Manipulating PNG Images

Specifying Transparency for PNG Images

One of the advantages of saving images in PNG format is that PNG can have transparent background. Aspose.Imaging for Python via .NET provides the feature for specifying transparency for the PngImage & RasterImage classes as demonstrated in the below section. Aspose.Imaging for Python via .NET API can be used to set any color as transparent while creating new PNG images or converting existing images to PNG format. For this purposes, the Aspose.Imaging for Python via .NET API has exposed TransparentColor property and PngColorType enumeration that can be set to specify any color to be rendered as transparent in the newly created PNG image. The below provided code snippet demonstrates how to convert an existing BMP image to PNG format by creating a PNG image from scratch while specifying the desired color as transparent.

import aspose.pycore as aspycore
from aspose.imaging import Image, RasterImage, Rectangle, Color
from aspose.imaging.imageoptions import PngOptions
from aspose.imaging.fileformats.png import PngColorType, PngImage
import os


if 'TEMPLATE_DIR' in os.environ:
	templates_folder = os.environ['TEMPLATE_DIR']
else:
	templates_folder = r"C:\Users\USER\Downloads\templates"

delete_output = 'SAVE_OUTPUT' not in os.environ
data_dir = templates_folder
# Initialize variables to hold width & height values
width = 0
height = 0
# Initialize an array of type Color to hold the pixel data
pixels = None
# Create an instance of RasterImage and load a BMP image
with aspycore.as_of(Image.load(os.path.join(data_dir, "template.png")), RasterImage) as raster:
	# Store the width & height in variables for later use
	width = raster.width
	height = raster.height
	# Load the pixels of RasterImage into the array of type Color
	pixels = raster.load_pixels(Rectangle(0, 0, width, height))

# Create & initialize an instance of PngImage while specifying size and PngColorType
with PngImage(width, height, PngColorType.TRUECOLOR_WITH_ALPHA) as png:
	# Save the previously loaded pixels on to the new PngImage and Set TransparentColor property to specify which color to be rendered as transparent
	png.save_pixels(Rectangle(0, 0, width, height), pixels)
	png.transparent_color = Color.black
	png.has_transparent_color = True
	png.save(os.path.join(data_dir, "result.jpg"))

if delete_output:
	os.remove(os.path.join(data_dir, "result.jpg"))

Setting Resolution for PNG Images

With Aspose.Imaging for Python via .NET 2.5.0, the API exposes the ResolutionSetting class which can be used to set the resolution for all image formats including PNG. This article demonstrates the usage of the Aspose.Imaging for Python via .NET API to set the horizontal & vertical resolution parameters for the PNG image format. The code snippet below loads an existing BMP image and converts it to PNG format also changing the resolution.

import aspose.pycore as aspycore
from aspose.imaging import Image, RasterImage, Rectangle, ResolutionSetting
from aspose.imaging.fileformats.png import PngImage
from aspose.imaging.imageoptions import PngOptions
import os


if 'TEMPLATE_DIR' in os.environ:
	templates_folder = os.environ['TEMPLATE_DIR']
else:
	templates_folder = r"C:\Users\USER\Downloads\templates"

delete_output = 'SAVE_OUTPUT' not in os.environ
data_dir = templates_folder
file_name = os.path.join(data_dir, "template.tiff")
# Initialize variables to hold width & height values
width = 0
height = 0
# Initialize an array of type Color to hold the pixel data
pixels = None
# Create an instance of RasterImage and load a BMP image
with aspycore.as_of(Image.load(os.path.join(data_dir, "template.png")), RasterImage) as raster:
	# Store the width & height in variables for later use
	width = raster.width
	height = raster.height
	# Load the pixels of RasterImage into the array of type Color
	pixels = raster.load_pixels(Rectangle(0, 0, width, height))

