render: Initial attempt at adding SDL_GenerateTextureMipmap().

This is only implemented for the OpenGL and OpenGL ES 2 backends currently,
and is completely untested, beyond it compiles on my laptop.

Fixes #4156.

include/SDL3/SDL_render.h

@@ -1230,6 +1230,25 @@ extern SDL_DECLSPEC int SDLCALL SDL_LockTextureToSurface(SDL_Texture *texture,
  */
 extern SDL_DECLSPEC void SDLCALL SDL_UnlockTexture(SDL_Texture *texture);
 
+/**
+ * Generate mipmaps for a texture.
+ *
+ * This may make this texture render better when downscaling for future
+ * draw calls. When updating a texture, call this function to regenerate
+ * mipmaps.
+ *
+ * This might not be a cheap operation, use it sparingly!
+ *
+ * Not every renderer supports this, but when unsupported, downscaled textures
+ * will just perhaps scale a little less well.
+ *
+ * \param texture a texture locked by SDL_LockTexture().
+ * \returns 0 on success, or -1 on error.
+ *
+ * \since This function is available since SDL 3.0.0.
+ */
+extern SDL_DECLSPEC int SDLCALL SDL_GenerateTextureMipmap(SDL_Texture *texture);
+
 /**
  * Set a texture as the current rendering target.
  *

src/dynapi/SDL_dynapi.sym

@@ -148,6 +148,7 @@ SDL3_0.0.0 {
     SDL_GamepadHasButton;
     SDL_GamepadHasSensor;
     SDL_GamepadSensorEnabled;
+    SDL_GenerateTextureMipmap;
     SDL_GetAndroidActivity;
     SDL_GetAndroidCachePath;
     SDL_GetAndroidExternalStoragePath;

src/dynapi/SDL_dynapi_overrides.h

@@ -173,6 +173,7 @@
 #define SDL_GamepadHasButton SDL_GamepadHasButton_REAL
 #define SDL_GamepadHasSensor SDL_GamepadHasSensor_REAL
 #define SDL_GamepadSensorEnabled SDL_GamepadSensorEnabled_REAL
+#define SDL_GenerateTextureMipmap SDL_GenerateTextureMipmap_REAL
 #define SDL_GetAndroidActivity SDL_GetAndroidActivity_REAL
 #define SDL_GetAndroidCachePath SDL_GetAndroidCachePath_REAL
 #define SDL_GetAndroidExternalStoragePath SDL_GetAndroidExternalStoragePath_REAL

src/dynapi/SDL_dynapi_procs.h

@@ -193,6 +193,7 @@ SDL_DYNAPI_PROC(SDL_bool,SDL_GamepadHasAxis,(SDL_Gamepad *a, SDL_GamepadAxis b),
 SDL_DYNAPI_PROC(SDL_bool,SDL_GamepadHasButton,(SDL_Gamepad *a, SDL_GamepadButton b),(a,b),return)
 SDL_DYNAPI_PROC(SDL_bool,SDL_GamepadHasSensor,(SDL_Gamepad *a, SDL_SensorType b),(a,b),return)
 SDL_DYNAPI_PROC(SDL_bool,SDL_GamepadSensorEnabled,(SDL_Gamepad *a, SDL_SensorType b),(a,b),return)
+SDL_DYNAPI_PROC(int,SDL_GenerateTextureMipmap,(SDL_Texture *a),(a),return)
 SDL_DYNAPI_PROC(void*,SDL_GetAndroidActivity,(void),(),return)
 SDL_DYNAPI_PROC(const char*,SDL_GetAndroidCachePath,(void),(),return)
 SDL_DYNAPI_PROC(const char*,SDL_GetAndroidExternalStoragePath,(void),(),return)

src/render/SDL_render.c

@@ -2410,6 +2410,21 @@ static int SDL_SetRenderTargetInternal(SDL_Renderer *renderer, SDL_Texture *text
     return 0;
 }
 
