From 9e9cb17a5d0d8ad0a8ef391356d986cc4b1164c0 Mon Sep 17 00:00:00 2001
From: Elias Naur <mail@eliasnaur.com>
Date: Fri, 27 Aug 2021 08:39:35 +0200
Subject: [PATCH] gpu: [compute] clip transformed images to their bounds

A follow-up change will cause some transformed images to render outside
their allocated atlas bounds. This change uses the GPU viewport to clip
them so they won't overwrite other atlas content.

Updates gio#219

Signed-off-by: Elias Naur <mail@eliasnaur.com>
---
 gpu/compute.go | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/gpu/compute.go b/gpu/compute.go
index 6528a826..70544708 100644
--- a/gpu/compute.go
+++ b/gpu/compute.go
@@ -917,6 +917,10 @@ func (g *compute) renderMaterials() error {
 			atlas    *textureAtlas
 			imgAtlas *textureAtlas
 		)
+		// A material is clipped to avoid drawing outside its atlas bounds.
+		// However, imprecision in the clipping may cause a single pixel
+		// overflow.
+		var padding = image.Pt(1, 1)
 		var allocStart int
 		for len(texOps) > 0 {
 			op := &texOps[0]
@@ -933,13 +937,10 @@ func (g *compute) renderMaterials() error {
 			}
 			imgAtlas = op.imgAlloc.atlas
 			quad, bounds := g.materialQuad(imgAtlas.size, op.key.transform, op.img, op.imgAlloc.rect.Min)
-			// A material is clipped to avoid drawing outside its atlas bounds.
-			// However, imprecision in the clipping may cause a single pixel
-			// overflow. Be safe.
-			size := bounds.Size().Add(image.Pt(1, 1))
+			size := bounds.Size()
 			alloc, fits := g.atlasAlloc(allocQuery{
 				atlas:    atlas,
-				size:     size,
+				size:     size.Add(padding),
 				format:   driver.TextureFormatRGBA8,
 				bindings: combinedBindings,
 			})
@@ -951,11 +952,15 @@ func (g *compute) renderMaterials() error {
 			}
 			atlas = alloc.atlas
 			alloc.cpu = g.useCPU
-			// Position quad to match place.
-			offsetf := layout.FPt(alloc.rect.Min.Sub(bounds.Min))
+			offsetf := layout.FPt(bounds.Min.Mul(-1))
+			scale := f32.Pt(float32(size.X), float32(size.Y))
 			for i := range quad {
+				// Position quad to match place.
 				quad[i].posX += offsetf.X
 				quad[i].posY += offsetf.Y
+				// Scale to match viewport [0, 1].
+				quad[i].posX /= scale.X
+				quad[i].posY /= scale.Y
 			}
 			// Draw quad as two triangles.
 			m.quads = append(m.quads, quad[0], quad[1], quad[3], quad[3], quad[1], quad[2])
@@ -980,7 +985,7 @@ func (g *compute) renderMaterials() error {
 		}
 		// Transform to clip space: [-1, -1] - [1, 1] and flip Y-axis to cancel the implied transformation
 		// between framebuffer and texture space.
-		m.vert.uniforms.scale = [2]float32{2 / float32(atlas.size.X), -2 / float32(atlas.size.Y)}
+		m.vert.uniforms.scale = [2]float32{2, -2}
 		m.vert.uniforms.pos = [2]float32{-1, +1}
 		m.vert.buf.Upload(byteslice.Struct(m.vert.uniforms))
 		g.ctx.BindVertexUniforms(m.vert.buf)
@@ -995,16 +1000,19 @@ func (g *compute) renderMaterials() error {
 			d.Action = driver.LoadActionClear
 		}
 		g.ctx.BindFramebuffer(atlas.fbo, d)
-		g.ctx.Viewport(0, 0, atlas.size.X, atlas.size.Y)
 		g.ctx.BindPipeline(m.pipeline)
 		g.ctx.BindVertexBuffer(m.buffer.buffer, 0)
-		g.ctx.DrawArrays(driver.DrawModeTriangles, 0, len(m.quads))
+		newAllocs := atlas.allocs[allocStart:]
+		for i, a := range newAllocs {
+			sz := a.rect.Size().Sub(padding)
+			g.ctx.Viewport(a.rect.Min.X, a.rect.Min.Y, sz.X, sz.Y)
+			g.ctx.DrawArrays(driver.DrawModeTriangles, i*6, 6)
+		}
 		if !g.useCPU {
 			continue
 		}
 		copyFBO := atlas.fbo
 		data := atlas.cpuImage.Data()
-		newAllocs := atlas.allocs[allocStart:]
 		for _, a := range newAllocs {
 			stride := atlas.size.X * 4
 			col := a.rect.Min.X * 4