Postshader: Let shaders use the previous frame

[unknownbrackets]

This is useful for i.e. simulating the slow update speed of the PSP's LCD screen, but could in theory be used for other effects.

Here's a test fragment shader I was using:

#ifdef GL_ES
precision mediump float;
precision mediump int;
#endif

uniform sampler2D sampler0;
uniform sampler2D sampler2;
varying vec2 v_texcoord0;

void main() {
	vec3 update = texture2D(sampler0, v_texcoord0.xy).rgb;
	vec3 prev = texture2D(sampler2, v_texcoord0.xy).rgb;

	gl_FragColor.rgb = mix(prev, update, 0.1);
	gl_FragColor.a = 1.0;
}

Using 0.1 makes the effect very obvious, but of course that varies by rendered FPS. For that reason I added a u_timeDelta:

Value	Meaning	Speeds up if unthrottled	Changes on load state
u_timeDelta.x	Fractional seconds, in real time, since last frame	No	No
u_timeDelta.y	Fractional seconds, in emulated time, since last frame (approx)	Yes	Yes
u_timeDelta.z	Number of vblanks (which occur 60 times every 1001ms, i.e. 59.94 times per second) since last frame	Yes	Yes
u_timeDelta.w	In practice, always 1.0 (delta number of flips)	N/A	N/A

With this, the above example can be amended with uniform vec4 u_timeDelta; at the top, and u_timeDelta.y * 6 in place of 0.1 to have a more stable effect regardless of game FPS. But you could also force 60 FPS output.

-[Unknown]

[hrydgard]

Nice refactoring too! Just nitpicking.

[hrydgard]

might as well change this array length to MAX_BOUND_TEXTURES since you're here poking around

[unknownbrackets]

Yep, agreed. Went ahead and cleaned up some other GL checks in Draw too along the same lines.

-[Unknown]

[hrydgard]

same

[LunaMoo]

Not sure if I'm doing something wrong, but even with a test shader posted here past frame is always black, so the test shader results in a 90% mix of black and 10% of current frame, so just a very dark image from current frame.
Does this have some special requirements? I have the same results on all backends.

[iota97]

Does this have some special requirements?

Have you set UsePreviousFrame=true in the ini?

[LunaMoo]

Aw, I completely missed that, it works fine now;3, thanks.