fix(opengles): clear layer texture for transparent screen refresh (#9987)

Signed-off-by: Borgeuzz <mmam2407@gmail.com>

src/core/lv_refr.c

@@ -21,6 +21,7 @@
 #include "../misc/lv_profiler.h"
 #include "../misc/lv_types.h"
 #include "../draw/lv_draw_private.h"
+#include "../draw/opengles/lv_draw_opengles.h"
 #include "../stdlib/lv_string.h"
 #include "lv_global.h"
 
@@ -1057,9 +1058,30 @@ static void refr_configured_layer(lv_layer_t * layer)
     }
     /*If the screen is transparent initialize it when the flushing is ready*/
     if(lv_color_format_has_alpha(disp_refr->color_format)) {
-        lv_area_t clear_area = layer->_clip_area;
-        lv_area_move(&clear_area, -layer->buf_area.x1, -layer->buf_area.y1);
-        lv_draw_buf_clear(layer->draw_buf, &clear_area);
+#if LV_USE_DRAW_OPENGLES
+        lv_layer_t * clear_target_layer = disp_refr->layer_head ? disp_refr->layer_head : layer;
+        /* TODO: this driver-specific branch is a temporary workaround.
+         * The proper fix may be a generic per-draw-unit clear callback (e.g.
+         * a `clear_area_cb` on `lv_draw_unit_t`) so `lv_refr` can just dispatch
+         * the right clear function. LVGL does not currently expose that hook now.
+         * This is a special-case for Draw_OpenGLES to fix issue #9912 (PR #9987).
+         */
+        /*With Draw_OpenGLES the layer's draw_buf is a dummy CPU buffer and the
+         *real pixels live in a GL texture. Clearing the CPU buffer is a no-op
+         *on the texture, so perform a GPU-side clear of the dirty area.
+         *Key this off the refreshing display's real backing layer instead of
+         *the current layer, because tiled rendering can use temporary tile
+         *layers with NULL user_data.*/
+        if(disp_refr->layer_head != NULL && disp_refr->layer_head->user_data != NULL) {
+            lv_draw_opengles_clear_layer_area(clear_target_layer, &layer->_clip_area);
+        }
+        else
+#endif
+        {
+            lv_area_t clear_area = layer->_clip_area;
+            lv_area_move(&clear_area, -layer->buf_area.x1, -layer->buf_area.y1);
+            lv_draw_buf_clear(layer->draw_buf, &clear_area);
+        }
     }
 
     lv_obj_t * top_act_scr = NULL;

src/draw/opengles/lv_draw_opengles.c

@@ -128,6 +128,48 @@ void lv_draw_opengles_deinit(void)
     g_unit = NULL;
 }
 
+void lv_draw_opengles_clear_layer_area(lv_layer_t * layer, const lv_area_t * area)
+{
+    if(g_unit == NULL) return;
+    unsigned int target_texture = layer_get_texture(layer);
+    if(target_texture == 0) return;
+
+    lv_area_t a;
+    if(!lv_area_intersect(&a, area, &layer->_clip_area)) return;
+    lv_area_move(&a, -layer->buf_area.x1, -layer->buf_area.y1);
+
+    int32_t w = lv_area_get_width(&a);
+    int32_t h = lv_area_get_height(&a);
+    if(w <= 0 || h <= 0) return;
+
+    int32_t tex_h = lv_area_get_height(&layer->buf_area);
+
+    GLint prev_fbo = 0;
+    GLint prev_scissor_box[4] = {0};
+    GLfloat prev_clear_color[4] = {0};
+    GLboolean prev_scissor_test = glIsEnabled(GL_SCISSOR_TEST);
+    GL_CALL(glGetIntegerv(GL_FRAMEBUFFER_BINDING, &prev_fbo));
+    GL_CALL(glGetIntegerv(GL_SCISSOR_BOX, prev_scissor_box));
+    GL_CALL(glGetFloatv(GL_COLOR_CLEAR_VALUE, prev_clear_color));
+
+    unsigned int framebuffer = get_framebuffer(g_unit);
+    GL_CALL(glBindFramebuffer(GL_FRAMEBUFFER, framebuffer));
+    GL_CALL(glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, target_texture, 0));
+
+    GL_CALL(glEnable(GL_SCISSOR_TEST));
+    GL_CALL(glScissor(a.x1, tex_h - a.y1 - h, w, h));
+    GL_CALL(glClearColor(0.0f, 0.0f, 0.0f, 0.0f));
+    GL_CALL(glClear(GL_COLOR_BUFFER_BIT));
+
+    /*Restore previous GL state.*/
+    if(!prev_scissor_test) {
+        GL_CALL(glDisable(GL_SCISSOR_TEST));
+    }
+    GL_CALL(glScissor(prev_scissor_box[0], prev_scissor_box[1], prev_scissor_box[2], prev_scissor_box[3]));
+    GL_CALL(glClearColor(prev_clear_color[0], prev_clear_color[1], prev_clear_color[2], prev_clear_color[3]));
+    GL_CALL(glBindFramebuffer(GL_FRAMEBUFFER, (GLuint)prev_fbo));
+}
+
 /**********************
  *   STATIC FUNCTIONS
  **********************/

src/draw/opengles/lv_draw_opengles.h

@@ -17,6 +17,9 @@ extern "C" {
 #include "../../lv_conf_internal.h"
 #if LV_USE_DRAW_OPENGLES
 
+#include "../lv_draw.h"
+#include "../../misc/lv_area.h"
+
 /*********************
  *      DEFINES
  *********************/
@@ -32,6 +35,18 @@ extern "C" {
 void lv_draw_opengles_init(void);
 void lv_draw_opengles_deinit(void);
 
+/**
+ * Clear a region of the GPU texture that backs the given layer.
+ * Needed because `lv_draw_buf_clear` operates on the CPU draw buffer,
+ * which is only a dummy placeholder when the layer is rendered to a
+ * GL texture. Without this, a transparent screen background leaves
+ * stale pixels on the texture between frames.
+ *
+ * @param layer layer whose backing texture must be cleared
+ * @param area  area (in layer/screen coordinates) to clear
+ */
+void lv_draw_opengles_clear_layer_area(lv_layer_t * layer, const lv_area_t * area);
+
 /**********************
  *      MACROS
  **********************/