# Create & initialize an instance of PngImage while specifying size and PngColorType
with PngImage(width, height) as png:
	# Save the previously loaded pixels on to the new PngImage
	png.save_pixels(Rectangle(0, 0, width, height), pixels)
	# Create an instance of PngOptions, Set the horizontal & vertical resolutions and Save the result on disc
	options = PngOptions()
	options.resolution_settings = ResolutionSetting(72, 96)
	png.save(os.path.join(data_dir, "result.png"), options)

if delete_output:
	os.remove(os.path.join(data_dir, "result.png"))

Compressing PNG Files

The Portable Network Graphic (PNG) is a lossless compression format for transmitting a bitmap over networks. When you save an image as a PNG file in any program, you may be asked to choose a compression level in a range from 0 to any max level. Setting this value actually compresses the file size and does not decrease the image quality. This article describes how Aspose.Imaging APIs allows you to control the PNG file size. Aspose.Imaging APIs can be used to set the Compression Levels for the PNG file format using the PngOptions class that has an int type CompressionLevel property. This property accepts a value from 0 to 9 where 9 is the maximum compression. The below provided code snippet demonstrates how to set the Compression Levels using Aspose.Imaging for Python via .NET API.

import aspose.pycore as aspycore
from aspose.imaging import Image
from aspose.imaging.imageoptions import PngOptions
import os


if 'TEMPLATE_DIR' in os.environ:
	templates_folder = os.environ['TEMPLATE_DIR']
else:
	templates_folder = r"C:\Users\USER\Downloads\templates"

delete_output = 'SAVE_OUTPUT' not in os.environ
data_dir = templates_folder
# Load an image from file (or stream)
with Image.load(os.path.join(data_dir, "template.png")) as image:
	# Loop over possible CompressionLevel range
	for i in range(9):
		# Create an instance of PngOptions for each resultant PNG, Set CompressionLevel and  Save result on disk
		options = PngOptions()
		options.compression_level = i
		file_name = os.path.join(data_dir, str(i) + "_out.png")
		image.save(file_name, options)
		if delete_output:
			os.remove(file_name)

Specifying Bit Depth for PNG Images

Bit depth in imaging is the number of bits used to indicate the color of a single pixel in a bitmap image. Like all other bitmap formats, PNG color depth is also represented in bit such as 1-bit (2 colors), 2-bit (4 colors), 4-bit (16 colors) and 8-bit (256 colors). Aspose.Imaging for Python via .NET API can be used to set bit depth for PNG images using BitDepth property exposed by the PngOptions class. At the moment, the BitDepth property can be set to 1, 2, 4 or 8 bits for grayscale and indexed color types. For all other color types only 8 bits are supported. The below provided code snippet demonstrates how to set the Bit Depth of an existing PNG.

import aspose.pycore as aspycore
from aspose.imaging import Image
from aspose.imaging.imageoptions import PngOptions
from aspose.imaging.fileformats.png import PngColorType, PngImage
import os


if 'TEMPLATE_DIR' in os.environ:
	templates_folder = os.environ['TEMPLATE_DIR']
else:
	templates_folder = r"C:\Users\USER\Downloads\templates"

delete_output = 'SAVE_OUTPUT' not in os.environ
data_dir = templates_folder
# Load an existing PNG image
with aspycore.as_of(Image.load(os.path.join(data_dir, "template.png")), PngImage) as png:
	# Create an instance of PngOptions, Set the desired ColorType, BitDepth 
	# according to the specified ColorType and save image
	options = PngOptions()
	options.color_type = PngColorType.GRAYSCALE
	options.bit_depth = 1
	png.save(os.path.join(data_dir, "result.png"), options)

if delete_output:
	os.remove(os.path.join(data_dir, "result.png"))

Applying Filter Methods on PNG Images

Aspose.Imaging for Python via .NET provides the functionality to apply filter methods (algorithms) before compression as demonstrated in the below section. The purpose of these filter methods is to prepare the image data for optimum compression. Aspose.Imaging for Python via .NET API can be used to apply filter methods before compressing any PNG image. For this purposes, the Aspose.Imaging for Python via .NET API has exposed a property named FilterType of PngOptions class. It accepts PngFilterType enumeration that can be used to specify the filter method (algorithm) among the five including 0 for none. The below provided code snippet demonstrates how to apply filter method on PNG image.

