MinimumSize | 1 | Searches the lossless options and writes the smallest file it finds; the pixels, the alpha values (the color of fully transparent pixels included), the frames, their durations, the poster and the metadata are stored exactly. Encoding is several times slower than with PngOptimization.None.- Layout. The color type and the bit depth are the ones the pixels need: grayscale (1, 2, 4, 8 or 16 bits), grayscale with alpha, RGB or RGBA (8 or 16 bits), a
tRNS color key when every transparent pixel has one color that no opaque pixel has, or a palette of at most 256 colors. The candidate layouts are compared by compressing every image known before the header is written, and the smallest total is kept. With PngPixelLayout.Indexed, only palettes are candidates. - Filters and compression. Every image is compressed with each filter strategy (
PngFilter, or only the fixed filter that PngEncoder.Filter selects) at the zlib levels CompressionLevel.SmallestSize, CompressionLevel.Optimal and CompressionLevel.Fastest (a lower level sometimes finds smaller data), and the smallest is written: PngEncoder.CompressionLevel is ignored. PngEncoder.Interlaced is respected. - Animation frames. Each frame stores the rectangle that differs from the canvas it is drawn on, with the blending and the disposal of the previous frame that give the smallest data. The frames are computed from the displayed frames, never from the delta information of a decoded source. Frame zero covers the canvas when it is the default image. For an animation saved at once, a layout without a transparent value is also tried with one (a
tRNS color key that no pixel has, or a transparent palette entry), which lets the frames skip their unchanged pixels: the layout trial compresses the frames as the regions that changed, so the transparent value is only written when it saves more than it costs. The samples and the palette indexes of an animation have 8 bits at least, because some decoders misplace the packed pixels of a frame region.
A file has a single layout, written before its first image. The eager save methods know every frame and the poster of an animation, so the layout fits all of them. A sequential ImageWriter<TPixel> only knows the first image it is given: a later frame that the layout cannot store (more colors, alpha values, or samples that need more bits) is rejected with an UnsupportedImageFeatureException before anything of it is written. A sequential writer also writes each animation frame when it is given the next one (or when it completes), because the control data of a frame holds the disposal chosen for the next one. The file may be read back with another pixel format than the one it was written from (for example PixelFormat.Rgb24 for an opaque PixelFormat.Rgba32 image, or PixelFormat.Rgba32 for a file with a color key or a palette); the pixel values are the same. |
Layout | 2 | Reads the pixels of a still image once and writes them in the narrowest exact layout they allow, instead of the layout of the pixel format; the filters follow PngEncoder.Filter and the zlib level PngEncoder.CompressionLevel, as with PngOptimization.None. Nothing is compressed twice, so encoding costs about one more pass over the pixels.- The candidates are the ones of
PngOptimization.MinimumSize without the tRNS color key: grayscale when every pixel is gray (1, 2, 4, 8 or 16 bits), without alpha when every pixel is opaque, 8-bit samples when the 16-bit ones are 8-bit values, and a palette of 1, 2, 4 or 8 bits (with tRNS for its translucent entries) when the image has at most 256 colors and they are 8-bit colors. The one with the smallest unfiltered data (its PLTE and tRNS chunks included) is written; on a tie, the layout without a palette. With PngPixelLayout.Indexed, only palettes are candidates. - The analysis stops as soon as nothing more can change the choice: once there are more than 256 colors, a pixel is not gray, a pixel is translucent (or the pixel format has no alpha) and a 16-bit sample is not an 8-bit value (or the samples are 8-bit). An opaque image of a pixel format with alpha is read to its last pixel.
- An ICC profile keeps its color space: a grayscale layout is a candidate only when the profile describes gray, and the RGB and palette layouts only when it describes RGB.
- An animated PNG is written as with
PngOptimization.None: the layout of a sequential writer would have to hold the frames that come after the header.
The pixel values are stored exactly, but the file may be read back with another pixel format than the one it was written from, as with PngOptimization.MinimumSize: for example PixelFormat.Rgb24 for an opaque PixelFormat.Rgba32 image, PixelFormat.Gray8 for a gray one, or PixelFormat.Rgba32 for a palette. The smallest unfiltered data is usually, not always, the smallest file: PngOptimization.MinimumSize compresses the candidates to find it. |
Meziantou.Framework