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Design pattern
for Image and text
composition in Go
Go Conference 2019 Spring
@__timakin__
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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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golang.org/x/image/font
> 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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github.com/golang/freetype/truetype
> 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
> github.com/rakyll/statik 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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No content
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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