tests/cogl: Migrate alpha test test

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2555>

src/tests/cogl/conform/meson.build

@@ -26,6 +26,7 @@ cogl_tests = [
   [ 'test-point-sprite-known-failure', ['gl', 'gl3', 'gles2'] ],
   [ 'test-no-gl-header', [] ],
   [ 'test-layer-remove', [] ],
+  [ 'test-alpha-test', [] ],
 ]
 
 cogl_test_conformance_includes = [

src/tests/cogl/conform/test-alpha-test.c

@@ -0,0 +1,76 @@
+#include <cogl/cogl.h>
+#include <string.h>
+
+#include "tests/cogl-test-utils.h"
+
+static CoglTexture2D *
+create_texture (CoglContext *context)
+{
+  static const uint8_t data[] =
+    {
+      0xff, 0x00, 0x00, 0xff,
+      0x00, 0xfa, 0x00, 0xfa
+    };
+
+  return cogl_texture_2d_new_from_data (context,
+                                        2, 1, /* width/height */
+                                        COGL_PIXEL_FORMAT_RGBA_8888_PRE,
+                                        4, /* rowstride */
+                                        data,
+                                        NULL /* error */);
+}
+
+static void
+test_alpha_test (void)
+{
+  CoglTexture *tex = create_texture (test_ctx);
+  CoglPipeline *pipeline = cogl_pipeline_new (test_ctx);
+  int fb_width = cogl_framebuffer_get_width (test_fb);
+  int fb_height = cogl_framebuffer_get_height (test_fb);
+  CoglColor clear_color;
+
+  cogl_pipeline_set_layer_texture (pipeline, 0, tex);
+  cogl_pipeline_set_layer_filters (pipeline, 0,
+                                   COGL_PIPELINE_FILTER_NEAREST,
+                                   COGL_PIPELINE_FILTER_NEAREST);
+  cogl_pipeline_set_alpha_test_function (pipeline,
+                                         COGL_PIPELINE_ALPHA_FUNC_GEQUAL,
+                                         254 / 255.0f /* alpha reference */);
+
+  cogl_color_init_from_4ub (&clear_color, 0x00, 0x00, 0xff, 0xff);
+  cogl_framebuffer_clear (test_fb,
+                          COGL_BUFFER_BIT_COLOR,
+                          &clear_color);
+
+  cogl_framebuffer_draw_rectangle (test_fb,
+                                   pipeline,
+                                   -1, -1,
+                                   1, 1);
+
+  cogl_object_unref (pipeline);
+  cogl_object_unref (tex);
+
+  /* The left side of the framebuffer should use the first pixel from
+   * the texture which is red */
+  test_utils_check_region (test_fb,
+                           2, 2,
+                           fb_width / 2 - 4,
+                           fb_height - 4,
+                           0xff0000ff);
+  /* The right side of the framebuffer should use the clear color
+   * because the second pixel from the texture is clipped from the
+   * alpha test */
+  test_utils_check_region (test_fb,
+                           fb_width / 2 + 2,
+                           2,
+                           fb_width / 2 - 4,
+                           fb_height - 4,
+                           0x0000ffff);
+
+  if (cogl_test_verbose ())
+    g_print ("OK\n");
+}
+
+COGL_TEST_SUITE (
+  g_test_add_func ("/alpha-test", test_alpha_test);
+)