R Shiny: fast reactive image display - r

Question

I'm trying to display images in my shiny app reactively. I've successfully done that in the server.R script with:
output$display.image <- renderImage({
image_file <- paste("www/",input$image.type,".jpeg",sep="")
return(list(
src = image_file,
filetype = "image/jpeg",
height = 520,
width = 696
))
}, deleteFile = FALSE)
BUT it's very slow.
However, it is VERY fast to embed one of the images into the ui.R script like so:
tabPanel("Live Images", img(src = "img_type1.jpeg"))
Why is there such a difference? Is there any way to make the reactive images appear faster?

Answer 1

Hi you can use conditionalPanel to do this, it embed all your images but only the one which have TRUE to the condition will be displayed :
tabPanel("Live Images",
conditionalPanel(condition = "input.image_type == 'img_type1'",
img(src = "img_type1.jpeg")
),
conditionalPanel(condition = "input.image_type == 'img_type2'",
img(src = "img_type2.jpeg")
)
)
And change the name of your input from image.type to image_type because . have special meaning in Javascript (as between input and image_type).
If you have a lot of images, you can always do something like that :
tabPanel("Live Images",
lapply(X = seq_len(10), FUN = function(i) {
conditionalPanel(condition = paste0("input.image_type == 'img_type", i, "'"),
img(src = paste0("img_type", i, ".jpeg"))
)
})
)
For example, with images from this post by tsperry (you can find it on rbloggers too), you can do :
library("shiny")
ui <- fluidPage(
tabsetPanel(
tabPanel("Live Images",
# 50 images to display
lapply(X = seq_len(50), FUN = function(i) {
# condition on the slider value
conditionalPanel(condition = paste0("input.slider == ", i),
# images are on github
img(src = paste0("https://raw.githubusercontent.com/pvictor/images/master/",
sprintf("%04d", i), "plot.png"))
)
}),
sliderInput(inputId = "slider", label = "Value", min = 1, max = 50, value = 1,
animate = animationOptions(interval = 100, loop = TRUE))
)
)
)
server <- function(input, output) {
}
shinyApp(ui = ui, server = server)