Gamut

Gamut (DUB package: gamut) is an image decoding/encoding library for D.

Inspired by the FreeImage design, the Image concept is monomorphic and can do it all.

Gamut tries to have the fastest and most memory-conscious image decoders available in pure D code. It is nothrow @nogc @safe for usage in -betterC and in disabled-runtime D.

Decoding

• PNG: 8-bit and 16-bit, L/LA/RGB/RGBA
• JPEG: 8-bit, L/RGB/RGBA, baseline and progressive
• JPEG XL: 8-bit, RGB (no alpha support), encoded with cjxl -e 4 or lower
• TGA: 8-bit, indexed, L/LA/RGB/RGBA
• GIF: indexed, animation support
• BMP: indexed 1/4/8-bit no-RLE, 8-bit RGB/RGBA
• QOI: 8-bit, RGB/RGBA
• QOIX: 8-bit, 10-bit, L/LA/RGB/RGBA. Improvement upon QOI. This format may change between major Gamut tags, so is not a storage format.

Encoding

• PNG. 8-bit, 16-bit, L/LA/RGB/RGBA
• JPEG: 8-bit, greyscale/RGB, baseline
• TGA: 8-bit, RGB/RGBA
• GIF: 8-bit, RGBA, animation support
• BMP: 8-bit, RGB/RGBA
• QOI: 8-bit, RGB/RGBA
• QOIX: 8-bit, 10-bit, L/LA/RGB/RGBA, premultiplied alpha
• DDS: BC7 encoded, 8-bit, RGB/RGBA

Changelog

• v3 Added premultiplied alpha pixel types. BREAKING.
  • Decoders are now allowed to return any type if you do not specify LOAD_PREMUL or LOAD_NO_PREMUL. Update your loading code.
  • Introduce image.premultiply() and image.unpremultiply().
  • QOIX supports encoding premultiplied. Saves space and decoding times for transparent overlays.
• v2.6 Added JPEG XL input. 8-bit, no alpha, cjxl --effort 4 or lower, raw streams not ISO BMFF.
• v2.5 Added BMP input.
• v2.4 Added BMP output.
• v2.3 Added GIF input and GIF output. Added multilayer images.
• v2.2 Added 16-bit PNG output.
• v2.1 Added TGA format support.
• v2 QOIX bitstream changed. Ways to disown and deallocate image allocation pointer. It's safe to update to latest tag in the same major version. Do keep a 16-bit source in case the bitstream changes.
• v1 Initial release.

Why QOIX?

Our benchmark results for 8-bit color images:

• QOIX and QOI generally outperforms PNG in decoding speed and encoding speed.
• QOIX outperforms QOI in compression efficiency at the cost of speed:
  • because it's based upon better intra predictors
  • because it is followed by LZ4, which removes some of the QOI worst cases.
• QOIX adds support for 8-bit greyscale and greyscale + alpha images, with a "QOI-plane" custom codec.
• QOIX adds support for 10-bit images, with a "QOI-10b" custom codec. It drops the last 6 bits of precision (lossy) to outperform PNG 16-bit in every way for some use cases.
• QOIX support for premultiplied alpha brings even more speed and compression for transparent images.

Use the convert tool to encode QOIX.

Gamut API documentation

1. Image basics

Key concept: The Image struct is where most of the public API resides.

1.1 Get the dimensions of an image:

  Image image = Image(800, 600);
  int w = image.width();
  int h = image.height();
  assert(w == 800 && h == 600);

1.2 Get the pixel format of an image:

  Image image = Image(800, 600);
  PixelType type = image.type();
  assert(type == PixelType.rgba8); // rgba8 is default if not provided

Key concept: PixelType completely describes the pixel format, for example PixelType.rgb8 is a 24-bit format with one byte for red, green and blue components each (in that order). Nothing is specified about the color space though.

Here are the possible PixelType:

  enum PixelType
  {
      l8,
      l16,
      lf32,

      la8,
      la16,
      laf32,
      lap8,
      lap16,
      lapf32,

      rgb8, 
      rgb16,
      rgbf32,

      rgba8,
      rgba16,
      rgbaf32
      rgbap8,
      rgbap16,
      rgbapf32
  }

For now, all pixels format have one to four components:

• 1 component is implicitely Greyscale
• 2 components is implicitely Greyscale + alpha
• 3 components is implicitely Red + Green + Blue

• 4 components is implicitely Red + Green + Blue + Alpha

  Bit-depth: Each of these components can be represented in 8-bit, 16-bit, or 32-bit floating-point (0.0f to 1.0f range).

  Alpha premultiplication: When an alpha channel exist, both premultiplied and non-premultiplied variants exist.

1.3 Create an image:

Different ways to create an Image:

• create() or regular constructor this() creates a new owned image filled with zeros.
• createNoInit() or setSize() creates a new owned uninitialized image.
• createView() creates a view into existing data.
• createNoData() creates a new image with no data pointed to (still has a type, size...).

  // Create with transparent black.
  Image image = Image(640, 480, PixelType.rgba8); 
  image.create(640, 480, PixelType.rgba8);

  // Create with no initialization.
  image.setSize(640, 480, PixelType.rgba8);
  image.createNoInit(640, 480, PixelType.rgba8);

  // Create view into existing data. Existing data is borrowed.
  image.createView(data.ptr, w, h, PixelType.rgb8, pitchbytes);

• At creation time, the Image forgets about its former life, and leaves any isError() state or former data/type
• Image.init is in isError() state
• isValid() can be used instead of !isError()
• Being valid == not being error == having a PixelType

2. Loading and saving an image

2.1 Load an Image from a file:

Another way to create an Image is to load an encoded image.

  Image image;
  image.loadFromFile("logo.png");
  if (image.isError)
      throw new Exception(image.errorMessage);

You can then read width(), height(), type(), etc...

There is no exceptions in Gamut. Instead the Image itself has an error API:

• bool isError() return true if the Image is in an error state. In an error state, the image can't be used anymore until recreated (for example, loading another file).
• const(char)[] errorMessage() is then available, and is guaranteed to be zero-terminated with an extra byte.

2.2 Load an image from memory:

  auto pngBytes = cast(const(ubyte)[]) import("logo.png"); 
  Image image;
  image.loadFromMemory(pngBytes);
  if (!image.isValid) 
      throw new Exception(image.errorMessage());

Key concept: You can force the loaded image to be a certain type using LoadFlags, or call convertTo() after load.

Here are the possible LoadFlags:

  LOAD_NORMAL      // Default: preserve type from original.

  LOAD_ALPHA       // Force one alpha channel.
  LOAD_NO_ALPHA    // Force zero alpha channel.

  LOAD_GREYSCALE   // Force greyscale.
  LOAD_RGB         // Force RGB values.

  LOAD_8BIT        // Force 8-bit `ubyte` per component.
  LOAD_16BIT       // Force 16-bit `ushort` per component.
  LOAD_FP32        // Force 32-bit `float` per component.

  LOAD_PREMUL      // Force premultiplied alpha representation (if alpha exists)
  LOAD_NO_PREMUL   // Force non-premultiplied alpha representation (if alpha exists)

Example:

  Image image;  
  image.loadFromMemory(pngBytes, LOAD_RGB | LOAD_ALPHA | LOAD_8BIT | LOAD_NO_PREMUL);  // force PixelType.rgba8 

Not all load flags are compatible, for example LOAD_8BIT and LOAD_16BIT cannot be used together.

2.3 Convert to another PixelType:

However, load flags are not the only way to select a PixelType, you can provide one explicitely with convertTo.

  // Convert to grey + one alpha channel, 16-bit
  image.convertTo(PixelType.la16); 

  // Convert to RGB + one alpha channel, 8-bit
  image.convertTo(PixelType.rgba8); 

2.4 Save an image to a file:

  Image image;
  if (!image.saveToFile("output.png"))
      throw new Exception("Writing output.png failed");

Key concept: ImageFormat is simply the codecs/containers files Gamut encode and decodes to.

  enum ImageFormat
  {
      unknown,
      JPEG,
      PNG,
      QOI,
      QOIX,
      DDS,
      TGA,
      GIF,
      JXL
  }

This can be used to avoid inferring the output format from the filename:

  Image image;
  if (!image.saveToFile(ImageFormat.PNG, "output.png"))
      throw new Exception("Writing output.png failed");

2.5 Save an image to memory:

  Image image;
  ubyte[] qoixEncoded = image.saveToMemory(ImageFormat.QOIX);
  scope(exit) freeEncodedImage(qoixEncoded);

The returned slice must be freed up with freeEncodedImage.

2.6 Convert an image to QOIX for faster load

  Image image;
  image.loadFromFile("input.png");
  image.saveToFromFile("output.qoix"); // .qoix loads faster

3. Accessing image pixels

3.1 Get the row pitch, in bytes:

  int pitch = image.pitchInBytes();

Key concept: The image pitch is the distance between the start of two consecutive scanlines, in bytes. IMPORTANT: This pitch can be negative.

3.2 Access a row of pixels:

  void* scan = image.scanptr(y);  // get pointer to start of pixel row
  void[] row = image.scanline(y); // get slice of pixel row

Key concept: The scanline is void* because the type it points to depends upon the PixelType. In a given scanline, the bytes scan[0..abs(pitchInBytes())] are all accessible, even if they may be outside of the image (trailing pixels, gap bytes for alignment, border pixels).

3.3 Iterate on pixels:

  assert(image.type == PixelType.rgba16);
  assert(image.hasData());
  for (int y = 0; y < image.height(); ++y)
  {
      ushort* scan = cast(ushort*) image.scanptr(y);
      for (int x = 0; x < image.width(); ++x)
      {
          ushort r = scan[4*x + 0];
          ushort g = scan[4*x + 1];
          ushort b = scan[4*x + 2];
          ushort a = scan[4*x + 3];
      }
  }

Key concept: The default is that you do not access pixels in a contiguous manner. See 4. for layout constraints that allow you to get all pixels at once.

3.4 Process pixels:

Here is how to process pixels of an rgba8 image in-place.

  void liftGammaGain(ref Image image, 
                     float lift,  // 0 to 1
                     float gamma, 
                     float gain)
  {
      assert(image.type == PixelType.rgba8);
      assert(image.hasData());
      for (int y = 0; y < image.height(); ++y)
      {
          byte* scan = cast(ubyte*) image.scanptr(y);
          for (int x = 0; x < image.width(); ++x)
          {
              float r = scan[4*x + 0] / 255.0f;
              float g = scan[4*x + 1] / 255.0f;
              float b = scan[4*x + 2] / 255.0f;
              float a = scan[4*x + 3] / 255.0f;
              r = (gain * (r + lift * (1-r)))^(1/gamma);
              g = (gain * (g + lift * (1-r)))^(1/gamma);
              b = (gain * (b + lift * (1-r)))^(1/gamma);
              if (r < 0) r = 0;
              if (g < 0) g = 0;
              if (b < 0) b = 0;
              if (r > 1) r = 1;
              if (g > 1) g = 1;
              if (b > 1) b = 1;
              scan[4*x+0] = cast(ubyte)(r * 255);
              scan[4*x+1] = cast(ubyte)(g * 255);
              scan[4*x+2] = cast(ubyte)(b * 255);
          }
      }
  }   

Key concept: .scanptr() pointers are untyped.

4. Layout constraints

One of the most interesting feature of Gamut! Images in Gamut can follow given constraints over the data layout.

Key concept: LayoutConstraint are carried by images all their life.

Example:

  // Do nothing in particular.
  LayoutConstraint constraints = LAYOUT_DEFAULT; // 0 = default

  // Layout can be given directly at image creation or afterwards.
  Image image;  
  image.loadFromMemory(pngBytes, constraints); 

  // Now the image has a 1 pixel border (at least).
  // Changing the layout only reallocates if needed.
  image.setLayout(LAYOUT_BORDER_1);

  // Those layout constraints are preserved.
  // (but: not the excess bytes content, if reallocated)
  image.convertToGreyscale();
  assert(image.layoutConstraints() == LAYOUT_BORDER_1);   

Important: Layout constraints are about the minimum guarantee you want. Your image may be more constrained than that in practice, but you can't rely on that.

• If you don't specify LAYOUT_VERT_STRAIGHT, you should expect your image to be possibly stored upside-down, and account for that possibility.
• If you don't specify LAYOUT_SCANLINE_ALIGNED_16, you should not expect your scanlines to be aligned on 16-byte boundaries, even though that can happen accidentally.

Beware not to accidentally reset constraints when resizing:

// If you do not provide layout constraints, 
// the one choosen is 0, the most permissive.
image.setSize(640, 480, PixelType.rgba8, LAYOUT_TRAILING_3);

4.1 Scanline alignment

Scanline alignment guarantees minimum alignment of each scanline.

LAYOUT_SCANLINE_ALIGNED_1 = 0
LAYOUT_SCANLINE_ALIGNED_2
LAYOUT_SCANLINE_ALIGNED_4
LAYOUT_SCANLINE_ALIGNED_8
LAYOUT_SCANLINE_ALIGNED_16
LAYOUT_SCANLINE_ALIGNED_32
LAYOUT_SCANLINE_ALIGNED_64
LAYOUT_SCANLINE_ALIGNED_128

4.2 Layout multiplicity

Multiplicity guarantees access to pixels 1, 2, 4 or 8 at a time. It does this with excess pixels at the end of the scanline, but they need not exist if the scanline has the right width.

