     glMultiTexCoord2f(GL_TEXTURE1,  2.0, y);     glVertex3f( 0.75,  0.0, 0.0);
+       glEnd();
 
        /* Right */
-       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();
-       }
+       glBindTexture(GL_TEXTURE_2D, texr);
+       glBegin(GL_QUADS);
+       glMultiTexCoord2f(GL_TEXTURE0, -1.0, y);     glMultiTexCoord2f(GL_TEXTURE1, -1.0, y);     glVertex3f(-0.75, 0.0, 0.0);
+       glMultiTexCoord2f(GL_TEXTURE0, -1.0, 1.0);   glMultiTexCoord2f(GL_TEXTURE1, -1.0, 1.0);   glVertex3f(-0.75, 0.5, 0.0);
+       glMultiTexCoord2f(GL_TEXTURE0,  1.0, 1.0);   glMultiTexCoord2f(GL_TEXTURE1,  1.0, 1.0);   glVertex3f( 0.75, 0.5, 0.0);
+       glMultiTexCoord2f(GL_TEXTURE0,  1.0, y);     glMultiTexCoord2f(GL_TEXTURE1,  1.0, y);     glVertex3f( 0.75, 0.0, 0.0);
+       glEnd();
+
+       glActiveTexture(GL_TEXTURE1);
+       glDisable(GL_TEXTURE_2D);
+       glActiveTexture(GL_TEXTURE0);
 
        /* Bottom */
        glBindTexture(GL_TEXTURE_2D, tex);
@@ -128,6 +118,26 @@ gboolean on_configure(GtkWidget *drawing, GdkEventConfigure *event, gpointer _)
        return FALSE;
 }
 
+guint load_mask(void)
+{
+       guint  tex  = 0;
+       guint8 byte = 0xff;
+
+       glGenTextures(1, &tex);
+       glBindTexture(GL_TEXTURE_2D, tex);
+
+       glTexImage2D(GL_TEXTURE_2D, 0, GL_ALPHA, 1, 1, 0,
+                       GL_ALPHA, GL_UNSIGNED_BYTE, &byte);
+
+       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);
+
+       return tex;
+}
+
 guint load_tex(gchar *filename)
 {
        GdkPixbuf *pixbuf = gdk_pixbuf_new_from_file(filename, NULL);
@@ -136,15 +146,18 @@ guint load_tex(gchar *filename)
        int        height = gdk_pixbuf_get_height(pixbuf);
        int        alpha  = gdk_pixbuf_get_has_alpha(pixbuf);
        guint      tex;
+
        glGenTextures(1, &tex);
        glBindTexture(GL_TEXTURE_2D, tex);
-       glTexImage2D(GL_TEXTURE_2D, 0, 4, width, height, 0,
+       glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0,
                        (alpha ? GL_RGBA : GL_RGB), GL_UNSIGNED_BYTE, 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);
+
        g_object_unref(pixbuf);
        return tex;
 }
@@ -172,6 +185,7 @@ int main(int argc, char **argv)
        gdk_gl_drawable_gl_begin(gldrawable, glcontext);
 
        /* Load texture */
+       mask = load_mask();
        texl = load_tex("texls.png");
        texr = load_tex("texrs.png");
        tex  = load_tex("tex.png");