Add GLES2 shader prologue infrastructure. (by @eloj)

There is supposedly an OpenGL ES2 target that does not support precision specifiers. However, the existing logic to detect this is currently broken in two ways:

1) There's a typo of the `#ifdef` as `#if`.
2) Checking for `GL_FRAGMENT_PRECISION_HIGH` can not be the correct way to detect this platform. Other targets, including some desktops, will also not have this defined (for various reasons).

Because some of the shader code is missing precision specifiers, and because a default is ONLY provided if `GL_FRAGMENT_PRECISION_HIGH` is set, these other targets break.

Instead of 'hard-coding' the prologue string into shaders in the C source, use our ability to provide a list of strings to `glShaderSource` instead, leaving the determination to run-time.

This commit closes https://github.com/libsdl-org/SDL/pull/6182

(cherry picked from commit 29f4a5ba0eba7bd543739520433a14cab79a3684)

src/render/opengles2/SDL_render_gles2.c

@@ -169,6 +169,7 @@ typedef struct GLES2_RenderData
 #endif
 
     GLES2_DrawStateCache drawstate;
+    GLES2_ShaderIncludeType texcoord_precision_hint;
 } GLES2_RenderData;
 
 #define GLES2_MAX_CACHED_PROGRAMS 8
@@ -494,16 +495,28 @@ GLES2_CacheShader(GLES2_RenderData *data, GLES2_ShaderType type, GLenum shader_t
 {
     GLuint id;
     GLint compileSuccessful = GL_FALSE;
-    const char *shader_src = (char *)GLES2_GetShader(type);
+    int num_src = 0;
+    const GLchar *shader_src_list[4] = { 0 };
+    const GLchar *shader_body = (const GLchar *)GLES2_GetShader(type);
 
-    if (!shader_src) {
-        SDL_SetError("No shader src");
+    if (!shader_body) {
+        SDL_SetError("No shader body src");
         return 0;
     }
 
+    if (shader_type == GL_FRAGMENT_SHADER) {
+        if (data->texcoord_precision_hint != GLES2_SHADER_FRAGMENT_INCLUDE_UNDEF_PRECISION) {
+            shader_src_list[num_src++] = (const GLchar*)GLES2_GetShaderInclude(GLES2_SHADER_FRAGMENT_INCLUDE_DEFAULT);
+        }
+        shader_src_list[num_src++] = (const GLchar*)GLES2_GetShaderInclude(data->texcoord_precision_hint);
+    }
+    shader_src_list[num_src++] = shader_body;
+
+    SDL_assert(num_src < SDL_arraysize(shader_src_list));
+
     /* Compile */
     id = data->glCreateShader(shader_type);
-    data->glShaderSource(id, 1, &shader_src, NULL);
+    data->glShaderSource(id, num_src, shader_src_list, NULL);
     data->glCompileShader(id);
     data->glGetShaderiv(id, GL_COMPILE_STATUS, &compileSuccessful);
 
@@ -2094,6 +2107,8 @@ GLES2_CreateRenderer(SDL_Window *window, Uint32 flags)
         data->debug_enabled = SDL_TRUE;
     }
 
+    data->texcoord_precision_hint = GLES2_GetTexCoordPrecisionEnumFromHint();
+
     value = 0;
     data->glGetIntegerv(GL_MAX_TEXTURE_SIZE, &value);
     renderer->info.max_texture_width = value;

src/render/opengles2/SDL_shaders_gles2.c

@@ -22,24 +22,44 @@
 
 #if SDL_VIDEO_RENDER_OGL_ES2 && !SDL_RENDER_DISABLED
 
+#include "SDL_hints.h"
 #include "SDL_video.h"
 #include "SDL_opengles2.h"
 #include "SDL_shaders_gles2.h"
 #include "SDL_stdinc.h"
 
