/// <summary>
/// For electronic books, we want to minimize the actual size of images since they'll
/// be displayed on small screens anyway. So before zipping up the file, we replace its
/// bytes with the bytes of a reduced copy of itself. If the original image is already
/// small enough, we return its bytes directly.
/// We also make png images have transparent backgrounds. This is currently only necessary
/// for cover pages, but it's an additional complication to detect which those are,
/// and doesn't seem likely to cost much extra to do.
/// </summary>
/// <returns>The bytes of the (possibly) reduced image.</returns>
internal static byte[] GetBytesOfReducedImage(string filePath)
{
using (var originalImage = PalasoImage.FromFileRobustly(filePath))
{
var image = originalImage.Image;
int originalWidth = image.Width;
int originalHeight = image.Height;
var appearsToBeJpeg = ImageUtils.AppearsToBeJpeg(originalImage);
if (originalWidth > kMaxWidth || originalHeight > kMaxHeight || !appearsToBeJpeg)
{
// Preserve the aspect ratio
float scaleX = (float)kMaxWidth / (float)originalWidth;
float scaleY = (float)kMaxHeight / (float)originalHeight;
// no point in ever expanding, even if we're making a new image just for transparency.
float scale = Math.Min(1.0f, Math.Min(scaleX, scaleY));
// New width and height maintaining the aspect ratio
int newWidth = (int)(originalWidth * scale);
int newHeight = (int)(originalHeight * scale);
var imagePixelFormat = image.PixelFormat;
switch (imagePixelFormat)
{
// These three formats are not supported for bitmaps to be drawn on using Graphics.FromImage.
// So use the default bitmap format.
// Enhance: if these are common it may be worth research to find out whether there are better options.
// - possibly the 'reduced' image might not be reduced...even though smaller, the indexed format
// might be so much more efficient that it is smaller. However, even if that is true, it doesn't
// necessarily follow that it takes less memory to render on the device. So it's not obvious that
// we should keep the original just because it's a smaller file.
// - possibly we don't need a 32-bit bitmap? Unfortunately the 1bpp/4bpp/8bpp only tells us
// that the image uses two, 16, or 256 distinct colors, not what they are or what precision they have.
case PixelFormat.Format1bppIndexed:
case PixelFormat.Format4bppIndexed:
case PixelFormat.Format8bppIndexed:
imagePixelFormat = PixelFormat.Format32bppArgb;
break;
}
// OTOH, always using 32-bit format for .png files keeps us from having problems in BloomReader
// like BL-5740 (where 24bit format files came out in BR with black backgrounds).
if (!appearsToBeJpeg)
{
imagePixelFormat = PixelFormat.Format32bppArgb;
}
using (var newImage = new Bitmap(newWidth, newHeight, imagePixelFormat))
{
// Draws the image in the specified size with quality mode set to HighQuality
using (Graphics graphics = Graphics.FromImage(newImage))
{
graphics.CompositingQuality = CompositingQuality.HighQuality;
graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
graphics.SmoothingMode = SmoothingMode.HighQuality;
using (var imageAttributes = new ImageAttributes())
{
// See https://stackoverflow.com/a/11850971/7442826
// Fixes the 50% gray border issue on bright white or dark images
imageAttributes.SetWrapMode(WrapMode.TileFlipXY);
// In addition to possibly scaling, we want PNG images to have transparent backgrounds.
if (!appearsToBeJpeg)
{
// This specifies that all white or very-near-white pixels (all color components at least 253/255)
// will be made transparent.
imageAttributes.SetColorKey(Color.FromArgb(253, 253, 253), Color.White);
}
var destRect = new Rectangle(0, 0, newWidth, newHeight);
graphics.DrawImage(image, destRect, 0, 0, image.Width, image.Height,
GraphicsUnit.Pixel, imageAttributes);
}
}
// Save the file in the same format as the original, and return its bytes.
using (var tempFile = TempFile.WithExtension(Path.GetExtension(filePath)))
{
// This uses default quality settings for jpgs...one site says this is
// 75 quality on a scale that runs from 0-100. For most images, this
// should give a quality barely distinguishable from uncompressed and still save
// about 7/8 of the file size. Lower quality settings rapidly lose quality
// while only saving a little space; higher ones rapidly use more space
// with only marginal quality improvement.
// See https://photo.stackexchange.com/questions/30243/what-quality-to-choose-when-converting-to-jpg
// for more on quality and https://docs.microsoft.com/en-us/dotnet/framework/winforms/advanced/how-to-set-jpeg-compression-level
// for how to control the quality setting if we decide to (RobustImageIO has
// suitable overloads).
RobustImageIO.SaveImage(newImage, tempFile.Path, image.RawFormat);
// Copy the metadata from the original file to the new file.
var metadata = SIL.Windows.Forms.ClearShare.Metadata.FromFile(filePath);
if (!metadata.IsEmpty)
{
metadata.Write(tempFile.Path);
}
return(RobustFile.ReadAllBytes(tempFile.Path));
}
}
}
}
return(RobustFile.ReadAllBytes(filePath));
}
