Drawing Transparent Images

Question

I'm writing a CSS sprite engine in C#, however I'm having a few issues. I create the master image, set all the properties to that then iterate the sprites and draw those to the master image. However when I come to save the master image it only appears to be just the empty master image with transparent background and none of the sprites. I'm very confused at where I'm going wrong.

The code I'm using is:

    // Work out the width/height required
    int max_width = 0;
    int max_height = 0;

    foreach(SpriteInformation sprite in sprites) {
        if (max_width < (sprite.Left + greatest_width)) max_width = sprite.Left + greatest_width;
        if (max_height < (sprite.Top + greatest_height)) max_height = sprite.Top + greatest_height;
    }

    // Create new master bitmap
    Bitmap bitmap = new Bitmap(max_width,max_height,PixelFormat.Format32bppArgb);
    Graphics graphics = Graphics.FromImage(bitmap);

    // Set background color
    SolidBrush brush;

    if (cbxBackground.Checked) {
        if (txtColor.Text == "") {
            brush = new SolidBrush(Color.Black);
        } else {
            brush = new SolidBrush(pnlColor.BackColor);
        }
    } else {
        if (txtColor.Text == "") {
            brush = new SolidBrush(Color.White);
        } else {
            brush = new SolidBrush(pnlColor.BackColor);
        }
    }

    //graphics.FillRectangle(brush,0,0,bitmap.Width,bitmap.Height);
    bitmap.MakeTransparent(brush.Color);
    graphics.Clear(brush.Color);

    // Copy images into place
    ImageAttributes attr = new ImageAttributes();

    //attr.SetColorKey(brush.Color,brush.Color);

    foreach(SpriteInformation sprite in sprites) {
        Rectangle source = new Rectangle(0,0,sprite.Width,sprite.Height);
        Rectangle dest = new Rectangle(sprite.Left,sprite.Top,sprite.Width,sprite.Height);

        graphics.DrawImage(sprite.Sprite,dest,0,0,sprite.Width,sprite.Height,GraphicsUnit.Pixel,attr);
    }

    // Save image
    string format = ddlFormat.Items[ddlFormat.SelectedIndex].ToString();

    if (format == "PNG") {
        dlgSave.Filter = "PNG Images|*.png|All Files|*.*";
        dlgSave.DefaultExt = ",png";

        if (dlgSave.ShowDialog() == DialogResult.OK) {
            bitmap.Save(dlgSave.FileName,ImageFormat.Png);
        }
    } else if (format == "JPEG") {

    } else {

    }

Answer 1

You draw to "graphics", but then then you save "bitmap"? I'm not sure if that graphics internally works with your original "bitmap" object...

Answer 2

What are sprite.Left and Top? If they aren't 0 that may be a problem. I think you have dest and source the wrong way round?

http://msdn.microsoft.com/en-us/library/ms142045.aspx

Try a simpler DrawImage variant if you haven't already. e.g. DrawImage(sprite.Sprite,0,0)