-#define SHADER_PRELOGUE "\n\
-#if GL_FRAGMENT_PRECISION_HIGH\n\
-     precision mediump float;\n\
-#else\n\
+/*************************************************************************************************
+ * Vertex/fragment shader source                                                                 *
+ *************************************************************************************************/
+
+static const Uint8 GLES2_Fragment_Include_Default[] = "\n\
+    #version 130\n\
+    precision mediump float;\n\
+";
+
+static const Uint8 GLES2_Fragment_Include_Best_Texture_Precision[] = "\n\
+    #ifdef GL_FRAGMENT_PRECISION_HIGH\n\
+    #define SDL_TEXCOORD_PRECISION highp\n\
+    #else\n\
+    #define SDL_TEXCOORD_PRECISION mediump\n\
+    #endif\n\
+";
+
+static const Uint8 GLES2_Fragment_Include_Medium_Texture_Precision[] = "\n\
+    #define SDL_TEXCOORD_PRECISION mediump\n\
+";
+
+static const Uint8 GLES2_Fragment_Include_High_Texture_Precision[] = "\n\
+    #define SDL_TEXCOORD_PRECISION highp\n\
+";
+
+static const Uint8 GLES2_Fragment_Include_Undef_Precision[] = "\n\
     #define mediump\n\
     #define highp\n\
     #define lowp\n\
-#endif\n\
-"
+    #define SDL_TEXCOORD_PRECISION\n\
+";
 
-/*************************************************************************************************
- * Vertex/fragment shader source                                                                 *
- *************************************************************************************************/
 static const Uint8 GLES2_Vertex_Default[] = " \
     uniform mat4 u_projection; \
     attribute vec2 a_position; \
@@ -57,8 +77,8 @@ static const Uint8 GLES2_Vertex_Default[] = " \
     } \
 ";
 
-static const Uint8 GLES2_Fragment_Solid[] = SHADER_PRELOGUE" \
-    varying vec4 v_color; \
+static const Uint8 GLES2_Fragment_Solid[] = " \
+    varying mediump vec4 v_color; \
     \
     void main() \
     { \
@@ -66,14 +86,10 @@ static const Uint8 GLES2_Fragment_Solid[] = SHADER_PRELOGUE" \
     } \
 ";
 
-static const Uint8 GLES2_Fragment_TextureABGR[] = SHADER_PRELOGUE" \
+static const Uint8 GLES2_Fragment_TextureABGR[] = " \
     uniform sampler2D u_texture; \
-    varying vec4 v_color;\n\
-    #ifdef GL_FRAGMENT_PRECISION_HIGH\n\
-    varying highp vec2 v_texCoord;\n\
-    #else\n\
-    varying vec2 v_texCoord;\n\
-    #endif\n\
+    varying mediump vec4 v_color;\n\
+    varying SDL_TEXCOORD_PRECISION vec2 v_texCoord;\n\
     \
     void main() \
     { \
@@ -83,18 +99,14 @@ static const Uint8 GLES2_Fragment_TextureABGR[] = SHADER_PRELOGUE" \
 ";
 
 /* ARGB to ABGR conversion */
-static const Uint8 GLES2_Fragment_TextureARGB[] = SHADER_PRELOGUE" \
+static const Uint8 GLES2_Fragment_TextureARGB[] = " \
     uniform sampler2D u_texture; \
-    varying vec4 v_color;\n\
-    #ifdef GL_FRAGMENT_PRECISION_HIGH\n\
-    varying highp vec2 v_texCoord;\n\
-    #else\n\
-    varying vec2 v_texCoord;\n\
-    #endif\n\
+    varying mediump vec4 v_color;\n\
+    varying SDL_TEXCOORD_PRECISION vec2 v_texCoord;\n\
     \
     void main() \
     { \
-        vec4 abgr = texture2D(u_texture, v_texCoord); \
+        mediump vec4 abgr = texture2D(u_texture, v_texCoord); \
         gl_FragColor = abgr; \
         gl_FragColor.r = abgr.b; \
         gl_FragColor.b = abgr.r; \
@@ -103,18 +115,14 @@ static const Uint8 GLES2_Fragment_TextureARGB[] = SHADER_PRELOGUE" \
 ";
 