+int SDL_GenerateTextureMipmap(SDL_Texture *texture)
+{
+    CHECK_TEXTURE_MAGIC(texture, -1);
+
+    SDL_Renderer *renderer = texture->renderer;
+
+    if (!renderer->GenMipmaps) {
+        return SDL_Unsupported();
+    } else if (FlushRenderCommandsIfTextureNeeded(texture) < 0) {
+        return -1;
+    }
+
+    return renderer->GenMipmaps(renderer, texture);
+}
+
 int SDL_SetRenderTarget(SDL_Renderer *renderer, SDL_Texture *texture)
 {
     if (!texture && renderer->logical_target) {

src/render/SDL_sysrender.h

@@ -201,6 +201,7 @@ struct SDL_Renderer
                        const SDL_Rect *rect, void **pixels, int *pitch);
     void (*UnlockTexture)(SDL_Renderer *renderer, SDL_Texture *texture);
     void (*SetTextureScaleMode)(SDL_Renderer *renderer, SDL_Texture *texture, SDL_ScaleMode scaleMode);
+    int (*GenMipmaps)(SDL_Renderer *renderer, SDL_Texture *texture);
     int (*SetRenderTarget)(SDL_Renderer *renderer, SDL_Texture *texture);
     SDL_Surface *(*RenderReadPixels)(SDL_Renderer *renderer, const SDL_Rect *rect);
     int (*RenderPresent)(SDL_Renderer *renderer);

src/render/opengl/SDL_render_gl.c

@@ -118,6 +118,9 @@ typedef struct
     PFNGLBINDFRAMEBUFFEREXTPROC glBindFramebufferEXT;
     PFNGLCHECKFRAMEBUFFERSTATUSEXTPROC glCheckFramebufferStatusEXT;
 
+    /* glGenerateMipmap support */
+    PFNGLGENERATEMIPMAPPROC glGenerateMipmap;
+
     /* Shader support */
     GL_ShaderContext *shaders;
 
@@ -843,6 +846,24 @@ static void GL_UnlockTexture(SDL_Renderer *renderer, SDL_Texture *texture)
     GL_UpdateTexture(renderer, texture, rect, pixels, data->pitch);
 }
 
+static int GL_GenMipmaps(SDL_Renderer *renderer, SDL_Texture *texture)
+{
+    GL_RenderData *renderdata = (GL_RenderData *)renderer->driverdata;
+    SDL_assert(renderdata->glGenerateMipmap != NULL);  // should have been set up at CreateRenderer time and we shouldn't be here if it isn't!
+
+    if (texture != renderdata->drawstate.texture) {
+        if (renderdata->GL_ARB_multitexture_supported) {
+            renderdata->glActiveTextureARB(GL_TEXTURE0_ARB);
+        }
+        const GL_TextureData *texturedata = (GL_TextureData *)texture->driverdata;
+        renderdata->glBindTexture(renderdata->textype, texturedata->texture);
+        renderdata->drawstate.texture = texture;
+    }
+
+    renderdata->glGenerateMipmap(renderdata->textype);
+    return 0;  // GL does not report failure here.
+}
+
 static void GL_SetTextureScaleMode(SDL_Renderer *renderer, SDL_Texture *texture, SDL_ScaleMode scaleMode)
 {
     GL_RenderData *renderdata = (GL_RenderData *)renderer->driverdata;
@@ -1719,23 +1740,21 @@ static int GL_CreateRenderer(SDL_Renderer *renderer, SDL_Window *window, SDL_Pro
         data->glEnable(GL_DEBUG_OUTPUT_SYNCHRONOUS_ARB);
     }
 
