cogl: Set LOD bias to -0.5 for single mipmap modes

So that whenever forced to choose between two levels of detail (two mipmaps)
we will land on the sharpest looking one (highest resolution). That's the
mipmap level equal to the floor of the current level of detail requested.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5920
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2644>

cogl/cogl/cogl-private.h

@@ -57,6 +57,7 @@ typedef enum
   COGL_PRIVATE_FEATURE_ALPHA_TEXTURES,
   COGL_PRIVATE_FEATURE_TEXTURE_SWIZZLE,
   COGL_PRIVATE_FEATURE_TEXTURE_MAX_LEVEL,
+  COGL_PRIVATE_FEATURE_TEXTURE_LOD_BIAS,
   COGL_PRIVATE_FEATURE_OES_EGL_SYNC,
   /* If this is set then the winsys is responsible for queueing dirty
    * events. Otherwise a dirty event will be queued when the onscreen

cogl/cogl/driver/gl/cogl-pipeline-opengl.c

@@ -682,6 +682,22 @@ _cogl_sampler_gl_init (CoglContext *context, CoglSamplerCacheEntry *entry)
       GE (context, glSamplerParameteri (entry->sampler_object,
                                         GL_TEXTURE_WRAP_T,
                                         entry->wrap_mode_t) );
+
+      /* While COGL_PRIVATE_FEATURE_SAMPLER_OBJECTS implies support for
+       * GL_TEXTURE_LOD_BIAS in GL, the same is not true in GLES. So check,
+       * and also only apply GL_TEXTURE_LOD_BIAS in mipmap modes:
+       */
+      if (_cogl_has_private_feature (context,
+                                     COGL_PRIVATE_FEATURE_TEXTURE_LOD_BIAS) &&
+          entry->min_filter != GL_NEAREST &&
+          entry->min_filter != GL_LINEAR)
+        {
+          GLfloat bias = _cogl_texture_min_filter_get_lod_bias (entry->min_filter);
+
+          GE (context, glSamplerParameterf (entry->sampler_object,
+                                            GL_TEXTURE_LOD_BIAS,
+                                            bias));
+        }
     }
   else
     {

cogl/cogl/driver/gl/cogl-texture-2d-gl.c

@@ -470,6 +470,15 @@ _cogl_texture_2d_gl_flush_legacy_texobj_filters (CoglTexture *tex,
                                    tex_2d->gl_texture);
   GE( ctx, glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, mag_filter) );
   GE( ctx, glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, min_filter) );
+
+  if (_cogl_has_private_feature (ctx, COGL_PRIVATE_FEATURE_TEXTURE_LOD_BIAS) &&
+      min_filter != GL_NEAREST &&
+      min_filter != GL_LINEAR)
+    {
+      GLfloat bias = _cogl_texture_min_filter_get_lod_bias (min_filter);
+
+      GE (ctx, glTexParameterf (GL_TEXTURE_2D, GL_TEXTURE_LOD_BIAS, bias));
+    }
 }
 
 void

cogl/cogl/driver/gl/cogl-texture-gl-private.h

@@ -62,4 +62,11 @@ _cogl_texture_gl_generate_mipmaps (CoglTexture *texture);
 GLenum
 _cogl_texture_gl_get_format (CoglTexture *texture);
 
+static inline GLfloat
+_cogl_texture_min_filter_get_lod_bias (GLenum min_filter)
+{
+  return (min_filter == GL_NEAREST_MIPMAP_NEAREST ||
+          min_filter == GL_LINEAR_MIPMAP_NEAREST) ? -0.5f : 0.0f;
+}
+
 #endif /* _COGL_TEXTURE_GL_PRIVATE_H_ */

cogl/cogl/driver/gl/cogl-util-gl-private.h

@@ -250,4 +250,8 @@ cogl_gl_get_gpu_time_ns (CoglContext *context);
 #define GL_QUERY_RESULT 0x8866
 #endif
 
+#ifndef GL_TEXTURE_LOD_BIAS
+#define GL_TEXTURE_LOD_BIAS 0x8501
+#endif
+
 #endif /* _COGL_UTIL_GL_PRIVATE_H_ */

cogl/cogl/driver/gl/gl/cogl-driver-gl.c

@@ -526,6 +526,13 @@ _cogl_driver_update_features (CoglContext *ctx,
   COGL_FLAGS_SET (private_features,
                   COGL_PRIVATE_FEATURE_TEXTURE_MAX_LEVEL, TRUE);
 
+  if (COGL_CHECK_GL_VERSION (gl_major, gl_minor, 3, 1) ||
+      _cogl_check_extension ("GL_EXT_texture_lod_bias", gl_extensions))
+    {
+      COGL_FLAGS_SET (private_features,
+                      COGL_PRIVATE_FEATURE_TEXTURE_LOD_BIAS, TRUE);
+    }
+
   if (ctx->glFenceSync)
     COGL_FLAGS_SET (ctx->features, COGL_FEATURE_ID_FENCE, TRUE);
 

cogl/cogl/driver/gl/gles/cogl-driver-gles.c

@@ -437,6 +437,12 @@ _cogl_driver_update_features (CoglContext *context,
                     COGL_FEATURE_ID_TEXTURE_RG,
                     TRUE);
 
+  if (_cogl_check_extension ("GL_EXT_texture_lod_bias", gl_extensions))
+    {
+      COGL_FLAGS_SET (private_features,
+                      COGL_PRIVATE_FEATURE_TEXTURE_LOD_BIAS, TRUE);
+    }
+
   if (context->glGenQueries && context->glQueryCounter && context->glGetInteger64v)
     COGL_FLAGS_SET (context->features, COGL_FEATURE_ID_TIMESTAMP_QUERY, TRUE);
 

cogl/cogl/gl-prototypes/cogl-all-functions.h

@@ -178,6 +178,10 @@ COGL_EXT_FUNCTION (void, glSamplerParameteri,
                    (GLuint sampler,
                     GLenum pname,
                     GLint param))
+COGL_EXT_FUNCTION (void, glSamplerParameterf,
+                   (GLuint sampler,
+                    GLenum pname,
+                    GLfloat param))
 COGL_EXT_END ()
 
 COGL_EXT_BEGIN (only_gl3, 3, 0,

cogl/cogl/gl-prototypes/cogl-core-functions.h

@@ -174,6 +174,8 @@ COGL_EXT_FUNCTION (void, glTexImage2D,
                     const GLvoid* pixels))
 COGL_EXT_FUNCTION (void, glTexParameteri,
                    (GLenum target, GLenum pname, GLint param))
+COGL_EXT_FUNCTION (void, glTexParameterf,
+                   (GLenum target, GLenum pname, GLfloat param))
 COGL_EXT_FUNCTION (void, glTexParameteriv,
                    (GLenum target, GLenum pname, const GLint* params))
 COGL_EXT_FUNCTION (void, glTexSubImage2D,