 /* RGB to ABGR conversion */
-static const Uint8 GLES2_Fragment_TextureRGB[] = SHADER_PRELOGUE" \
+static const Uint8 GLES2_Fragment_TextureRGB[] = " \
     uniform sampler2D u_texture; \
-    varying vec4 v_color;\n\
-    #ifdef GL_FRAGMENT_PRECISION_HIGH\n\
-    varying highp vec2 v_texCoord;\n\
-    #else\n\
-    varying vec2 v_texCoord;\n\
-    #endif\n\
+    varying mediump vec4 v_color;\n\
+    varying SDL_TEXCOORD_PRECISION vec2 v_texCoord;\n\
     \
     void main() \
     { \
-        vec4 abgr = texture2D(u_texture, v_texCoord); \
+        mediump vec4 abgr = texture2D(u_texture, v_texCoord); \
         gl_FragColor = abgr; \
         gl_FragColor.r = abgr.b; \
         gl_FragColor.b = abgr.r; \
@@ -124,18 +132,14 @@ static const Uint8 GLES2_Fragment_TextureRGB[] = SHADER_PRELOGUE" \
 ";
 
 /* BGR to ABGR conversion */
-static const Uint8 GLES2_Fragment_TextureBGR[] = SHADER_PRELOGUE" \
+static const Uint8 GLES2_Fragment_TextureBGR[] = " \
     uniform sampler2D u_texture; \
-    varying vec4 v_color;\n\
-    #ifdef GL_FRAGMENT_PRECISION_HIGH\n\
-    varying highp vec2 v_texCoord;\n\
-    #else\n\
-    varying vec2 v_texCoord;\n\
-    #endif\n\
+    varying mediump vec4 v_color;\n\
+    varying SDL_TEXCOORD_PRECISION vec2 v_texCoord;\n\
     \
     void main() \
     { \
-        vec4 abgr = texture2D(u_texture, v_texCoord); \
+        mediump vec4 abgr = texture2D(u_texture, v_texCoord); \
         gl_FragColor = abgr; \
         gl_FragColor.a = 1.0; \
         gl_FragColor *= v_color; \
@@ -173,16 +177,11 @@ static const Uint8 GLES2_Fragment_TextureBGR[] = SHADER_PRELOGUE" \
 
 
 #define YUV_SHADER_PROLOGUE                                     \
-SHADER_PRELOGUE                                                 \
 "uniform sampler2D u_texture;\n"                                \
 "uniform sampler2D u_texture_u;\n"                              \
 "uniform sampler2D u_texture_v;\n"                              \
-"varying vec4 v_color;\n"                                       \
-"#ifdef GL_FRAGMENT_PRECISION_HIGH\n"                           \
-"varying highp vec2 v_texCoord;\n"                              \
-"#else\n"                                                       \
-"varying vec2 v_texCoord;\n"                                    \
-"#endif\n"                                                      \
+"varying mediump vec4 v_color;\n"                               \
+"varying SDL_TEXCOORD_PRECISION vec2 v_texCoord;\n"             \
 "\n"                                                            \
 
 #define YUV_SHADER_BODY                                         \
@@ -331,14 +330,9 @@ static const Uint8 GLES2_Fragment_TextureNV21BT709[] = \
 /* Custom Android video format texture */
 static const Uint8 GLES2_Fragment_TextureExternalOES[] = " \
     #extension GL_OES_EGL_image_external : require\n\
-    precision mediump float; \
     uniform samplerExternalOES u_texture; \
-    varying vec4 v_color;\n\
-    #ifdef GL_FRAGMENT_PRECISION_HIGH\n\
-    varying highp vec2 v_texCoord;\n\
-    #else\n\
-    varying vec2 v_texCoord;\n\
-    #endif\n\
+    varying mediump vec4 v_color;\n\
+    varying SDL_TEXCOORD_PRECISION vec2 v_texCoord;\n\
     \
     void main() \
     { \
@@ -352,6 +346,37 @@ static const Uint8 GLES2_Fragment_TextureExternalOES[] = " \
  * Shader selector                                                                               *
  *************************************************************************************************/
 
