vulkan/swapchain: add pl_vulkan_swapchain_params.{alpha,color}_bits

Author: kasper93

meson.build

@@ -12,6 +12,7 @@ project('libplacebo', ['c', 'cpp'],
     7,
     # API version
     {
+      '359': 'add pl_vulkan_swapchain_params.{alpha,color}_bits',
       '358': 'add PL_COLOR_SYSTEM_YCGCO_{RE,RO}',
       '357': 'add pl_daylight_from_temp, pl_blackbody_from_temp, and generalize pl_white_from_temp',
       '356': 'pl_avframe_set_repr now sets frame.alpha_mode',

src/include/libplacebo/vulkan.h

@@ -356,6 +356,18 @@ struct pl_vulkan_swapchain_params {
     // drivers which don't dither properly when outputting high bit depth
     // SDR backbuffers to 8-bit screens.
     bool disable_10bit_sdr;
+
+    // Minimum number of alpha bits requested in the swapchain format. In
+    // practice, this can be used to prefer formats with higher alpha bit depth
+    // (e.g. rgba8/rgba16 over rgb10a2), potentially trading 2 bits per color
+    // channel for increased alpha precision. This is a hint, if no matching
+    // format is found, lower bit depths will be accepted.
+    uint8_t alpha_bits;
+
+    // Minimum number of color bits requested in the swapchain format. This can
+    // be used to prefer formats with higher color bit depth. This is a hint,
+    // if no matching format is found, lower bit depths will be accepted.
+    uint8_t color_bits;
 };
 
 #define pl_vulkan_swapchain_params(...) (&(struct pl_vulkan_swapchain_params) { __VA_ARGS__ })

src/vulkan/swapchain.c

@@ -159,13 +159,20 @@ static bool map_color_space(VkColorSpaceKHR space, struct pl_color_space *out)
     }
 }
 
+static inline int quant_score(int depth, int requested)
+{
+    pl_assert(depth >= 0 && depth <= 16);
+    pl_assert(requested >= 0 && requested <= 16);
+    return requested <= depth ? 0 : 17 * (16 - depth) + requested - depth;
+}
+
 static bool pick_surf_format(pl_swapchain sw, const struct pl_color_space *hint)
 {
     struct priv *p = PL_PRIV(sw);
     struct vk_ctx *vk = p->vk;
     pl_gpu gpu = sw->gpu;
 
-    int best_score = 0, best_id;
+    int best_score = -1000, best_id;
     bool wide_gamut = pl_color_primaries_is_wide_gamut(hint->primaries);
     bool prefer_hdr = pl_color_transfer_is_hdr(hint->transfer);
 
@@ -227,6 +234,12 @@ static bool pick_surf_format(pl_swapchain sw, const struct pl_color_space *hint)
                 default: // skip any other format
                     continue;
             }
+            int alpha_depth = plfmt->num_components < 4 ? 0 : PL_MIN(plfmt->component_depth[3], 16);
+            int err = quant_score(plfmt->component_depth[0], PL_MIN(p->params.color_bits, 16)) +
+                      quant_score(alpha_depth, PL_MIN(p->params.alpha_bits, 16));
+            // Reset score if we don't meet the requested bit depth
+            if (err)
+                score = -err;
 #ifdef __APPLE__
             // On Apple hardware, only these formats allow direct-to-display
             // rendering, so give them a slight score boost to tie-break against
@@ -274,7 +287,7 @@ static bool pick_surf_format(pl_swapchain sw, const struct pl_color_space *hint)
         }
     }
 
-    if (!best_score) {
+    if (best_score == -1000) {
         PL_ERR(vk, "Failed picking any valid, renderable surface format!");
         return false;
     }