Module: Libpng::BytesPerPixel

Defined in:
lib/libpng/bytes_per_pixel.rb

Overview

Maps pixel formats to bytes-per-pixel. Used by both the simplified and standard encoders / decoders to compute row stride.

Pure data class -- no I/O, no allocations beyond the lookup itself.

Constant Summary collapse

CHANNEL_BYTES_8_BIT =

Per-channel byte count for the supported bit depths. Linear (16-bit) formats aren't currently exposed via the simplified API path; the standard encoder uses this table when bit_depth: 16 is requested.

1
CHANNEL_BYTES_16_BIT = 
2
CHANNELS_BY_COLOR_TYPE =

Channels per PNG_COLOR_TYPE_* (png.h).

{
  Libpng::COLOR_TYPE_GRAY => 1,
  Libpng::COLOR_TYPE_GRAY_ALPHA => 2,
  Libpng::COLOR_TYPE_RGB => 3,
  Libpng::COLOR_TYPE_RGB_ALPHA => 4,
  Libpng::COLOR_TYPE_PALETTE => 1
}.freeze

Class Method Summary collapse

Class Method Details

.for_color_type(color_type, bit_depth: 8) ⇒ Object 



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# File 'lib/libpng/bytes_per_pixel.rb', line 36

def self.for_color_type(color_type, bit_depth: 8)
  channels = CHANNELS_BY_COLOR_TYPE[color_type] ||
             raise(Libpng::Error, "unknown color_type #{color_type}")
  channel_bytes = bit_depth == 16 ? CHANNEL_BYTES_16_BIT : CHANNEL_BYTES_8_BIT
  channels * channel_bytes
end

.for_format(format_value) ⇒ Object 



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# File 'lib/libpng/bytes_per_pixel.rb', line 24

def self.for_format(format_value)
  case format_value
  when Libpng::FORMAT_GRAY then 1
  when Libpng::FORMAT_GA, Libpng::FORMAT_AG then 2
  when Libpng::FORMAT_RGB, Libpng::FORMAT_BGR then 3
  when Libpng::FORMAT_RGBA, Libpng::FORMAT_ARGB,
    Libpng::FORMAT_BGRA, Libpng::FORMAT_ABGR then 4
  else
    raise Libpng::Error, "no bytes-per-pixel mapping for format 0x#{format_value.to_s(16)}"
  end
end