alpha channel test

ConsoleApp2.csproj

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
  <PropertyGroup>
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    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <ProjectGuid>{6561C4EB-18A8-482D-8C28-C5C7C6E9BF07}</ProjectGuid>
    <OutputType>Exe</OutputType>
    <RootNamespace>ConsoleApp2</RootNamespace>
    <AssemblyName>ConsoleApp2</AssemblyName>
    <TargetFrameworkVersion>v4.6.1</TargetFrameworkVersion>
    <FileAlignment>512</FileAlignment>
    <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <PlatformTarget>AnyCPU</PlatformTarget>
    <DebugSymbols>true</DebugSymbols>
    <DebugType>full</DebugType>
    <Optimize>false</Optimize>
    <OutputPath>bin\Debug\</OutputPath>
    <DefineConstants>DEBUG;TRACE</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
    <PlatformTarget>AnyCPU</PlatformTarget>
    <DebugType>pdbonly</DebugType>
    <Optimize>true</Optimize>
    <OutputPath>bin\Release\</OutputPath>
    <DefineConstants>TRACE</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <ItemGroup>
    <Reference Include="System" />
    <Reference Include="System.Core" />
    <Reference Include="System.Drawing" />
    <Reference Include="System.Xml.Linq" />
    <Reference Include="System.Data.DataSetExtensions" />
    <Reference Include="Microsoft.CSharp" />
    <Reference Include="System.Data" />
    <Reference Include="System.Net.Http" />
    <Reference Include="System.Xml" />
  </ItemGroup>
  <ItemGroup>
    <Compile Include="Program.cs" />
  </ItemGroup>
  <ItemGroup>
    <Folder Include="Properties\" />
  </ItemGroup>
  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
</Project>

ConsoleApp2.sln


Microsoft Visual Studio Solution File, Format Version 12.00
# Visual Studio 15
VisualStudioVersion = 15.0.26730.16
MinimumVisualStudioVersion = 10.0.40219.1
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ConsoleApp2", "ConsoleApp2.csproj", "{6561C4EB-18A8-482D-8C28-C5C7C6E9BF07}"
EndProject
Global
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		Release|Any CPU = Release|Any CPU
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		{6561C4EB-18A8-482D-8C28-C5C7C6E9BF07}.Debug|Any CPU.Build.0 = Debug|Any CPU
		{6561C4EB-18A8-482D-8C28-C5C7C6E9BF07}.Release|Any CPU.ActiveCfg = Release|Any CPU
		{6561C4EB-18A8-482D-8C28-C5C7C6E9BF07}.Release|Any CPU.Build.0 = Release|Any CPU
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	GlobalSection(SolutionProperties) = preSolution
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	GlobalSection(ExtensibilityGlobals) = postSolution
		SolutionGuid = {8585E7F7-679D-4957-87C8-0258DB0EAC28}
	EndGlobalSection
EndGlobal

opaque.png

Program.cs

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.IO;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            var width = 256;
            var height = 256;
            var filenames = new string[] {
                "opaque.png",
                "transparent.png"
            };

            Directory.CreateDirectory("output");

            foreach (var filename in filenames)
            {
                var src = Path.Combine("..", "..", filename);
                var dst = Path.Combine("output", filename);

                using (var originalImg = Image.FromFile(src))
                {
                    using (var resizedImg = new Bitmap(width, height))
                    {
                        using (var graphics = Graphics.FromImage(resizedImg))
                        {
                            graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
                            graphics.PixelOffsetMode = PixelOffsetMode.HighQuality;
                            graphics.DrawImage(originalImg, new Rectangle(0, 0, width, height));
                            resizedImg.Save(dst);
                        }
                        Console.WriteLine("input: " + originalImg.PixelFormat + ", output: " + resizedImg.PixelFormat);
                    }
                }
            }
        }
    }
}

transparent.png