diff --git a/src/rasterimage/image.hpp b/src/rasterimage/image.hpp
index c900dbd..a114ab4 100644
--- a/src/rasterimage/image.hpp
+++ b/src/rasterimage/image.hpp
@@ -46,7 +46,6 @@ class image : public dimensioned
 	using image_span_type = image_span<channel_type, num_channels>;
 	using const_image_span_type = const_image_span<channel_type, num_channels>;
 	using pixel_type = r4::vector<channel_type, num_channels>;
-	using value_type = typename pixel_type::value_type;
 
 	static_assert(sizeof(pixel_type) == sizeof(channel_type) * number_of_channels, "pixel_type has padding");
 
@@ -130,7 +129,7 @@ class image : public dimensioned
 
 	static image make(
 		dimensions_type dims, //
-		const value_type* data,
+		const typename pixel_type::value_type* data,
 		size_t stride_in_values = 0
 	)
 	{
@@ -151,7 +150,7 @@ class image : public dimensioned
 				// NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-pointer-arithmetic)
 				src_row + num_values_per_row,
 				// NOLINTNEXTLINE(cppcoreguidelines-pro-type-reinterpret-cast)
-				reinterpret_cast<value_type*>(row.data())
+				reinterpret_cast<typename pixel_type::value_type*>(row.data())
 			);
 			// NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-pointer-arithmetic)
 			src_row += stride_in_values;
@@ -162,9 +161,9 @@ class image : public dimensioned
 		return im;
 	}
 
-	constexpr static value_type value(float f)
+	constexpr static typename pixel_type::value_type value(float f)
 	{
-		return rasterimage::value<value_type>(f);
+		return rasterimage::value<pixel_type::value_type>(f);
 	}
 };
 
diff --git a/src/rasterimage/image_span.hpp b/src/rasterimage/image_span.hpp
index 24e15fc..f2f115a 100644
--- a/src/rasterimage/image_span.hpp
+++ b/src/rasterimage/image_span.hpp
@@ -51,7 +51,6 @@ class image_span : public dimensioned
 		is_const_span, //
 		const r4::vector<channel_type, num_channels>,
 		r4::vector<channel_type, num_channels>>;
-	using value_type = typename pixel_type::value_type;
 
 	static_assert(sizeof(pixel_type) == sizeof(channel_type) * number_of_channels, "pixel_type has padding");
 
@@ -96,6 +95,7 @@ class image_span : public dimensioned
 		using iterator_category = std::input_iterator_tag;
 
 		using difference_type = std::ptrdiff_t;
+
 		using value_type = decltype(line);
 		using reference = value_type;
 		using pointer = void;
@@ -238,6 +238,7 @@ class image_span : public dimensioned
 	using const_iterator = iterator_internal<true>;
 	using reverse_iterator = std::reverse_iterator<iterator>;
 	using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+	using value_type = typename iterator::value_type;
 
 	explicit image_span(
 		dimensions_type dimensions, //
@@ -299,6 +300,15 @@ class image_span : public dimensioned
 		return this->buffer == nullptr;
 	}
 
+	/**
+	 * @brief Get number of lines.
+	 * @return Number of lines in this image span.
+	 */
+	size_t size() const noexcept
+	{
+		return this->dims().y();
+	}
+
 	iterator begin() noexcept
 	{
 		return iterator(utki::make_span(this->buffer, this->dims().x()), this->stride_px);
@@ -426,14 +436,11 @@ class image_span : public dimensioned
 		ASSERT(!src_span.empty())
 		ASSERT(src_span.dims() == dst_span.dims())
 
-		// TODO: use zip_view
-		auto src_line = src_span.begin();
-		auto dst_line = dst_span.begin();
-		for (; src_line != src_span.end(); ++src_line, ++dst_line) {
+		for (auto [s, d] : utki::views::zip(src_span, dst_span)) {
 			std::copy(
-				src_line->begin(), //
-				src_line->end(),
-				dst_line->begin()
+				s.begin(), //
+				s.end(),
+				d.begin()
 			);
 		}
 	}
diff --git a/src/rasterimage/image_variant.cpp b/src/rasterimage/image_variant.cpp
index 86bd491..7ce1b4d 100644
--- a/src/rasterimage/image_variant.cpp
+++ b/src/rasterimage/image_variant.cpp
@@ -203,7 +203,7 @@ void image_variant::write_png(const papki::file& fi) const
 		// get bits per channel
 		std::visit(
 			[](const auto& im) {
-				using value_type = typename std::remove_reference_t<decltype(im)>::value_type;
+				using value_type = typename std::remove_reference_t<decltype(im)>::pixel_type::value_type;
 				if constexpr (!std::is_same_v<value_type, uint8_t> && !std::is_same_v<value_type, uint16_t>) {
 					throw std::invalid_argument("write_png(): PNG supports only 8 bit or 16 bit per channel images");
 				}