How to save a Jpg file with quality optionThe result
First let's see the results of images in different quality levels. The highest quality is 100, the lowest quality is 0.
Original image: 
Quality 100 image: (Maybe because the image is too big, 2.6mb, we can't upload it now. Sorry.)
Quality 75 image: 
Quality 50 image: 
Quality 25 image: 
Quality 0 image: 
If you compare the images above carefully, you will find noise is lesser when the quality is higher. We can also find the image size is larger when the quality is higher.
In the example above:
original image's size is: 1810kb
quality 100 image's size is: 2619kb
quality 75 image's size is: 329kb
quality 50 image's size is: 186kb
quality 25 image's size is: 127kb
quality 0 image's size is: 82kb
How to use it
-- Test of usage of saving image with quality parameter
require
valid: 0 <= a_quality and a_quality <= 100
local
l_source: WEL_GDIP_BITMAP
l_paras: WEL_GDIP_IMAGE_ENCODER_PARAMETERS
l_para: WEL_GDIP_IMAGE_ENCODER_PARAMETER
l_format: WEL_GDIP_IMAGE_ENCODER
do
create l_source.make_with_size (1, 1)
l_source.load_image_from_file ("Goleta_Flower_orignal.jpg")
-- Start create parameters used by image saving feature
create l_paras.make (1)
-- We set quality parameter here.
create l_para.make ((create {WEL_GDIP_IMAGE_ENCODER}.make_empty).quality, a_quality)
l_paras.parameters.extend (l_para)
create l_format.make_empty
l_source.save_image_to_file_with_encoder_and_parameters (
"Goleta_Flower_with_quality_" + a_quality.out + ".jpg", l_format.jpg, l_paras)
end
The key is feature {WEL_GDIP_IMAGE}.save_image_to_file_with_encoder_and_parameters. This feature will use {WEL_GDIP_IMAGE_ENCODER_PARAMETERS} as last parameter. When saving a image we can not only set quality parameter, but also compression, color_depth, scan_method, version, render_method, transformation, luminance_table, chrominance_table and save_flag parameters. For the parameter type enumeration, please see {WEL_GDIP_IMAGE_ENCODER}.
PS:
Quality parameter has effects on Jpeg format images. For Png format images, the quality parameter will not work.
Saving image with parameters feature is added in revision#72923.
