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Design pattern 
 for Image and text composition in Go Go Conference 2019 Spring

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SEIJI TAKAHASHI Software Engineer at Gunosy Inc.

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Agenda > Image and font packages > Design pattern for composition > Conclusions

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There are so much stuff to do when you composite image resources and generate a new one.

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Scope for this talk is … > Workflow of image composition > How to implement them along with the separation of concerns.

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Scope for this talk is … Timakin Timakin

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Not scope for this talk ... > The detail of the image package > Image processing except composition

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Image and font packages

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Quick Question Have you ever used the image package?

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package “image” > Package image implements a basic 2-D image library. • Encodes/decodes formats such as GIF, JPEG or PNG. > Provides below interfaces • An inspection of data in the pixels • Adds a color to the specified range • Composition, mask and quantization.

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e.g. Draw a red rectangle

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Slide 12 text > Package which defines an interface for font faces, for drawing text on an image. > font.Face is an interface that represents a content of the file of font face. > font.Drawer reads it and draws text on a destination image. > Opentype is not suitable for font.Drawer.

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Slide 13 text > Package which provides a parser for the TTF and TTC file formats. > When you would like to flexibly set a font face to font.Drawer, this package will really help you. > A lot of .ttf/.ttc files don’t pass the parse process. • PostScript font type • Hiragino Sans…

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Design pattern of image composition

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Workflow of composition > To generate this name card, 
 you have to • Initialize a frame • Draw images • Attach labels • Encode a jpeg output

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Interfaces Processor Initialize a frame Draw images Attach labels Encode a jpeg output Framer Drawer Labeler Encoder

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Roles of each structs Framer Drawer Labeler Encoder

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Processor > Loads fixture images and pass
 them to Drawer. > Includes Framer, Drawer,
 Encoder as fields.

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Loading fixtures > is better if your fixtures are not so large to save the loading time.

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Insertion of dependent structs > Optional values • Main Thumbnail • Sub image • Background image • Labels • Color • Font family/size • Output path

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Framer > Initializes *image.RGBA 
 as a canvas. > Width & height, and 
 a background color are required.

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Fill the RGBA frame with colors

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Drawer > Receiving the frame, drawer draws fixture and specified images.

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Call drawing and labeling functions inside > Labeler is called inside of Drawer. • The positions of the texts are calculated by relative value to the images.

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Call drawing and labeling functions inside Sometimes you have to guard an unexpected-sized images, so drawing function often contains a resizer.

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Labeler > Labeler attaches a text label with the options. (coordinate, font- family, text alignment etc)

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Loading assets and font.Drawer > After loading font assets, call truetype font parser with options of size and color.

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A minor adjustment of text position 
 with fixed.PointXX_xx

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Encoder > Encoder just calls encode function the image package. > The role of this interface is to abstract the difference between each formats.

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Conclusion > Image and text compression becomes really messy if you don’t care about the separation of concerns. • Separation by the role becomes easy if you simplify the process. > Composition process deeply depends on a coordinate calculation. Designing optional values is the essential for a simple code.

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Thank you for listening