ChafaKittyCanvas: If alpha threshold is at max, composite on solid BG

This allows us to show artifact-free animations with transparency using the basic Kitty protocol.
hpjansson · Sep 19, 2022 · 6fa53ad · 6fa53ad
1 parent 8b3426e
commit 6fa53ad
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Showing 5 changed files with 57 additions and 5 deletions.
diff --git a/chafa/chafa-canvas.c b/chafa/chafa-canvas.c
@@ -1561,6 +1561,11 @@ chafa_canvas_draw_all_pixels (ChafaCanvas *canvas, ChafaPixelType src_pixel_type
     }
     else if (canvas->config.pixel_mode == CHAFA_PIXEL_MODE_KITTY)
     {
+        ChafaColor bg_color;
+
+        chafa_unpack_color (canvas->config.bg_color_packed_rgb, &bg_color);
+        bg_color.ch [3] = canvas->config.alpha_threshold < 1 ? 0x00 : 0xff;
+
         /* Kitty mode */
 
         canvas->fg_palette.alpha_threshold = canvas->config.alpha_threshold;
@@ -1570,7 +1575,8 @@ chafa_canvas_draw_all_pixels (ChafaCanvas *canvas, ChafaPixelType src_pixel_type
                                             src_pixel_type,
                                             src_pixels,
                                             src_width, src_height,
-                                            src_rowstride);
+                                            src_rowstride,
+                                            bg_color);
     }
     else  /* if (canvas->config.pixel_mode == CHAFA_PIXEL_MODE_ITERM2) */
     {

diff --git a/chafa/internal/chafa-kitty-canvas.c b/chafa/internal/chafa-kitty-canvas.c
@@ -26,6 +26,7 @@
 #include "internal/chafa-bitfield.h"
 #include "internal/chafa-indexed-image.h"
 #include "internal/chafa-kitty-canvas.h"
+#include "internal/chafa-pixops.h"
 #include "internal/chafa-string-util.h"
 
 typedef struct
@@ -39,6 +40,8 @@ typedef struct
 {
     ChafaKittyCanvas *kitty_canvas;
     SmolScaleCtx *scale_ctx;
+    ChafaColor bg_color;
+    gboolean flatten_alpha;
 }
 DrawCtx;
 
@@ -47,7 +50,7 @@ chafa_kitty_canvas_new (gint width, gint height)
 {
     ChafaKittyCanvas *kitty_canvas;
 
-    kitty_canvas = g_new (ChafaKittyCanvas, 1);
+    kitty_canvas = g_new0 (ChafaKittyCanvas, 1);
     kitty_canvas->width = width;
     kitty_canvas->height = height;
     kitty_canvas->rgba_image = g_malloc (width * height * sizeof (guint32));
@@ -69,12 +72,21 @@ draw_pixels_worker (ChafaBatchInfo *batch, const DrawCtx *ctx)
                            ((guint32 *) ctx->kitty_canvas->rgba_image) + (ctx->kitty_canvas->width * batch->first_row),
                            batch->first_row,
                            batch->n_rows);
+
+    /* FIXME: Smolscale should be able to do this */
+    if (ctx->flatten_alpha)
+        chafa_composite_rgba_on_solid_color (ctx->bg_color,
+                                             ctx->kitty_canvas->rgba_image,
+                                             ctx->kitty_canvas->width,
+                                             batch->first_row,
+                                             batch->n_rows);
 }
 
 void
 chafa_kitty_canvas_draw_all_pixels (ChafaKittyCanvas *kitty_canvas, ChafaPixelType src_pixel_type,
                                     gconstpointer src_pixels,
-                                    gint src_width, gint src_height, gint src_rowstride)
+                                    gint src_width, gint src_height, gint src_rowstride,
+                                    ChafaColor bg_color)
 {
     DrawCtx ctx;
 
@@ -100,6 +112,8 @@ chafa_kitty_canvas_draw_all_pixels (ChafaKittyCanvas *kitty_canvas, ChafaPixelTy
                                          kitty_canvas->width * sizeof (guint32),
                                          NULL,
                                          &ctx);
+    ctx.bg_color = bg_color;
+    ctx.flatten_alpha = bg_color.ch [3] == 0;
 
     chafa_process_batches (&ctx,
                            (GFunc) draw_pixels_worker,

diff --git a/chafa/internal/chafa-kitty-canvas.h b/chafa/internal/chafa-kitty-canvas.h
@@ -34,9 +34,11 @@ ChafaKittyCanvas;
 ChafaKittyCanvas *chafa_kitty_canvas_new (gint width, gint height);
 void chafa_kitty_canvas_destroy (ChafaKittyCanvas *kitty_canvas);
 
-void chafa_kitty_canvas_draw_all_pixels (ChafaKittyCanvas *kitty_canvas, ChafaPixelType src_pixel_type,
+void chafa_kitty_canvas_draw_all_pixels (ChafaKittyCanvas *kitty_canvas,
+                                         ChafaPixelType src_pixel_type,
                                          gconstpointer src_pixels,
-                                         gint src_width, gint src_height, gint src_rowstride);
+                                         gint src_width, gint src_height, gint src_rowstride,
+                                         ChafaColor bg_color);
 void chafa_kitty_canvas_build_ansi (ChafaKittyCanvas *kitty_canvas, ChafaTermInfo *term_info, GString *out_str,
                                     gint width_cells, gint height_cells);
 

diff --git a/chafa/internal/chafa-pixops.c b/chafa/internal/chafa-pixops.c
@@ -620,6 +620,33 @@ composite_alpha_on_bg (ChafaColor bg_color,
     }
 }
 
+/* FIXME: Could we always destroy the alpha channel and eliminate the other
+ * variant? */
+static void
+composite_alpha_on_solid (ChafaColor bg_color,
+                          ChafaPixel *pixels, gint width, gint first_row, gint n_rows)
+{
+    ChafaPixel *p0, *p1;
+
+    p0 = pixels + first_row * width;
+    p1 = p0 + n_rows * width;
+
+    for ( ; p0 < p1; p0++)
+    {
+        p0->col.ch [0] += (bg_color.ch [0] * (255 - (guint32) p0->col.ch [3])) / 255;
+        p0->col.ch [1] += (bg_color.ch [1] * (255 - (guint32) p0->col.ch [3])) / 255;
+        p0->col.ch [2] += (bg_color.ch [2] * (255 - (guint32) p0->col.ch [3])) / 255;
+        p0->col.ch [3] = 0xff;
+    }
+}
+
+void
+chafa_composite_rgba_on_solid_color (ChafaColor color,
+                                     ChafaPixel *pixels, gint width, gint first_row, gint n_rows)
+{
+    composite_alpha_on_solid (color, pixels, width, first_row, n_rows);
+}
+
 static void
 prepare_pixels_2_worker (ChafaBatchInfo *batch, PrepareContext *prep_ctx)
 {

diff --git a/chafa/internal/chafa-pixops.h b/chafa/internal/chafa-pixops.h
@@ -43,6 +43,9 @@ void chafa_sort_pixel_index_by_channel (guint8 *index,
                                         const ChafaPixel *pixels, gint n_pixels,
                                         gint ch);
 
+void chafa_composite_rgba_on_solid_color (ChafaColor color,
+                                          ChafaPixel *pixels, gint width, gint first_row, gint n_rows);
+
 G_END_DECLS
 
 #endif /* __CHAFA_PIXOPS_H__ */