diff --git a/internal/graphicscommand/command.go b/internal/graphicscommand/command.go
index a80b6e7a8..b829bc9ee 100644
--- a/internal/graphicscommand/command.go
+++ b/internal/graphicscommand/command.go
@@ -418,13 +418,27 @@ func (c *drawTrianglesCommand) Exec(indexOffset int) error {
 
 	if c.shader != nil {
 		us := make([]interface{}, len(c.uniforms))
-		// TODO: Adjust the source sizes in uniform variables.
-		for k, v := range c.uniforms {
-			switch v := v.(type) {
+
+		firstImage := true
+		for i := 0; i < len(c.uniforms); i++ {
+			switch v := c.uniforms[i].(type) {
 			case *Image:
-				us[k] = v.image.ID()
+				us[i] = v.image.ID()
+				if !firstImage {
+					// Convert pixels to texels.
+					w, h := v.InternalSize()
+					i++
+					region := c.uniforms[i].([]float32)
+					us[i] = []float32{
+						region[0] / float32(w),
+						region[1] / float32(h),
+						region[2] / float32(w),
+						region[3] / float32(h),
+					}
+				}
+				firstImage = false
 			default:
-				us[k] = v
+				us[i] = v
 			}
 		}
 		return theGraphicsDriver.DrawShader(c.dst.image.ID(), c.shader.shader.ID(), c.nindices, indexOffset, c.mode, us)
diff --git a/internal/shareable/image.go b/internal/shareable/image.go
index ba632d36a..836fea0c5 100644
--- a/internal/shareable/image.go
+++ b/internal/shareable/image.go
@@ -324,16 +324,18 @@ func (i *Image) DrawTriangles(img *Image, vertices []float32, indices []uint16,
 	firstImage := true
 	us := make([]interface{}, len(uniforms))
 	for i := 0; i < len(uniforms); i++ {
-		switch v := us[i].(type) {
+		switch v := uniforms[i].(type) {
 		case *Image:
 			us[i] = v.backend.restorable
 			if !firstImage {
 				i++
-				region := us[i].([]float32)
-				region[0] += oxf
-				region[1] += oyf
-				region[2] += oxf
-				region[3] += oyf
+				region := uniforms[i].([]float32)
+				us[i] = []float32{
+					region[0] + oxf,
+					region[1] + oyf,
+					region[2] + oxf,
+					region[3] + oyf,
+				}
 			}
 			firstImage = false
 		default: