[retro-gtk/wip/aplazas/gl-display: 9/10] colorful blinking square

[Adrien Plazas <aplazas src gnome org>]

commit 9770671e9385ab3ee7ee65055a12eac6b52577ff
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Mon Nov 13 13:13:33 2017 +0100

    colorful blinking square

 retro-gtk/retro-gl-display.c |   20 ++++++++++++++++----
 1 files changed, 16 insertions(+), 4 deletions(-)
---
diff --git a/retro-gtk/retro-gl-display.c b/retro-gtk/retro-gl-display.c
index 9b1e448..c61ba40 100644
--- a/retro-gtk/retro-gl-display.c
+++ b/retro-gtk/retro-gl-display.c
@@ -70,9 +70,15 @@ retro_gl_display_get_video_box (RetroGLDisplay *self,
 }
 
 float vertices[] = {
-     0.0f,  0.5f, 1.0f, 0.0f, 0.0f, // Vertex 1: Red
-     0.5f, -0.5f, 0.0f, 1.0f, 0.0f, // Vertex 2: Green
-    -0.5f, -0.5f, 0.0f, 0.0f, 1.0f  // Vertex 3: Blue
+    -0.5f,  0.5f, 1.0f, 0.0f, 0.0f, // Top-left
+     0.5f,  0.5f, 0.0f, 1.0f, 0.0f, // Top-right
+     0.5f, -0.5f, 0.0f, 0.0f, 1.0f, // Bottom-right
+    -0.5f, -0.5f, 1.0f, 1.0f, 1.0f, // Bottom-left
+};
+
+GLuint elements[] = {
+    0, 1, 2,
+    2, 3, 0,
 };
 
 static void
@@ -80,6 +86,7 @@ retro_gl_display_realize (RetroGLDisplay *self)
 {
   GLuint vertex_buffer_object;
   GLuint vertex_array_object;
+  GLuint element_buffer_object;
   GLuint vertex_shader;
   GLuint fragment_shader;
   GLuint shader_program;
@@ -95,6 +102,10 @@ retro_gl_display_realize (RetroGLDisplay *self)
   glGenVertexArrays (1, &vertex_array_object);
   glBindVertexArray (vertex_array_object);
 
+  glGenBuffers (1, &element_buffer_object);
+  glBindBuffer (GL_ELEMENT_ARRAY_BUFFER, element_buffer_object);
+  glBufferData (GL_ELEMENT_ARRAY_BUFFER, sizeof (elements), elements, GL_STATIC_DRAW);
+
   // Prepare the vertex shader
   shader_source_bytes = g_resources_lookup_data ("/org/gnome/Retro/retro-gl-display-vertex.glsl", 0, NULL);
   shader_source = g_bytes_get_data (shader_source_bytes, NULL);
@@ -179,7 +190,8 @@ retro_gl_display_render (RetroGLDisplay *self)
                      filter);
   glBindFramebuffer (GL_READ_FRAMEBUFFER, 0);
 
-  glDrawArrays (GL_TRIANGLES, 0, 3);
+  /* glDrawArrays (GL_TRIANGLES, 0, 3); */
+  glDrawElements (GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
 
   return FALSE;
 }