GDI+ and drawing on a Transparent PNG

eddie_jordan

21y5.3k

Hi, There has been a lot of great tutorials written on this subject, however they all seem to work with JPGs. I myself have no difficultly drawing on a JPG, however when I try to draw on a PNG the drawing will not show up. I have tried numerous things, my code looks like the following: Image image( txtString ); IMAGE_WIDTH = image.GetWidth(); IMAGE_HEIGHT =image.GetHeight(); graphics.DrawImage(&image, 0, 0, IMAGE_WIDTH, IMAGE_HEIGHT ); Graphics imageGraphics(&image); DrawRoute(&imageGraphics); // this function draws lines on the first image. graphics.DrawImage(&image, 0, 0, IMAGE_WIDTH, IMAGE_HEIGHT); SaveImage(&image); The lines are drawn with Pens and Brushes that specify ARGB values, with A being 255, though I tried change the alpha levels, but that doesn't help. I've noticed one thing strange, if I take the PNG into photoshop and drop the the fill level or the opacity level by 1, the PNG will then work perfectly.. I have also tried the ColorMatrix and ImageAttributes etc.. however this doesn't look very good in what I'm trying to do.. why can't I just draw directly on top of the image like a JPG? Any help is appreciated.. Thanks in advance, Edward.

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RSPERL21y ago
the reason for not being able to draw directly to the png and hey I am probably wrong my be this: You cannot create a Graphics object from an image with an indexed pixel format. If you want to open this image and draw on it you need to do the following... size the new bitmap to the source bitmaps dimensions draw the old bitmaps contents to the new bitmap paint the entire region of the old bitmap to the new bitmap..use the rectangle type to select area of the source image
RSPERL21y ago
I don't know if you have figured this out or not but I thought I would add my 2 cents. basically I get my image from the file system but it could be from anywhere Sql Blob field etc.. then I just created a very bare helper method to do the work. [code] private void BtnLoadImage_Click( object sender, EventArgs e ) { Image image = new Bitmap(FilePath.Text); image = AddToImage(image); pictureBox.Image = image; } private Image AddToImage( Image image ) { Image bmpNew = image; Graphics canvas; try { canvas = Graphics.FromImage(image); } catch( Exception e ) { Bitmap bmp = new Bitmap(image.Width, image.Height); canvas = Graphics.FromImage(bmp); canvas.DrawImage(bmp, new Rectangle(0, 0, image.Width, image.Height), 0, 0, bmp.Width, bmp.Height, GraphicsUnit.Pixel); bmpNew = bmp; } canvas.DrawString("Hello From .NET\nGDI", new Font("Tahoma", 14, FontStyle.Bold), new SolidBrush(Color.White), 5, 5); canvas.Dispose(); return bmpNew; } [/code]