Love2d: How to retain window content? - loops

Question

In love2d, the contents of the screen are reset between the draw calls. So that I cannot add something to the screen created during the last draw operation, for example, print another line below the line printed during the previous iteration.
How can I do that with the love game engine. Specifically I have a debug area alongside the main game area, and I want to print the logged messages in that window.
Other use cases are drawing some effects over the game area when the player wins(or looses), blurring the background when a dialog is displayed.

Answer 1

Love2ds default loop, which calls love.draw, automatically clears the screen.
see https://love2d.org/wiki/love.run
To avoid this you can use your own run function and remove love.graphics.clear() or you can use a canvas https://love2d.org/wiki/Canvas.

Answer 2

I don't know about a visual debug console. However, manipulating pixels during draw calls can be done using some sort of Shader approach. Here is a sample of a Gaussian Blur pixel effect:
local graphics = love.graphics
function love.load()
local program = ([[
const float kernel[5] = float[](0.2270270270, 0.1945945946, 0.1216216216, 0.0540540541, 0.0162162162);
vec4 effect(vec4 color, sampler2D tex, vec2 tex_coords, vec2 pos) {
color = texture2D(tex, tex_coords) * kernel[0];
for(int i = 1; i < 5; i++) {
color += texture2D(tex, vec2(tex_coords.x + i * %f, tex_coords.y)) * kernel[i];
color += texture2D(tex, vec2(tex_coords.x - i * %f, tex_coords.y)) * kernel[i];
}
return color;
}
]]):format(1 / graphics.getWidth(), 1 / graphics.getWidth())
fx = graphics.newPixelEffect(program)
local program = ([[
const float kernel[5] = float[](0.2270270270, 0.1945945946, 0.1216216216, 0.0540540541, 0.0162162162);
vec4 effect(vec4 color, sampler2D tex, vec2 tex_coords, vec2 pos) {
color = texture2D(tex, tex_coords) * kernel[0];
for(int i = 1; i < 5; i++) {
color += texture2D(tex, vec2(tex_coords.x, tex_coords.y + i * %f)) * kernel[i];
color += texture2D(tex, vec2(tex_coords.x, tex_coords.y - i * %f)) * kernel[i];
}
return color;
}
]]):format(1 / graphics.getHeight(), 1 / graphics.getHeight())
fy = graphics.newPixelEffect(program)
print(fx:getWarnings())
print(fy:getWarnings())
canvas_x = graphics.newCanvas(graphics.width, graphics.height)
canvas_y = graphics.newCanvas(graphics.width, graphics.height)
end
t = 0
function love.draw()
t = t + 0.02
local x = 400 + math.sin(t) * 400
local y = 300 + math.sin(t * 0.8) * 300
graphics.setCanvas(canvas_x)
graphics.push()
graphics.translate(x, y)
graphics.rotate(t * 1.3)
graphics.rectangle("fill", -10, -50, 20, 100)
graphics.pop()
graphics.setPixelEffect(fx)
graphics.setCanvas(canvas_y)
graphics.draw(canvas_x, 0, 0)
graphics.setPixelEffect(fy)
graphics.setCanvas(canvas_x)
graphics.draw(canvas_y, 0, 0)
graphics.setPixelEffect()
graphics.setCanvas()
graphics.draw(canvas_x, 0, 0)
end