colorspace: match input/output luminance for SDR->SDR case

When doing SDR->SDR rendering, we in fact map display referred transfer,
we don't really care about absolute luminance, all that matters is the
lumiannce matches between input and output so we map image into full
range.

Note how in SDR->PQ case we still allow to specify arbitrary luminance,
which is than effectively and hdr reference white value, at which
luminance level this SDR input will be rendered at.

Author: kasper93

src/colorspace.c

@@ -952,6 +952,7 @@ void pl_color_space_infer_map(struct pl_color_space *src,
 {
     bool unknown_src_contrast = !src->hdr.min_luma;
     bool unknown_dst_contrast = !dst->hdr.min_luma;
+    bool unknown_src_luminance = !src->hdr.max_luma;
 
     infer_both_ref(dst, src);
 
@@ -970,6 +971,14 @@ void pl_color_space_infer_map(struct pl_color_space *src,
     if (unknown_dst_contrast && src_is_sdr && dst_is_sdr)
         dst->hdr.min_luma = src->hdr.min_luma;
 
+    // If both src and dst are SDR, match luminance, this input is display-referred.
+    if (unknown_src_luminance && src_is_sdr && dst_is_sdr)
+        src->hdr.max_luma = dst->hdr.max_luma;
+
+    // If SDR source has luminance, say from ICC profile, use it for target.
+    if (!unknown_src_luminance && src_is_sdr && dst_is_sdr)
+        dst->hdr.max_luma = src->hdr.max_luma;
+
     // If the src is HLG and the output is HDR, tune the HLG peak to the output
     if (src->transfer == PL_COLOR_TRC_HLG && pl_color_space_is_hdr(dst))
         src->hdr.max_luma = dst->hdr.max_luma;