在 C# 中在 PNG 图像上绘制文本时出现颜色问题

Question

I want to draw black text over with grey opacity PNG file so text is BLACK .

What I am getting is the text is some % of grey:

Even if I use Brushes.Black the text is still grey;

My code is following:

List<string> GenerateDeviceIcon(string backgroundImageFile, string deviceImageFile, string deviceNumber, int deviceID, string saveNewFilePath, string fontName, int fontSize, Brush textColor)
{
    var r = new List<string>();

    try
    {
        Image background = Image.FromFile(backgroundImageFile);
        Image logo = Image.FromFile(deviceImageFile);

        PointF firstLocation = new PointF(2f, 2f);

        using (background)
        {
           using (var bitmap = new Bitmap(background.Width, background.Height))
           {
                using (var canvas = Graphics.FromImage(bitmap))
                {
                     using (Font arialFont = new Font(fontName, fontSize))
                     {
                          canvas.DrawString(deviceNumber, arialFont, textColor, firstLocation);                            
                     }

                     canvas.InterpolationMode = InterpolationMode.HighQualityBicubic;
                     canvas.DrawImage(background, new Rectangle(0, 0, background.Width, background.Height), new Rectangle(0, 0, background.Width, background.Height), GraphicsUnit.Pixel);
                     canvas.DrawImage(logo, (bitmap.Width / 2) - (logo.Width / 2), (bitmap.Height / 2) - (logo.Height / 2));
                     canvas.Save();
                }

                try
                {
                   var filename = Path.Combine(saveNewFilePath, deviceID.ToString() + ".png");

                    if (File.Exists(filename)) 
                    {
                         File.Delete(filename);
                    }

                    bitmap.Save(filename, System.Drawing.Imaging.ImageFormat.Png);
                }
                catch (Exception ex)
                {
                   r.Add(ex.Message);
                }
           }
        }
   }
   catch (Exception ex)
   {
      r.Add(ex.Message);
   }

   return r;
}

How to fix it?

Many thanks!

Answer 1

Well I found the bug: dont draw text BEFORE you draw a background! And I've improved the code so it draws multiple lines of a transport ID.

Enjoy if you need create complex icons in .NET!

Code:

   static List<string> GenerateDeviceIcon2(string backgroundImageFile, string deviceImageFile,
            string deviceNumber, int deviceID, string saveNewFilePath, string fontName, int fontSize, Color textColor)
        {
            var r = new List<string>();

            try
            {
                Image background = Image.FromFile(backgroundImageFile);
                Image logo = Image.FromFile(deviceImageFile);
                PointF firstLocation = new PointF(2f, 2f);

                #region Create text as Image with Transparancy

                //first, create a dummy bitmap just to get a graphics object
                Image img = new Bitmap(1, 1);
                Graphics drawingText = Graphics.FromImage(img);
                //measure the string to see how big the image needs to be
                int maxWidth = background.Width - 2;
                var font = new Font(fontName, fontSize, new FontStyle());
                SizeF textSize = drawingText.MeasureString(deviceNumber, font, maxWidth);

                //set the stringformat flags to rtl
                StringFormat sf = new StringFormat
                {
                    //uncomment the next line for right to left languages
                    //sf.FormatFlags = StringFormatFlags.DirectionRightToLeft;
                    Trimming = StringTrimming.Word
                };

                //free up the dummy image and old graphics object
                img.Dispose();
                drawingText.Dispose();

                //create a new image of the right size
                img = new Bitmap((int)textSize.Width, (int)textSize.Height);
                // drawingText = Graphics.FromImage(img);
                #endregion
                //create a brush for the text
                Brush textBrush = new SolidBrush(textColor);

                using (background)
                {
                    using (var bitmap = new Bitmap(background.Width, background.Height))
                    {
                        using (var canvas = Graphics.FromImage(bitmap))
                        {
                            //Adjust for high quality
                            canvas.CompositingQuality = CompositingQuality.HighQuality;
                            canvas.InterpolationMode = InterpolationMode.HighQualityBilinear;
                            canvas.PixelOffsetMode = PixelOffsetMode.HighQuality;
                            canvas.SmoothingMode = SmoothingMode.HighQuality;
                            canvas.TextRenderingHint = TextRenderingHint.AntiAliasGridFit; 

                            //paint the background
                            canvas.Clear(Color.Transparent);

                            // First - draw a background!
                            canvas.DrawImage(background, new Rectangle(0, 0, background.Width, background.Height),
                                                new Rectangle(0, 0, background.Width, background.Height), GraphicsUnit.Pixel);

                            // Second - draw the text in multiple rows over background
                            canvas.DrawImage(logo, (bitmap.Width / 2) - (logo.Width / 2), (bitmap.Height / 2) - (logo.Height / 2));

                            // Third - draw the logo over background
                            canvas.DrawString(deviceNumber, font, textBrush, new RectangleF(0, 0, textSize.Width, textSize.Height), sf);

                            canvas.Save();
                        }

                        try
                        {
                            var filename = Path.Combine(saveNewFilePath, deviceID.ToString() + ".png");

                            if (File.Exists(filename))
                            {
                                File.Delete(filename);
                            }

                            bitmap.Save(filename, System.Drawing.Imaging.ImageFormat.Png);
                        }
                        catch (Exception ex)
                        {
                            r.Add(ex.Message);
                        }
                    }
                }

                textBrush.Dispose();
                img.Dispose();
            }
            catch (Exception ex)
            {
                r.Add(ex.Message);
            }

            return r;
        }