LAYOUT_MULTIPLICITY_1 = 0
LAYOUT_MULTIPLICITY_2
LAYOUT_MULTIPLICITY_4
LAYOUT_MULTIPLICITY_8

Together with scanline alignment, this allow processing a scanline using aligned SIMD without processing the last few pixels differently.

4.3 Trailing pixels

Trailing pixels gives you up to 7 excess pixels after each scanline.

LAYOUT_TRAILING_0 = 0
LAYOUT_TRAILING_1
LAYOUT_TRAILING_3
LAYOUT_TRAILING_7

Allows unaligned SIMD access by itself.

4.4 Pixel border

Border gives you up to 3 excess pixels around an image, eg. for filtering.

LAYOUT_BORDER_0 = 0
LAYOUT_BORDER_1
LAYOUT_BORDER_2
LAYOUT_BORDER_3

4.5 Forcing pixels to be upside down or straight

Vertical constraint forces the image to be stored in a certain vertical direction (by default: any).

LAYOUT_VERT_FLIPPED
LAYOUT_VERT_STRAIGHT

4.6 Gapless pixel access

The Gapless constraint force the image to have contiguous scanlines without excess bytes.

LAYOUT_GAPLESS

If you have both LAYOUT_GAPLESS and LAYOUT_VERT_STRAIGHT, then you can access a slice of all pixels at once, with the ubyte[] allPixelsAtOnce() method.

  image.setSize(640, 480, PixelType.rgba8, LAYOUT_GAPLESS | LAYOUT_VERT_STRAIGHT);
  ubyte[] allpixels = image.allPixelsAtOnce(y);

LAYOUT_GAPLESS is incompatible with constraints that needs excess bytes, like borders, scanline alignment, trailing pixels...

5. Geometric transforms

Gamut provides a few geometric transforms.

Image image;
image.flipHorizontal(); // Flip image pixels horizontally.
image.flipVertical();   // Flip image vertically (pixels or logically, depending on layout)

6. Multi-layer images

6.1 Create multi-layer images

All Image have a number of layers.

Image image;
image.create(640 ,480);
assert(image.layers == 1); // typical image has one layer
assert(image.hasOneLayer);

• Create a multi-layer image, cleared with zeroes:

  // This single image has 24 black layers.
  image.createLayered(800, 600, 24); 
  assert(image.layers == 24);

• Create a multi-layer uninitialized image:

  // Make space for 24 800x600 rgba8 different images.
  image.createLayeredNoInit(800, 600, 24);
  assert(image.layers == 24);

• Create a multi-layer as a view into existing data:

  // Create view into existing data.
  // layerOffsetBytes is byte offset between first scanlines 
  // of two consecutive layers.
  image.createLayeredViewFromData(data.ptr, 
                              w, h, numLaters, 
                              PixelType.rgb8, 
                              pitchbytes,
                              layerOffsetBytes);

Gamut Image is secretly similar to 2D Array Texture in OpenGL. Each layer is store consecutively in memory.

6.2 Get individual layer

image.layer(int index) return non-owning view of a single-layer.

Image image;
image.create(640, 480, 5);
assert(image.layer(4).width  == 640);
assert(image.layer(4).height == 480);
assert(image.layer(4).layers ==   1);

Key concept: All image operations work on all layers by default.

Regarding layout: Each layer has its own border, trailing bytes... and follow the same layout constraints. Moreover, LAYOUT_GAPLESS also constrain the layers to be immediately next in memory, without any byte (like it constrain the scanlines). The layers cannot be stored in reverse order.

6.2 Get sub-range of layers

image.layerRange(int start, int stop) return non-owning view of a several layers.

6.3 Access layer pixels

• Get a pointer to a scanline:

// Get the 160th scanline of layer 2.
void* scan = image.layerptr(2, 160);

• Get a slice of a whole scanline:

// Get the 160th scanline of layer 2.
void[] line = image.layerline(2, 160);

Actually, scanptr(y) and scanline(y) only access the layer index 0.

// Get the 160th scanline of layer 0.
void* scan = image.scanptr(160);
void[] line = image.scanline(160);

Key concept: First layer has index 0.

Consequently, there are two ways to access pixel data in Image:

// Two different ways to access layer pixels.
assert(image.layer(2).scanline(160) == image.layerline(2, 160)

*The calls:*

• image.layerptr(layer, y)
• image.layerline(layer, y)

are like:

• image.scanptr(y)
• image.scanline(y)

but take a layer index.