LibGfx+LibPDF: Create filter_type() for converting u8 to FilterType

...and use it in LibPDF.

No behavior change.

Userland/Libraries/LibGfx/ImageFormats/PNGLoader.cpp

@@ -698,18 +698,18 @@ static ErrorOr<void> decode_png_bitmap_simple(PNGLoadingContext& context, ByteBu
     Streamer streamer(decompression_buffer.data(), decompression_buffer.size());
 
     for (int y = 0; y < context.height; ++y) {
-        PNG::FilterType filter;
-        if (!streamer.read(filter)) {
+        u8 filter_byte;
+        if (!streamer.read(filter_byte)) {
             context.state = PNGLoadingContext::State::Error;
             return Error::from_string_literal("PNGImageDecoderPlugin: Decoding failed");
         }
 
-        if (to_underlying(filter) > 4) {
+        if (filter_byte > 4) {
             context.state = PNGLoadingContext::State::Error;
             return Error::from_string_literal("PNGImageDecoderPlugin: Invalid PNG filter");
         }
 
-        context.scanlines.append({ filter });
+        context.scanlines.append({ MUST(PNG::filter_type(filter_byte)) });
         auto& scanline_buffer = context.scanlines.last().data;
         auto row_size = context.compute_row_size_for_width(context.width);
         if (row_size.has_overflow())
@@ -784,18 +784,18 @@ static ErrorOr<void> decode_adam7_pass(PNGLoadingContext& context, Streamer& str
         return {};
 
     for (int y = 0; y < subimage_context.height; ++y) {
-        PNG::FilterType filter;
-        if (!streamer.read(filter)) {
+        u8 filter_byte;
+        if (!streamer.read(filter_byte)) {
             context.state = PNGLoadingContext::State::Error;
             return Error::from_string_literal("PNGImageDecoderPlugin: Decoding failed");
         }
 
-        if (to_underlying(filter) > 4) {
+        if (filter_byte > 4) {
             context.state = PNGLoadingContext::State::Error;
             return Error::from_string_literal("PNGImageDecoderPlugin: Invalid PNG filter");
         }
 
-        subimage_context.scanlines.append({ filter });
+        subimage_context.scanlines.append({ MUST(PNG::filter_type(filter_byte)) });
         auto& scanline_buffer = subimage_context.scanlines.last().data;
 
         auto row_size = context.compute_row_size_for_width(subimage_context.width);

Userland/Libraries/LibGfx/ImageFormats/PNGShared.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/Array.h>
+#include <AK/Error.h>
 #include <AK/SIMD.h>
 
 namespace Gfx::PNG {
@@ -32,6 +33,13 @@ enum class FilterType : u8 {
     Paeth,
 };
 
+inline ErrorOr<FilterType> filter_type(u8 byte)
+{
+    if (byte <= 4)
+        return static_cast<FilterType>(byte);
+    return Error::from_string_literal("PNGImageDecoderPlugin: Invalid PNG filter");
+}
+
 // https://www.w3.org/TR/PNG/#9Filter-type-4-Paeth
 ALWAYS_INLINE u8 paeth_predictor(u8 a, u8 b, u8 c)
 {

Userland/Libraries/LibPDF/Filter.cpp

@@ -162,19 +162,19 @@ PDFErrorOr<ByteBuffer> Filter::decode_png_prediction(Bytes bytes, int bytes_per_
     for (int row_index = 0; row_index < number_of_rows; ++row_index) {
         auto row = bytes.data() + row_index * bytes_per_row;
 
-        u8 algorithm_tag = row[0];
-        switch (algorithm_tag) {
-        case 0:
+        auto filter = TRY(Gfx::PNG::filter_type(row[0]));
+        switch (filter) {
+        case Gfx::PNG::FilterType::None:
             break;
-        case 1:
+        case Gfx::PNG::FilterType::Sub:
             for (int i = 2; i < bytes_per_row; ++i)
                 row[i] += row[i - 1];
             break;
-        case 2:
+        case Gfx::PNG::FilterType::Up:
             for (int i = 1; i < bytes_per_row; ++i)
                 row[i] += previous_row[i];
             break;
-        case 3:
+        case Gfx::PNG::FilterType::Average:
             for (int i = 1; i < bytes_per_row; ++i) {
                 u8 left = 0;
                 if (i > 1)
@@ -183,7 +183,7 @@ PDFErrorOr<ByteBuffer> Filter::decode_png_prediction(Bytes bytes, int bytes_per_
                 row[i] += (left + above) / 2;
             }
             break;
-        case 4:
+        case Gfx::PNG::FilterType::Paeth:
             for (int i = 1; i < bytes_per_row; ++i) {
                 u8 left = 0;
                 u8 upper_left = 0;
@@ -195,8 +195,6 @@ PDFErrorOr<ByteBuffer> Filter::decode_png_prediction(Bytes bytes, int bytes_per_
                 row[i] += Gfx::PNG::paeth_predictor(left, above, upper_left);
             }
             break;
-        default:
-            return AK::Error::from_string_literal("Unknown PNG algorithm tag");
         }
 
         previous_row = row;