+const Uint8 *GLES2_GetShaderInclude(GLES2_ShaderIncludeType type) {
+    switch (type) {
+    case GLES2_SHADER_FRAGMENT_INCLUDE_DEFAULT:
+        return GLES2_Fragment_Include_Default;
+    case GLES2_SHADER_FRAGMENT_INCLUDE_UNDEF_PRECISION:
+        return GLES2_Fragment_Include_Undef_Precision;
+    case GLES2_SHADER_FRAGMENT_INCLUDE_BEST_TEXCOORD_PRECISION:
+        return GLES2_Fragment_Include_Best_Texture_Precision;
+    case GLES2_SHADER_FRAGMENT_INCLUDE_MEDIUM_TEXCOORD_PRECISION:
+        return GLES2_Fragment_Include_Medium_Texture_Precision;
+    case GLES2_SHADER_FRAGMENT_INCLUDE_HIGH_TEXCOORD_PRECISION:
+        return GLES2_Fragment_Include_High_Texture_Precision;
+    default:
+        return (Uint8*)"";
+    }
+}
+
+GLES2_ShaderIncludeType GLES2_GetTexCoordPrecisionEnumFromHint() {
+    const char *texcoord_hint = SDL_GetHint("SDL_RENDER_OPENGLES2_TEXCOORD_PRECISION");
+    GLES2_ShaderIncludeType value = GLES2_SHADER_FRAGMENT_INCLUDE_BEST_TEXCOORD_PRECISION; // "best"
+    if (texcoord_hint) {
+        if (SDL_strcmp(texcoord_hint, "undefined") == 0)
+            return GLES2_SHADER_FRAGMENT_INCLUDE_UNDEF_PRECISION;
+        if (SDL_strcmp(texcoord_hint, "high") == 0)
+            return GLES2_SHADER_FRAGMENT_INCLUDE_HIGH_TEXCOORD_PRECISION;
+        if (SDL_strcmp(texcoord_hint, "medium") == 0)
+            return GLES2_SHADER_FRAGMENT_INCLUDE_MEDIUM_TEXCOORD_PRECISION;
+    }
+    return value;
+}
+
 const Uint8 *GLES2_GetShader(GLES2_ShaderType type)
 {
     switch (type) {

src/render/opengles2/SDL_shaders_gles2.h

@@ -25,6 +25,16 @@
 
 #if SDL_VIDEO_RENDER_OGL_ES2
 
+typedef enum
+{
+    GLES2_SHADER_FRAGMENT_INCLUDE_NONE = 0,
+    GLES2_SHADER_FRAGMENT_INCLUDE_DEFAULT,
+    GLES2_SHADER_FRAGMENT_INCLUDE_BEST_TEXCOORD_PRECISION,
+    GLES2_SHADER_FRAGMENT_INCLUDE_MEDIUM_TEXCOORD_PRECISION,
+    GLES2_SHADER_FRAGMENT_INCLUDE_HIGH_TEXCOORD_PRECISION,
+    GLES2_SHADER_FRAGMENT_INCLUDE_UNDEF_PRECISION,
+    GLES2_SHADER_FRAGMENT_INCLUDE_COUNT
+} GLES2_ShaderIncludeType;
 
 typedef enum
 {
@@ -52,6 +62,8 @@ typedef enum
 } GLES2_ShaderType;
 
 const Uint8 *GLES2_GetShader(GLES2_ShaderType type);
+const Uint8 *GLES2_GetShaderInclude(GLES2_ShaderIncludeType type);
+GLES2_ShaderIncludeType GLES2_GetTexCoordPrecisionEnumFromHint();
 
 #endif /* SDL_VIDEO_RENDER_OGL_ES2 */