import aspose.pycore as aspycore
from aspose.imaging import Image
from aspose.imaging.fileformats.png import PngImage, PngFilterType
from aspose.imaging.imageoptions import PngOptions
import os


if 'TEMPLATE_DIR' in os.environ:
	templates_folder = os.environ['TEMPLATE_DIR']
else:
	templates_folder = r"C:\Users\USER\Downloads\templates"

delete_output = 'SAVE_OUTPUT' not in os.environ
data_dir = templates_folder
with aspycore.as_of(Image.load(os.path.join(data_dir, "template.png")), PngImage) as png:
	# Create an instance of PngOptions, Set the PNG filter method and Save changes to the disc
	options = PngOptions()
	options.filter_type = PngFilterType.PAETH
	png.save(os.path.join(data_dir, "result.png"), options)

if delete_output:
	os.remove(os.path.join(data_dir, "result.png"))

Changing Background Color of a Transparent PNG Image

PNG format images can have transparent background. Aspose.Imaging for Python via .NET provides the feature to change the background color of a PNG image that has transparent background. Aspose.Imaging for Python via .NET API can be used to set/change color of a transparent PNG image. The below provided code snippet demonstrates how to set/change the background color of a transparent PNG image.

import aspose.pycore as aspycore
from aspose.imaging import Image, RasterImage, Color
import os


if 'TEMPLATE_DIR' in os.environ:
	templates_folder = os.environ['TEMPLATE_DIR']
else:
	templates_folder = r"C:\Users\USER\Downloads\templates"

delete_output = 'SAVE_OUTPUT' not in os.environ
data_dir = templates_folder
# Create an instance of Image class and load a PNG image
with Image.load(os.path.join(data_dir, "template.png")) as img:
	# Create an instance of RasterImage and get the pixels array by calling method LoadArgb32Pixels.
	raster_img = aspycore.as_of(img, RasterImage)
	if raster_img is not None:
		pixels = raster_img.load_argb_32_pixels(img.bounds)
		if pixels is not None:
			# Iterate through the pixel array and Check the pixel information that if it is a transparent color pixel and Change the pixel color to white
			transparent_color = raster_img.transparent_color.to_argb()
			white_color = Color.white.to_argb()
			for i in range(pixels.length):
				if pixels[i] == transparent_color:
					pixels[i] = white_color

			# Replace the pixel array into the image.
			raster_img.save_argb_32_pixels(img.bounds, pixels)

	# Save the updated image to disk.
	if raster_img is not None:
		raster_img.save(os.path.join(data_dir, "result.png"))

if delete_output:
	os.remove(os.path.join(data_dir, "result.png"))

Optimizing Memory Usage

When reading a big PNG file, the total amount of RAM the process will take is always a concern. There are measures which can be adopted to cope with the challenge. Aspose.Imaging provides some relevant options and API calls to lower, reduce and optimize memory use. Also, it can help the process work more efficiently and run faster.

The following example shows how to read a large PNG file in optimized mode.

import aspose.pycore as aspycore
from aspose.imaging import Image
from aspose.imaging.imageoptions import JpegOptions
from aspose.imaging.memorymanagement import Configuration
import os


if 'TEMPLATE_DIR' in os.environ:
	templates_folder = os.environ['TEMPLATE_DIR']
else:
	templates_folder = r"C:\Users\USER\Downloads\templates"

delete_output = 'SAVE_OUTPUT' not in os.environ
data_dir = templates_folder
# Enabling the global memory usage optimization strategy.
# Setting a global memory limit of 50 megabytes 
Configuration.set_buffer_size_hint(50)
with Image.load(os.path.join(data_dir, "template.png")) as image:
	image.save(os.path.join(data_dir, "result.jpg"), JpegOptions())

if delete_output:
	os.remove(os.path.join(data_dir, "result.jpg"))

# Resetting a global memory limit
Configuration.set_buffer_size_hint(0)