+    int glmajorver = 1;
+    const char *glverstr = (const char *)data->glGetString(GL_VERSION);
+    if (glverstr) {
+        char verbuf[16];
+        SDL_strlcpy(verbuf, glverstr, sizeof(verbuf));
+        char *ptr = SDL_strchr(verbuf, '.');
+        if (ptr) {
+            *ptr = '\0';
+            glmajorver = SDL_atoi(verbuf);
+        }
+    }
+
     hint = SDL_getenv("GL_ARB_texture_non_power_of_two");
     if (!hint || *hint != '0') {
-        SDL_bool isGL2 = SDL_FALSE;
-        const char *verstr = (const char *)data->glGetString(GL_VERSION);
-        if (verstr) {
-            char verbuf[16];
-            char *ptr;
-            SDL_strlcpy(verbuf, verstr, sizeof(verbuf));
-            ptr = SDL_strchr(verbuf, '.');
-            if (ptr) {
-                *ptr = '\0';
-                if (SDL_atoi(verbuf) >= 2) {
-                    isGL2 = SDL_TRUE;
-                }
-            }
-        }
-        if (isGL2 || SDL_GL_ExtensionSupported("GL_ARB_texture_non_power_of_two")) {
+        if ((glmajorver >= 2) || SDL_GL_ExtensionSupported("GL_ARB_texture_non_power_of_two")) {
             non_power_of_two_supported = SDL_TRUE;
         }
     }
@@ -1765,6 +1784,11 @@ static int GL_CreateRenderer(SDL_Renderer *renderer, SDL_Window *window, SDL_Pro
         }
     }
 
+    if (glmajorver >= 3) {
+        data->glGenerateMipmap = (PFNGLGENERATEMIPMAPPROC)SDL_GL_GetProcAddress("glGenerateMipmap");
+        renderer->GenMipmaps = (data->glGenerateMipmap != NULL) ? GL_GenMipmaps : NULL;
+    }
+
     /* Check for shader support */
     data->shaders = GL_CreateShaderContext();
     SDL_LogInfo(SDL_LOG_CATEGORY_RENDER, "OpenGL shaders: %s",

src/render/opengles2/SDL_gles2funcs.h

@@ -76,3 +76,4 @@ SDL_PROC(void, glDeleteBuffers, (GLsizei, const GLuint *))
 SDL_PROC(void, glBindBuffer, (GLenum, GLuint))
 SDL_PROC(void, glBufferData, (GLenum, GLsizeiptr, const GLvoid *, GLenum))
 SDL_PROC(void, glBufferSubData, (GLenum, GLintptr, GLsizeiptr, const GLvoid *))
+SDL_PROC(void, glGenerateMipmap, (GLenum))

src/render/opengles2/SDL_render_gles2.c

@@ -1906,6 +1906,22 @@ static void GLES2_SetTextureScaleMode(SDL_Renderer *renderer, SDL_Texture *textu
     renderdata->glTexParameteri(data->texture_type, GL_TEXTURE_MAG_FILTER, glScaleMode);
 }
 
+static int GLES2_GenMipmaps(SDL_Renderer *renderer, SDL_Texture *texture)
+{
+    GLES2_RenderData *renderdata = (GLES2_RenderData *)renderer->driverdata;
+    SDL_assert(renderdata->glGenerateMipmap != NULL); // this should have been set up at CreateRenderer time and we shouldn't be here if it isn't!
+
+    const GLES2_TextureData *tdata = (GLES2_TextureData *)texture->driverdata;
+    if (texture != renderdata->drawstate.texture) {
+        renderdata->glActiveTexture(GL_TEXTURE0);
+        renderdata->glBindTexture(tdata->texture_type, tdata->texture);
+        renderdata->drawstate.texture = texture;
+    }
+
+    renderdata->glGenerateMipmap(tdata->texture_type);
+    return 0;  // GLES2 does not report failure here.
+}
+
 static int GLES2_SetRenderTarget(SDL_Renderer *renderer, SDL_Texture *texture)
 {
     GLES2_RenderData *data = (GLES2_RenderData *)renderer->driverdata;
@@ -2152,6 +2168,7 @@ static int GLES2_CreateRenderer(SDL_Renderer *renderer, SDL_Window *window, SDL_
     renderer->LockTexture = GLES2_LockTexture;
     renderer->UnlockTexture = GLES2_UnlockTexture;
     renderer->SetTextureScaleMode = GLES2_SetTextureScaleMode;
+    renderer->GenMipmaps = GLES2_GenMipmaps;  // always available since OpenGL ES 2.0.
     renderer->SetRenderTarget = GLES2_SetRenderTarget;
     renderer->QueueSetViewport = GLES2_QueueNoOp;
     renderer->QueueSetDrawColor = GLES2_QueueNoOp;