OPENGL: Handle GLES2 and GL shaders uniformly.

GLES2 requires precision qualifiers to be set and allows use of precision
qualifiers. For GLES2 we define a default precision now. Since precision
qualifiers are not supported in the GLSL version we use for GL, we
introduce compatibility #defines.

3 files changed, 28 insertions, 23 deletions

diff --git a/backends/graphics/opengl/opengl-graphics.cpp b/backends/graphics/opengl/opengl-graphics.cpp
index df939fb3d8..7f4fcf73bb 100644
--- a/backends/graphics/opengl/opengl-graphics.cpp
+++ b/backends/graphics/opengl/opengl-graphics.cpp
@@ -905,14 +905,14 @@ void OpenGLGraphicsManager::notifyContextCreate(const Graphics::PixelFormat &def
 		if (g_context.type == kContextGLES2) {
 #endif
 #if !USE_FORCED_GL
-			_shader = new Shader(g_defaultVertexShader, g_defaultFragmentShaderGLES2);
+			_shader = new Shader(g_defaultVertexShader, g_defaultFragmentShader);
 #endif
 #if !USE_FORCED_GL && !USE_FORCED_GLES2
 		} else {
 #endif
 #if !USE_FORCED_GLES2
 			if (g_context.shadersSupported) {
-				_shader = new Shader(g_defaultVertexShader, g_defaultFragmentShaderGL);
+				_shader = new Shader(g_defaultVertexShader, g_defaultFragmentShader);
 			}
 #endif
 #if !USE_FORCED_GL && !USE_FORCED_GLES2
diff --git a/backends/graphics/opengl/shader.cpp b/backends/graphics/opengl/shader.cpp
index c251ec1ed9..e699262b8d 100644
--- a/backends/graphics/opengl/shader.cpp
+++ b/backends/graphics/opengl/shader.cpp
@@ -25,6 +25,7 @@
 #if !USE_FORCED_GLES
 
 #include "common/textconsole.h"
+#include "common/util.h"
 
 namespace OpenGL {
 
@@ -44,8 +45,7 @@ const char *const g_defaultVertexShader =
 	"\tgl_Position = projection * position;\n"
 	"}\n";
 
-#if !USE_FORCED_GLES2
-const char *const g_defaultFragmentShaderGL =
+const char *const g_defaultFragmentShader =
 	"varying vec2 texCoord;\n"
 	"varying vec4 blendColor;\n"
 	"\n"
@@ -54,19 +54,24 @@ const char *const g_defaultFragmentShaderGL =
 	"void main(void) {\n"
 	"\tgl_FragColor = blendColor * texture2D(texture, texCoord);\n"
 	"}\n";
-#endif
 
-#if !USE_FORCED_GL
-const char *const g_defaultFragmentShaderGLES2 =
-	"varying lowp vec2 texCoord;\n"
-	"varying lowp vec4 blendColor;\n"
-	"\n"
-	"uniform sampler2D texture;\n"
-	"\n"
-	"void main(void) {\n"
-	"\tgl_FragColor = blendColor * texture2D(texture, texCoord);\n"
-	"}\n";
-#endif
+namespace {
+
+// Taken from: https://en.wikibooks.org/wiki/OpenGL_Programming/Modern_OpenGL_Tutorial_03#OpenGL_ES_2_portability
+const char *const g_precisionDefines =
+	"#ifdef GL_ES\n"
+	"\t#if defined(GL_FRAGMENT_PRECISION_HIGH) && GL_FRAGMENT_PRECISION_HIGH == 1\n"
+	"\t\tprecision highp float;\n"
+	"\t#else\n"
+	"\t\tprecision mediump float;\n"
+	"\t#endif\n"
+	"#else\n"
+	"\t#define highp\n"
+	"\t#define mediump\n"
+	"\t#define lowp\n"
+	"#endif\n";
+
+} // End of anonymous namespace
 
 Shader::Shader(const Common::String &vertex, const Common::String &fragment)
     : _vertex(vertex), _fragment(fragment), _program(0), _projectionLocation(-1), _textureLocation(-1) {
@@ -188,7 +193,12 @@ GLshader Shader::compileShader(const char *source, GLenum shaderType) {
 		return 0;
 	}
 
-	GL_CALL(glShaderSource(handle, 1, &source, nullptr));
+	const char *const sources[2] = {
+		g_precisionDefines,
+		source
+	};
+
+	GL_CALL(glShaderSource(handle, ARRAYSIZE(sources), sources, nullptr));
 	GL_CALL(glCompileShader(handle));
 
 	GLint result;
diff --git a/backends/graphics/opengl/shader.h b/backends/graphics/opengl/shader.h
index 458ecb9f16..98a2a2859b 100644
--- a/backends/graphics/opengl/shader.h
+++ b/backends/graphics/opengl/shader.h
@@ -38,12 +38,7 @@ enum {
 };
 
 extern const char *const g_defaultVertexShader;
-#if !USE_FORCED_GLES2
-extern const char *const g_defaultFragmentShaderGL;
-#endif
-#if !USE_FORCED_GL
-extern const char *const g_defaultFragmentShaderGLES2;
-#endif
+extern const char *const g_defaultFragmentShader;
 
 class Shader {
 public: