Glob2 renders a white square instead of semi-transparent building when placing a school

[Quipyowert2]

When I start tutorial 4 and start to place a school in a spot where there is already a building, the game shows a white square with a red outline instead of a semi-transparent building indicator like when placing a hospital. Happens with commit c196fa5. This is probably my fault. I think the bug is from my PR #120.

Please disregard the tutorial messages in the screenshots, the "A unique string" is from when I was testing something.

[stephanemagnenat]

Hum, it feels to me like the per-object-draw transparency is not correct when using the new drawing path. I do not have the full new drawing code in mind, but I feel that somehow the transparency of each draw should be stored somewhere and used in the shader when drawing each object, even from the same sprite sheet.

[Quipyowert2]

Looks like I need to somehow pass the transparency values to glDrawArrays instead of falling back to the slow path (glBegin/glEnd) when the sprite is being drawn with transparency. From a quick search, it looks like this can be done with vertex array objects or maybe shaders? I will have to look up how to do that when I have time, hopefully in the next few days.

[stephanemagnenat]

In Candli I'm doing something similar using a simple pair af vertex/fragment shaders. But it is in WebGL so the setup API calls differ a bit. I can copy/paste the shaders here if that is helpfu (later, I'm on mobile now)l, although they might need some adaptation for OpenGL 2.

[stephanemagnenat]

Here are the Candli object shaders for WebGL 1.0 (based on OpenGL ES 2.0 API):

Vertex

attribute vec2 aVertexPosition;
attribute vec2 aTextureCoord;

uniform mat3 uWorldToViewport;
uniform mat3 uPoseToWorld;

varying lowp vec2 vTextureCoord;

void main() {
	gl_Position = vec4(uWorldToViewport * uPoseToWorld * vec3(aVertexPosition, 1.0), 1.0);
	vTextureCoord = aTextureCoord;
}

In Glob2 you might not need both uWorldToViewport and uPoseToWorld.

Fragment

varying lowp vec2 vTextureCoord;

uniform sampler2D uSampler;
precision lowp float;
uniform float uAlpha;

void main(void) {
	vec4 col = texture2D(uSampler, vTextureCoord);
	col.rgba *= uAlpha;
	gl_FragColor = col;
}

Hopefully these can serve as a helpful inspiration, uAlpha allows for per draw alpha.