Fix antialiasing for downsampling

Use the average color filter to smooth the image in downsampling mode. Resolves #183. Signed-off-by: Artem Senichev <[email protected]>
artemsen · Sep 15, 2024 · d170c3c · d170c3c
1 parent fecd704
commit d170c3c
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Showing 4 changed files with 90 additions and 7 deletions.
diff --git a/src/image.c b/src/image.c
@@ -64,7 +64,13 @@ void image_thumbnail(struct image* image, size_t size, bool fill,
     size_t thumb_width = scale * full->width;
     size_t thumb_height = scale * full->height;
     ssize_t offset_x, offset_y;
-    const enum pixmap_scale sf = antialias ? pixmap_bicubic : pixmap_nearest;
+    enum pixmap_scale scaler;
+
+    if (antialias) {
+        scaler = (scale > 1.0) ? pixmap_bicubic : pixmap_average;
+    } else {
+        scaler = pixmap_nearest;
+    }
 
     if (fill) {
         offset_x = size / 2 - thumb_width / 2;
@@ -80,7 +86,7 @@ void image_thumbnail(struct image* image, size_t size, bool fill,
     if (!pixmap_create(&thumb, thumb_width, thumb_height)) {
         return;
     }
-    pixmap_scale(sf, full, &thumb, offset_x, offset_y, scale, image->alpha);
+    pixmap_scale(scaler, full, &thumb, offset_x, offset_y, scale, image->alpha);
 
     image_free_frames(image);
     frame = image_create_frames(image, 1);

diff --git a/src/pixmap.c b/src/pixmap.c
@@ -102,6 +102,63 @@ static void scale_nearest(struct scale_param* sp, size_t start, size_t step)
     }
 }
 
+/** Average scale filter, see `scale_fn` for details. */
+static void scale_average(struct scale_param* sp, size_t start, size_t step)
+{
+    const ssize_t left = max(0, sp->x);
+    const ssize_t top = max(0, sp->y);
+    const ssize_t right =
+        min((ssize_t)sp->dst->width, sp->x + sp->scale * sp->src->width);
+    const ssize_t bottom =
+        min((ssize_t)sp->dst->height, sp->y + sp->scale * sp->src->height);
+
+    const ssize_t iscale = (1.0 / sp->scale) / 2;
+    const ssize_t delta_x = left - sp->x;
+    const ssize_t delta_y = top - sp->y;
+
+    for (ssize_t dst_y = top + start; dst_y < bottom; dst_y += step) {
+        ssize_t src_y = (double)(dst_y - top + delta_y) / sp->scale;
+        const ssize_t src_y0 = max(0, src_y - iscale);
+        const ssize_t src_y1 = min((ssize_t)sp->src->height, src_y + iscale);
+        argb_t* dst_line = &sp->dst->data[dst_y * sp->dst->width];
+
+        for (ssize_t dst_x = left; dst_x < right; ++dst_x) {
+            ssize_t src_x = (double)(dst_x - left + delta_x) / sp->scale;
+            const ssize_t src_x0 = max(0, src_x - iscale);
+            const ssize_t src_x1 = min((ssize_t)sp->src->width, src_x + iscale);
+
+            size_t a = 0, r = 0, g = 0, b = 0;
+            size_t count = 0;
+
+            for (src_y = src_y0; src_y <= src_y1; ++src_y) {
+                for (src_x = src_x0; src_x <= src_x1; ++src_x) {
+                    const argb_t color =
+                        sp->src->data[src_y * sp->src->width + src_x];
+                    a += ARGB_GET_A(color);
+                    r += ARGB_GET_R(color);
+                    g += ARGB_GET_G(color);
+                    b += ARGB_GET_B(color);
+                    ++count;
+                }
+            }
+            if (count) {
+                a /= count;
+                r /= count;
+                g /= count;
+                b /= count;
+            }
+
+            const argb_t color = ARGB(a, r, g, b);
+
+            if (sp->alpha) {
+                alpha_blend(color, &dst_line[dst_x]);
+            } else {
+                dst_line[dst_x] = ARGB_SET_A(0xff) | color;
+            }
+        }
+    }
+}
+
 /** Bicubic scale filter, see `scale_fn` for details. */
 static void scale_bicubic(struct scale_param* sp, size_t start, size_t step)
 {
@@ -449,9 +506,7 @@ void pixmap_scale(enum pixmap_scale scaler, const struct pixmap* src,
     const size_t threads_num = min(16, max(cpus, 1)) - 1;
     struct scale_task* tasks = NULL;
 
-    const scale_fn scaler_fn =
-        scaler == pixmap_nearest ? scale_nearest : scale_bicubic;
-
+    // scaling parameters
     struct scale_param sp = {
         .src = src,
         .dst = dst,
@@ -460,6 +515,21 @@ void pixmap_scale(enum pixmap_scale scaler, const struct pixmap* src,
         .scale = scale,
         .alpha = alpha,
     };
+    scale_fn scaler_fn;
+
+    switch (scaler) {
+        case pixmap_nearest:
+            scaler_fn = scale_nearest;
+            break;
+        case pixmap_bicubic:
+            scaler_fn = scale_bicubic;
+            break;
+        case pixmap_average:
+            scaler_fn = scale_average;
+            break;
+        default:
+            return;
+    }
 
     // create task for each CPU core
     if (threads_num) {

diff --git a/src/pixmap.h b/src/pixmap.h
@@ -50,6 +50,7 @@ struct pixmap {
 enum pixmap_scale {
     pixmap_nearest, ///< Nearest filter, poor quality but fast
     pixmap_bicubic, ///< Bicubic filter, good quality but slow
+    pixmap_average, ///< Average color, downsampling image
 };
 
 /**

diff --git a/src/viewer.c b/src/viewer.c
@@ -498,8 +498,14 @@ static void draw_image(struct pixmap* wnd)
     if (ctx.scale == 1.0) {
         pixmap_copy(img_pm, wnd, ctx.img_x, ctx.img_y, img->alpha);
     } else {
-        pixmap_scale(ctx.antialiasing ? pixmap_bicubic : pixmap_nearest, img_pm,
-                     wnd, ctx.img_x, ctx.img_y, ctx.scale, img->alpha);
+        enum pixmap_scale scaler;
+        if (ctx.antialiasing) {
+            scaler = (ctx.scale > 1.0) ? pixmap_bicubic : pixmap_average;
+        } else {
+            scaler = pixmap_nearest;
+        }
+        pixmap_scale(scaler, img_pm, wnd, ctx.img_x, ctx.img_y, ctx.scale,
+                     img->alpha);
     }
 }